Structural refinement, growth process, photoluminescence and photocatalytic properties of (Ba_1-xPr_2x/3)WO₄ crystals synthesized by the coprecipitation method

Abstract

Barium praseodymium tungstate (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01, and 0.02) were prepared by the coprecipitation method. These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier-transform Raman (FT-Raman) and Fourier-transform infrared (FT-IR) spectroscopies. The shape and size of these crystals were observed by field emission scanning electron microcopy (FE-SEM). Their optical properties were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. Moreover, we have studied the photocatalytic (PC) activity of crystals for degradation of rhodamine B (RhB) dye. XRD patterns, Rietveld refinements data, FT-Raman and FT-IR spectroscopies indicate that all crystals exhibit a tetragonal structure without deleterious phases. FT-Raman spectra exhibited 13 Raman-active modes in a range from 50 to 1000 cm⁻¹, while FT-IR spectra have 8 infrared active modes in a range from 200 to 1050 cm⁻¹. FE-SEM images showed different shapes (bonbon-, spindle-, rice- and flake-like) as well as a reduction in the crystal size with an increase in Pr³⁺ ions. A possible growth process was proposed for these crystals. UV-vis absorption measurements revealed a decrease in optical band gap values with an increase of Pr³⁺ into the matrix. An intense green PL emission was noted for (Ba_1-xPr_2x/3)WO₄ crystals (x = 0), while crystals with (x = 0.01 and 0.02) produced a reduction in the wide band PL emission and the narrow band PL emission which is related to f–f transitions from Pr³⁺ ions. High photocatalytic efficiency was verified for the bonbon-like BaWO₄ crystals as a catalyst in the degradation of the RhB dye after 25 min under UV-light. Finally, we discuss possible mechanisms for PL and PC properties of these crystals.

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Introduction

Barium tungstate (BaWO₄) crystals were traditionally prepared by different methods: an oxide mixture or a solid state reaction,^1–3 flux evaporation,⁴ the Czochralski method,^5–8 top-seeded slow-cooling⁹ and melting at a high temperature.¹⁰ However, these preparation methods require high temperatures, long processing times and sophisticated equipment with high maintenance costs as well as the formation of deleterious phases.

In recent years, several synthesis methods have been developed and used in the preparation of micro-, nano- and BaWO₄ mesocrystals.^11–16 Different synthesis methods reported in the literature to obtain BaWO₄ crystals are: coprecipitation,^17,18 electrochemical,^19,20 a biomimetic system of a supported liquid membrane,²¹ sonochemical,²² conventional hydrothermal,^23–25 solvothermal,^26–28 microwave-hydrothermal^29,30 and cyclic-microwave.³¹ These methods facilitate the minimization of problems encountered in traditional methods. However, new synthesis routes and processing can lead to the formation of crystals with different sizes, shapes and structures.^32,33 In particular, the conventional hydrothermal method has recently been widely used in the preparation of various tungstates with different sizes and shapes.^34,35 However, these method still require high temperatures (160–220 °C) and long processing times (12–48 h).³⁶

Regarding their crystalline structure, tungstates (AWO₄, A = Ca, Sr, Ba and Pb) have a scheelite-type tetragonal structure which is characterized by a space group of I4₁/a) and a point group symmetry of (C⁶_4h)^37–40 while tungstates (AWO₄, A = Zn, Mn, Co, Fe and Ni) exhibit a wolframite-type monoclinic structure with a space group of (P2/c) and a point group symmetry of (C⁴_2h).^41–44 Consequently, both “scheelite and wolframite” structures follow a general formula (AWO₄) since they are composed of cations (A²⁺) in the lattice. Depending on the electron density of these cations, the structure can be tetragonal or monoclinic.

In relation to crystal-growth mechanisms in aqueous solution systems, it is well known that the “Ostwald ripening process”⁴⁵ and the “oriented attachment”⁴⁶ are the two main processes involved. Ostwald ripening is a thermodynamically-driven spontaneous process that occurs because larger particles are more energetically favored than smaller particles. In this growth process, all smaller crystals are dissolved and deposited on other intermediate crystals, while larger crystals continue to grow. After a length of time, all crystals in this system are already grown to minimize the total surface area. In the “oriented attachment” mechanism, the bigger crystals are grown from small primary nanocrystals through an oriented merger process where adjacent nanocrystals are self-assembled by sharing a common crystallographic orientation and the docking of these crystals at a planar interface.^46,47

In particular, the tungstates (AWO₄; A = Ca, Sr, Ba, Sr) with scheelite-type tetragonal structures have recently attracted the attention of several researchers, mainly because of their excellent photoluminescence (PL) properties at room temperature.^48–50 Among these tungstates, BaWO₄ crystals have the best PL, cathodoluminescence and photocatalytic (PC) properties reported in the literature.^51–53

In this paper, for the first time, we report the synthesis of barium praseodymium tungstate (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01, and 0.02) prepared by the coprecipitation (CP) method at room temperature. The structural refinement, morphology, optical properties and PC activity for the degradation of rhodamine B (RhB) are discussed in detail. Finally, we propose models to explain the growth mechanism, PL and PC properties of these crystals.

Experimental details

Synthesis of (Ba_1-xPr_2x/3)WO₄ crystals by the CP method

(Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01, and 0.02) were prepared by the CP method at room temperature without surfactants in aqueous solutions. The typical BaWO₄ crystal synthesis procedure is described as follows: 2 × 10⁻³ mol tungstate(VI) sodium dihydrate (Na₂WO₄·2H₂O) (99% purity, Vetec) and 2 × 10⁻³ mol barium(II) acetate [Ba(CH₃COO)₂] (99% purity, Aldrich) were dissolved separately into two plastic tubes (Falcon) with 50 mL capacity of deionized water. The first solution with (2Na⁺ and WO²⁻₄) ions was transferred to a beaker of 250 mL capacity under constant stirring for 2 min. Then the second solution with Ba²⁺ and 2CH₃COO⁻ ions was added to this system where a suspension rapidly formed, and a white powder precipitated. Reactions between (Ba²⁺ ←:WO²⁻₄) ions resulted in the formation of crystalline BaWO₄ precipitates. (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0.01 and 0.02) were synthesized as follows: 2 × 10⁻³ mol Na₂WO₄·2H₂O, 99%, and 98% of the 2 × 10⁻³ mol [Ba(CH₃COO)₂] were dissolved separately as described above. However, 1% and 2% of the 2 × 10⁻³ mol praseodymium(III) nitrate hexahydrate [Pr(NO₃)₃·6H₂O] was previously prepared by the dissolution of praseodymium (III,IV) oxide [Pr₆O₁₁] (99.999% purity, Aldrich) with 4 drops of nitric acid (65% Suprapur®, Merck) into a beaker with 5 mL (H₂O) at 85 °C and 100 mL capacity. After the complete dissolution of [Pr₆O₁₁ → Pr₂O₃ (III) + 4PrO₂ (IV)], these oxides in an aqueous solution are decomposed in the Pr³⁺ ions in acid medium and form only Pr(NO₃)₃·6H₂O with a more stable valence state. Then 50 mL of the Ba²⁺ ion solution was added to the system which resulted in the homogenization of the two Pr³⁺ and Ba²⁺ ions. In sequence, these systems were transferred into a beaker of 250 mL capacity with WO²⁻₄ ions under constant stirring for 2 min. Reactions between (Pr³⁺/Ba²⁺ ←:WO²⁻₄) ions resulted in the formation of crystalline (Ba,Pr)WO₄ precipitates as shown in eqn (1)–(5) below:After the CP at room temperature:

The resulting suspensions were washed with deionized water several times to remove any Na⁺ ions. Finally, these white powdered precipitates were collected and dried with acetone at 25 °C for some hours.

Characterization

After the CP at 25 °C, these (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01, and 0.02) were structurally characterized by different techniques (ESI†).

Photocatalytic activity measurement

After, the PC properties of these crystals (as an agent catalyst) for the degradation of [9-(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene]-diethylammonium chloride, which is known as tetraethylated rhodamine or Rhodamine B (RhB) dye with a molecular formula [C₂₈H₃₁ClN₂O₃] (99.5% purity, Mallinckrodt) in an aqueous solution were tested under UV-light illumination. 50 mg catalyst crystals were placed in a 250 mL beaker; 50 mL of RhB solution (1 × 10⁻⁵ mol L⁻¹) with pH = 4 was added. These suspensions were ultrasonicated for 10 min in a Branson (Model 1510) ultrasonic cleaner with a frequency of 42 kHz before illumination then were stored in the dark for 5 min to allow the saturated absorption of RhB onto the catalyst. The beakers were then placed in a photo-reactor at 20 °C and illuminated by six UV lamps (TUV Philips, 15 W, with a maximum intensity at 254 nm). The power light was measured by Coherent Power Max model No PM10 and the value for the optical energy density was 20 mW cm⁻². At five-minute intervals, one 3 mL aliquot of these suspensions was removed and centrifuged at 9000 rpm for 5 min to remove crystals in suspension. Finally, variations of the absorption band maximum of supernatant solutions were monitored by UV-vis absorbance spectra measurements using a double-beam spectrophotometer with a double monochromator and a photomultiplier tube detector of JASCO (Model V-660, USA).

Results and discussion

XRD and Rietveld refinement analyses

Fig. 1(a,b) show XRD patterns for (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01, and 0.02) precipitates obtained by CP at 25 °C in a range from 10° to 70° and zoomed in normalized XRD patterns (from 27° to 35°), respectively.

Fig. 1 (a) XRD patterns ranging from 10° to 70° of the (Ba_1-xPr_2x/3)WO₄ crystals (x = 0, 0.01, and 0.02) precipitated at room temperature. Inset show the zoomed in XRD patterns from 53° to 55.5° (2θ range). The vertical lines indicate the position and relative intensity of the ICSD card N°. 155 511 for BaWO₄ phase and (b) normalized XRD patterns (from 27° to 35°), respectively.

Fig. 1(a) demonstrates that precipitated BaWO₄ crystals are crystalline and structurally ordered at long-range. When Pr³⁺ ions are incorporated into the BaWO₄ lattice, a splitting of the XRD peaks occurs which is related to (107) and (224) planes (see Fig. 1(a) and the inset of Fig. 1(a)). This same figure also verifies that (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0.02) have a higher XRD pattern signal/noise ratio which infers possible roughness in these surface crystals. According to Robinson,⁵⁴ it is possible to determine the surface roughness of crystals by X-ray crystallographic determinations. Also, we verified that all XRD patterns of these crystals correspond to a scheelite-type tetragonal structure which is in agreement with the respective Inorganic Crystal Structure Database (ICSD) N°. 155 511.⁵⁵ To verify and confirm that the structure of these (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01, and 0.02) are really tetragonal, a structural refinement was conducted by the Rietveld method.⁵⁶Fig. 1(b) shows that (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0) have a different normalized intensity between (004) and (200) planes in relation to (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0.01, and 0.02). Since these crystals are polycrystalline, it is possible that the synthesis method of CP and dopant can cause different effects in the crystallographic orientation. Therefore, the differences of normalized intensity between the (004) and (200) planes for these crystals were compared to verify that the (Ba_1-xPr_2x/3)WO₄ crystals with x = 0 have a I_norm(004) > I_norm(200) plane, while (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 possess a I_norm(004) < I_norm(200) plane. This characteristic indicates that the pure BaWO₄ crystal has a crystallographic orientation different from standard diffraction patterns (ICSD N°. 155 511) while (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01, and 0.02 have very similar XRD patterns as can be verified in the following section from Rietveld refinement data.

The lattice parameters, unit cell volume and atomic positions were obtained from a GSAS program.^57,58 Structural refinement plots for (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01 and 0.02) crystals prepared by the CP method at 25 °C for 2 min are illustrated in Fig. 2(a–c).

Fig. 2 Rietveld refinement plots of the (Ba_1-xPr_2x/3)WO₄ crystals precipitate at room temperature: (a) x = 0; (b) x = 0.01, and (c) x = 0.02.

The results obtained from the Rietveld refinement method show good agreement between observed XRD patterns and theoretical results (see Fig. 2(a–c)). Moreover, the difference between XRD pattern profiles experimentally observed and theoretically calculated data display small differences near to zero in the intensity scale as illustrated by a line (Y_observed − Y_calculated). More details regarding structural refinement are shown in Table 1.

Table 1 Lattice parameters, unit cell volume, atomic coordinates and site occupation obtained by Rietveld refinement data for the (Ba_1-xPr_2x/3)WO₄ crystals with x = 0; 0.01, and 0.02 at room temperature^a

■Atoms	Wycroff	Site	S.O.F		x		y		z
a S.O.F = Site occupancy factor; (Ba1-xPr2x/3)WO4 crystals with (x = 0; ■), (x = 0.01; ●), and (x = 0.02; ▲) obtained by CP method; *generated from Rietveld refinements in ESI,† Table S1(a–c)).
Ba	4b	−4	1		0		0.25		0.625
W	4a	−4	1		0		0.25		0.125
O	16f	1	1		0.24566		0.12581		0.03991
a = b = 5.60433(1) Å; c = 12.7338(4) Å; V = 399.949(2) Å^3; Rwp = 10.82%; Rp = 8.12%; Rb = 2.80%; χ^2 = 5.299 and S = 2.032
●Atoms	Wycroff	Site	S.O.F		x		y			z
Ba	4b	−4	0.9813		0		0.25			0.625
Pr	4b	−4	0.01247		0		0.25			0.625
W	4a	−4	1		0		0.25			0.125
O	16f	1	1		0.25209		0.14395			0.04443
a = b = 5.61125(9) Å; c = 12.73071(3) Å; V = 400.841(1) Å^3; Rwp = 10.39%; Rp = 7.50%; Rb = 3.46%; χ^2 = 4.388 and S = 2.095
▲Atoms	Wycroff	Site		S.O.F		x		y			z
Ba	4b	−4		0.97616		0		0.25			0.625
Pr	4b	−4		0.02256		0		0.25			0.625
W	4ª	−4		1		0		0.25			0.125
O	16f	1		1		0.23923		0.12208			0.05307
a = b = 5.61140(7) Å; c = 12.7288(4) Å; V = 400.802(1) Å^3; Rwp = 10.64%; Rp = 8.04%; Rb = 4.54%; χ^2 = 4.018 and S = 2.004

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In this table, fitting parameters (R_wp, R_p, R_b, χ², and S) indicate good agreement between refined and observed XRD patterns for (Ba_1-xPr_2x/3)WO₄ crystals with x = 0, 0.01, and 0.02 crystals. Structural refinement data confirm that these crystals are crystallized in a scheelite-type tetragonal structure with a space group of (I4₁/a). However, some variations in the atomic positions related to oxygen atoms were observed while barium, praseodymium and tungsten atoms have fixed atomic positions. These results indicate that the position of oxygen atoms is very disturbed in the lattice. Therefore, we believe these variations in atomic positions of oxygen atoms can lead to the formation of three types of distortions on [Ba–O], [Pr–O] and/or [W–O] bonds and consequently promotes different levels of distortions on the [BaO₈], [PrO₈] and/or [WO₄] clusters in the lattice.

Representation of the (Ba_1-xPr_2x/3)WO₄ unit cells

Fig. 3(a–c) illustrate schematic representations for tetragonal (Ba_1-xPr_2x/3)WO₄ unit cells with (x = 0, 0.01 and 0.02) modeled from Rietveld refinement data.

Fig. 3 Schematic representation of tetragonal (Ba_1-xPr_2x/3)WO₄ unit cells with (a) x = 0; (b) x = 0.01, (c) x = 0.02 and its [BaO₈]–[PrO₈]–[WO₄] clusters, respectively.

These unit cells were modeled through the Diamond Crystal and Molecular Structure Visualization (Version 3.2 g for Windows) program⁵⁹ using lattice parameters and atoms positions obtained from Rietveld refinement data presented in Table 1. BaWO₄ crystals belong to scheelite-type tetragonal structures with a space group of (I4₁/a), a point-group symmetry of (C⁶_4h) and four molecular formula per unit cell (Z = 4).⁶⁰Fig. 3(a) shows that bonds between O–W–O and O–Ba–O atoms were projected out of the unit cell. In these unit cells, tungsten (W) atoms are coordinated to four oxygen atoms which form [WO₄] clusters with a tetrahedral configuration, a symmetry group (T_d) and tetrahedron polyhedra (4 vertices, 4 faces and 6 edges).⁶¹ These [WO₄] clusters are slightly distorted in the lattice and for each (Ba_1-xPr_2x/3)WO₄ crystal. These differences in the (O–W–O) bond angles can lead to different levels of order-disorder and/or distortions in the (Ba_1-xPr_2x/3)WO₄ crystal lattice with x = 0.01 and 0.02; (see Fig. 3(b,c)). We believe that this behavior can be due to the effect of Pr in the BaWO₄ crystal lattice. In addition, for all unit cells, barium (Ba) atoms are bonded to eight oxygen atoms, which result in [BaO₈] clusters with a deltahedral configuration, a symmetry group of (D_2d) and snub-dispenoide polyhedra (8 vertices, 12 faces and 18 edges).^62,63 Thus, [PrO₈] clusters have the same electronic configuration as [BaO₈] clusters in the A-site. Moreover, we note possible distortions on [BaO₈] clusters through different bond angles between the (O–Ba–O). However, this situation is much more complicated and needs to be analyzed and explained in more detail.

FT-Raman/infrared spectroscopies analyses

According to group theory calculations, tungstates with a scheelite-type tetragonal exhibit 26 different (Raman and infrared) vibrational modes which are represented by eqn (6) below:⁶⁴

Γ_{(Raman+Infrared)} = 3A_g + 5A_u + 5B_g + 3B_u + 5E_g + 5E_u (6)

where A_g, B_g, and E_g are Raman-active modes, A and B modes are nondegenerate and the E modes are doubly degenerate. The subscripts “g and u” indicate the parity under inversion in centrosymmetric BaWO₄ crystals. A_u and E_u modes correspond to the zero frequency of acoustic modes while the others are optic modes. In addition, the A_g, B_g and E_g modes arise from the same motion in a BaWO₄ phase. Thus, we expect that there are 13 zone-center Raman-active modes for BaWO₄ crystals as described in eqn (7):^65,66

Γ_(Raman) = 3A_g + 5B_g + 5E_g (7)

According to the literature,^67,68 vibrational modes detected in tungstate Raman spectra can be classified into two groups: external and internal modes. Vibrational external modes are related to the lattice phonon or motion of [BaO₈] clusters, and vibrational internal modes are caused by the vibration of [WO₄] clusters (the mass center is in the stationary state). An isolated [WO₄] cluster has a cubic symmetry point (T_d),⁶⁹ and its vibrations are composed of four modes (ν₁(A₁), ν₂(E₁), ν₃(F₂)) and ν₄(F₂)), one free rotation mode ν_f.r(F₁) and one translational mode (F₂). On the other hand, when [WO₄] clusters are located in the scheelite structure, its point symmetry is reduced to S₄.⁷⁰

In infrared spectra, we can expect 13 infrared vibrational modes after excluding 13 Raman vibrational modes (see eqn (6)). Thus, the infrared modes can be described as shown in eqn (8) below:

Γ_(Infrared) = 5A_u + 3B_u + 5E_u (8)

However, in vibrational infrared spectra, (1A_u and 1E_u) are acoustic infrared modes, and (3B_u) are forbidden infrared modes; therefore, they are infrared-inactive modes. Thus, only 8 infrared-active vibrational modes remain which are 4A_u modes that are perpendicular to the c-axis and 4E_u modes with the electric vector parallel to the c-axis (see eqn (9)):^71,72

Γ_(Infrared) = 4A_u + 4E_u (9)

Fig. 4 shows FT-Raman spectra for (Ba_1-xPr_2x/3)WO₄ crystal precipitate obtained at 25 °C by the CP method.

Fig. 4 FT-Raman spectra in the range from 50 to 1000 cm⁻¹ of the (Ba_1-xPr_2x/3)WO₄ crystals precipitated at room temperature: (a) x = 0; (b) x = 0.01, and (c) x = 0.02. The vertical lines indicate the positions and relative intensities of Raman-active modes and inset shows the symmetric stretching of ←O←W→O→ bonds.

Fig. 4(a–c) show that 13 Raman-active vibrational modes were detected experimentally. According to the literature,⁷³ Raman spectra provide information on the degree of structural order-disorder at local short-range in ABO₄ materials. Sharp and intense Raman-active modes indicate that (Ba_1-xPr_2x/3)WO₄ crystals are structurally ordered at short-range with a strong interaction between clusters which arise from the symmetric stretching (←O←W→O→) (see inset in Fig. 4). Raman peak positions refer to active vibrational modes which are shown in Table 2 and are compared with active vibrational modes by other methods as reported in the literature.^{14,30,68,74,75}

Table 2 Comparative results between the experimental Raman-active modes of (Ba_1-xPr_2x/3)WO₄ crystals obtained in this work with those reported in the literature^a

M	T	t	B g	E g	E g	B g	A g	E g	A g	B g	B g	E g	E g	B g	A g	Ref.
	(°C)	(min)
a M = method; T = temperature; t = time; Raman modes = (cm^−1); MH = microwave-hydrothermal; CZ = Czochralski; CRM = chemical reaction peroxide; CP = coprecipitation of (Ba1-xPr2x/3)WO4 crystals (x = 0.0; * = 0.01 and ** = 0.02); and ^× = this work.
MH	140	6	62	74	101	132	149	190	330	332	344	354	793	830	924	14
MH	140	120	62	74	101	132	149	190	330	332	344	354	793	830	924	30
MH	1200	2400	63	75	101	132	150	191	332	332	345	353	795	831	925	68
CZ	1000	1440	63	74	101	133	150	191	331	332	344	352	795	831	926	74
CRP	25	180	60	72	98	—	—	188	330	—	—	—	790	829	920	75
CP	25	2	72	84	101	134	151	192	332	334	344	354	795	830	924	×
CP*	25	2	72	84	101	133	150	191	332	334	344	353	795	830	924	×
CP**	25	2	72	84	101	133	150	191	332	334	344	353	795	830	924	×

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The results reported in Table 2 indicate that all Raman-active modes of (Ba_1-xPr_2x/3)WO₄ crystals prepared by the CP method are related to a scheelite-type tetragonal structure which is in agreement with the research reported in the literature.^{14,30,68,74,75} In this table verifies that some relative positions of Raman modes have small shifts which can be caused by different factors such as: preparation methods, average crystal size, distortions on the (O–W–O)/(O–Ba–O) bonds, interaction forces between [WO₄]–[BaO₈]–[WO₄] clusters and/or different degrees of structural order-disorder in the lattice at short-range. Relative positions for two Raman-active modes of (Ba_1-xPr_2x/3)WO₄ crystals demonstrate a small shift in free rotation ν_f.r.(F₁) Raman modes. These results can be linked to the preparation method at room temperature which is in agreement with Raman positions reported in ref. 75.

Fig. 5 illustrates FT-IR spectra for (Ba_1-xPr_2x/3)WO₄ crystal precipitates obtained at 25 °C by the CP method.

Fig. 5 FT-IR spectra in the range 50 to 1000 cm⁻¹ of the (Ba_1-xPr_2x/3)WO₄ crystals precipitated at room temperature: (a) x = 0; (b) x = 0.01, and (c) x = 0.02. The vertical lines dotted indicate the positions and relative intensities of infrared-active modes and inset shows the anti-symmetric stretching →O→W→O→ bonds.

As previously discussed in the text, tungstates with a scheelite-type tetragonal structure show 8 stretching and/or bending vibrational modes in FT-IR spectra.^76,77 In our case, it was possible to identify no more than 6 modes [2(A_u), 1(E_u)/1(A_u) and 2(E_u)] which were found and identified in specific positions in the spectra (see Fig. 5(a–c)). First, strong absorption bands with two modes located at 788/792 cm⁻¹ and 816/827 cm⁻¹ are visible for our (Ba_1-xPr_2x/3)WO₄ crystal (x = 0, 0.01, and 0.02) precipitates, respectively. Vertical lines are used to for easier identification (see Fig. 5(a–c)). These two bands are related to ν₃[(1E_u) and (1A_u)] internal modes which are assigned to (→O→W→O→) anti-symmetric stretching vibrations in [WO₄] clusters (see inset in Fig. 5). The ν₄[1(E_u)/1(A_u)] modes are ascribed to the anti-symmetric bending of bonds in [WO₄] clusters.⁷⁷ These modes are located in the same position at 380–378 cm⁻¹ for our (Ba_1-xPr_2x/3)WO₄ crystal (x = 0, 0.01, and 0.02) precipitates, respectively. The ν₂(A_u) internal mode located at the same position (313 cm⁻¹) is relative to the symmetric bending of [WO₄] clusters. Finally, for other R_x,y(1E_u) external modes, R is related to the torsional motion of [WO₄] clusters in the lattice. The relative positions for infrared-actives modes are shown in Table 3 and are compared with infrared-active modes for crystals prepared by other methods as reported in the literature.^14,78,79

Table 3 Comparative results between the experimental infrared-active modes of (Ba_1-xPr_2x/3)WO₄ crystals obtained in this work with those reported in the literature^a

M	T/°C	t/min	(Eu)	(Au)	(Eu)	(Au)	(Eu)	(Au)	Ref.
a M = method; T = temperature; t = time; Raman modes = (cm^−1); MH = microwave-hydrothermal, CZ = Czochralski; SSR = solid state reaction; CP = coprecipitation of (Ba1-xPr2x/3)WO4 crystals (x = 0.0; * = 0.01 and ** = 0.02); and × = this work.
MH	140	6	—	—	—	—	808	889	14
CZ	1250	1440	280	310	380	380	795	835	78
SSR	850	180	—	—	—	—	798	919	79
CP	25	2	276	313	380	380	788	816	×
CP*	25	2	275	313	378	378	792	827	×
CP**	25	2	274	313	378	378	792	827	×

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These data (shown in Table 3) indicate a slight shift in the position of the stretching modes between (Ba_1-xPr_2x/3)WO₄ crystal precipitates with x = 0 and 0.01 and 0.02. We believe that this behavior can possibly be related to different interaction strengths and stretching or angles between [O–W–O] bonds due to distortions on [WO₄] clusters into the lattice. Other factors can be considered such as the replacement of [BaO₈] by [PrO₈] clusters which can influence bonds of neighboring [WO₄] clusters. Moreover, we can verify a good agreement between the IR-active mode obtained in this work with IR-active mode reported in the literature.^78,79

FE-SEM analyses

Fig. 6(a–f) show FE-SEM images of the (Ba_1-xPr_2x/3)WO₄ crystal precipitates at room temperature.

Fig. 6 FE-SEM images of the (Ba_1-xPr_2x/3)WO₄ crystals precipitate at room temperature: (a,b) x = 0; (c,d) x = 0.01, and (e,f) x = 0.02.

The FE-SEM images indicate several bonbon-like BaWO₄ microcrystals with an agglomerate nature as well as quasi-monodisperse in shape and polydisperse shapes in a size distribution (see Fig. 6(a) and ESI† in Fig. SD1(a)). These images also indicate that these microcrystals are formed and grow quickly in an aqueous solution after the precipitation reaction. Fig. 6(b) clearly shows these superstructures in high magnification FE-SEM images. The inset in Fig. 6(b) displays a microcrystal intermediate with its two extremities elongated before enabling the growth of the cuttings. After the final precipitation reaction stage occurs, the growth of these cuttings on the center part with a quasi-spherical shape for bonbon-like BaWO₄ microcrystals. Bonbon-like BaWO₄ microcrystals exhibit an average size distribution in the range from 5 to 6.75 μm. In this system, it was estimated that 30% of these superstructures have an average size of approximately 5.75 μm (see ESI,† Fig. SD-2(a)). When 1% of Ba²⁺ is replaced by Pr³⁺ in the BaWO₄ lattice, FE-SEM images reveal a change in the shape to spindle-like microcrystals/flake-like nanocrystals and a reduction in the average microcrystal size (see Fig. 6(c)). These spindle-like microcrystals have an average size distribution in the range from 3 to 4.75 μm. In this system, it was estimated that 28% of these superstructures have an average size of approximately 3.75 μm (see Fig. 6(c) and ESI,† Fig. SD-1,2(b)). Moreover, FE-SEM images illustrated in Fig. 6(d) verify that a probable dissolution of microcrystals can occur due to small flake-like nanocrystals on the microcrystals surfaces (highlighted by rectangles; see inset in Fig. 6(d)). We believe that this behavior can be caused by the use of 4 drops of HNO₃ at the start of the synthesis. Another possible explain is related to the effect of the introduction of earth-rare Pr³⁺ into an A-site where [BaO₈] clusters are located, since [PrO₈] clusters have a minor electronic density which promotes a reduction in the average crystal size. These results are in agreement with recent research reported in the literature for tungstates and molybdates doped with earth-rare.^80,81 Finally, FE-SEM images show that (Ba_1-xPr_2x/3)WO₄ crystal precipitates with x = 0.02 were dissolved due to a large quantity of small flake-like nanocrystals, imperfections on crystal surfaces and defects in shape (Fig. 6(e)). Moreover, the FE-SEM images shown in Fig. 6(f) indicate the presence of two shapes (rice-like microcrystals and flake-like nanocrystals). A high magnification of FE-SEM images (highlighted by rectangles, see inset of Fig. 6(f)) verifies that the increase of Pr³⁺ in the BaWO₄ lattice promotes a clear reduction in the average crystal size. These rice-like microcrystals have an average size distribution in the range from 2 to 3.75 μm. In this system, it was verified that 25% of these superstructures have an average size of approximately 2.75 μm (see Fig. 6(c) and ESI,† Fig. SD-1,2(c)).

Growth process

Fig. 7(a–d) is a schematic representation of all stages involved in the synthesis and growth process of (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01 and 0.02) synthesized by the CP method at room temperature.

Fig. 7 Schematic representation of the growth mechanism for the (Ba_1-xPr_2x/3)WO₄ crystals precipitated at room temperature: (a) chemical synthesis (coprecipitation method) and all the stages involved in the growth process of the [(b) x = 0, (c) x = 0.01, (d) x = 0.02)] bonbon-, spindle-, rice-like and flake-like crystals precipitate.

Fig. 7(a) shows the initial synthesis method employed in the preparation of (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0, 0.01, and 0.02) by the addition of stoichiometric amounts of respective starting reagents [Ba(CH₃CO₂)₂, Pr(NO₃)₃·6H₂O and Na₂WO₄·2H₂O]. These reagents were dissolved in order in two Falcon tubes containing 50 mL of deionized water under constant stirring. In this solution, the H₂O molecule solvation energy promotes a fast salt dissociation where the Ba²⁺, Pr³⁺ and WO²⁻₄ ions are quickly solvated by H₂O molecules. Partial negative charges on the H₂O molecules are electrostatically attracted by Ba²⁺/Pr³⁺ ions while the partial positive charges on the H₂O molecules are electrostatically attracted by WO²⁻₄ ions. However, due to the difference in the electronic density between Ba²⁺/Pr³⁺ and WO²⁻₄ ions, a strong electrostatic force attraction occurs between both ion, which results in the formation of the first (Ba_1-xPr_2x/3)WO₄ nuclei with (x = 0, 0.01, and 0.02) (see Fig. 7(b–d)). The increase in the precipitation rate leads to the aggregation process for small nuclei, and these small nuclei self-assemble due to specific conditions of the chemical reaction such as: the counter-ion effect, pH and solvent (see Fig. 7(b–d)). In the first case, these n⋯units of small BaWO₄ crystals are formed and can be merged through an assembly process that leads to the growth of intermediate BaWO₄ superstructures (see inset in Fig. 6(b) and Fig. 7(b)). In second and third case, the n⋯units of particles of these (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 which are formed as previously described have an assembly process as well as an Ostwald-ripening (OR) process. As can be observed in intermediate superstructures and nanostructures, a possible OR mechanism also governs the growth process of these crystals (see Fig. 7(c,d)). Since we verified a dissolution process of the microcrystals and a later recrystallization of nanocrystals and microcrystals, the OR process verifies that small crystals are dissolved and deposited on larger crystals. This thermodynamically-driven spontaneous process occurs because larger crystals are more energetically favored than are smaller crystals. In this mechanism, atoms on the crystal surface are energetically less stable than atoms in the interior.^82,83 These results are in agreement with the explanations reported by Tian et al.⁸⁴ about the growth mechanism of nanostructured CaWO₄ microcrystals. Finally, after further growth of these intermediate (Ba_1-xPr_2x/3)WO₄ crystals with x = 0, 0.01 and 0.02 for around 2 min, bonbon-, spindle-, and rice-like microcrystal precipitates are formed (see Fig. 7(b–d)).

UV-vis absorption spectroscopy analyses

The optical band gap energy (E_gap) was calculated by the method proposed by Kubelka and Munk.⁸⁵ This methodology is based on the transformation of diffuse reflectance measurements to estimate E_gap values with good accuracy⁸⁶ and is particularly effective in limiting cases of an infinitely thick sample layer. The Kubelka–Munk equation for any wavelength is described as:where F(R_∞) is the Kubelka–Munk function or absolute reflectance of the sample. In our case, magnesium oxide (MgO) was the standard sample in reflectance measurements. R_∞ = R_sample/R_MgO (R_∞ is the reflectance when the sample is infinitely thick), k is the molar absorption coefficient, and s is the scattering coefficient. In a parabolic band structure, the optical band gap and absorption coefficient of semiconductor oxides⁸⁷ can be calculated by the following equation:

αhv = C₁(hv − E_gap)ⁿ (11)

where α is the linear absorption coefficient of the material, hν is the photon energy, C₁ is a proportionality constant, E_gap is the optical band gap and n is a constant associated with different kinds of electronic transitions (n = 0.5 → direct allowed, n = 2 → indirect allowed, n = 1.5 → direct forbidden and n = 3 → indirect forbidden). According to the literature, tungstates (AWO₄; A = Ca, Sr, Ba) exhibit an optical absorption spectrum governed by direct electronic transitions.⁸⁸ In this phenomenon, after the electronic absorption process, electrons located in maximum-energy states in the valence band return to minimum energy states in the conduction band under the same point in the Brillouin zone.⁸⁹ Based on this information, E_gap values of (Ba_1-xPr_2x/3)WO₄ were calculated using n = 0.5 in eqn (9). Finally, using the remission function described in eqn (8) and with the term k = 2α, we obtain the modified Kubelka–Munk equation as indicated in eqn (10):

[F(R_∞)hv]² = C₂(hv − E_gap) (12)

Therefore, by finding the F(R_∞) value from eqn (10) and plotting a graph of [F(R_∞)hv]² against hν and C₂, it was possible to determine the E_gap of (Ba_1-xPr_2x/3)WO₄ crystals.

Fig. 8(a–c) show UV-vis spectra of (Ba_1-xPr_2x/3)WO₄ crystals with: (a) x = 0, (b) x = 0.01, and (c) x = 0.02 which were synthesized by the CP method.

Fig. 8 UV-vis absorbance spectra of (Ba_1-xPr_2x/3)WO₄ crystals with (a) x = 0; (b) x = 0.01, and (c) x = 0.02.

Fig. 8(a–c) illustrate different E_gap values with a reduction in values because of the replacement of Ba²⁺ by Pr³⁺ in the BaWO₄ lattice. The profile of UV-vis absorption curves indicates that Pr³⁺ ions induce the appearance of new intermediary energy levels within the optical band gaps, since the praseodymium shows (4f orbitals), while the barium exhibits (6s orbitals) in the valence band.⁹⁰ This effect is promoted by 4f orbitals of [PrO₈] clusters which leads to distortions on [BaO₈]/[WO₄] clusters that are interlinked in the tetragonal lattice ([BaO₈]⋯[PrO₈]⋯[WO₄]). Fig. 8(a) shows that pure BaWO₄ crystals have an E_gap of 4.9 eV while (Ba_1-xPr_2x/3)WO₄ crystals (x = 0.01) have a E_gap = 4.0 eV (see Fig. 8(b)). A further increase in Pr³⁺ in the BaWO₄ tetragonal lattice leads to a decrease in the E_gap value to 3.82 eV (see Fig. 8(c)) which indicates the influence of [PrO₈] clusters on the electronic structure of these crystals; data are listed in Table 4. Moreover, this table illustrates a comparison between the E_gap obtained in our work with the E_gap values reported in the literature.^{13,60,90–93}

Table 4 Comparative results between the E_gap values experimental of (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0; 0.01, and 0.02) obtained in this work with those reported in the literature^a

M	Shape	T/°C	t/min	E gap /eV	Ref.
a M = method; T = temperature; t = time; Egap = energy band gap; MF = molten flux, CZ = Czochralski; SSR = solid state reaction; PP = polymeric precursors, PP = solid state metathetic; CP = coprecipitation of (Ba1-xPr2x/3)WO4 crystals (x = 0; ■), (x = 0.01; ●), and (x = 0.02; ▲); and ^× = this work.
MF	Octahedrons-like microcrystals	550	720	3.8	13
CZ	Monocrystals	1000	600	4.8	60
CZ	Monocrystals	1200	1440	4.9	90
SSR	Bulk ceramics	1400	2880	3.8	91
PPM	Nanopowders	600	120	4.1	92
SSM	Rods-like microcrystals	—	—	4.8	93
CP■	Bonbon-like microcrystals	25	2	4.9	×
CP●	Spindle-like microcrystals	25	2	4.0	×
CP▲	Rice-like microcrystals	25	2	3.82	×

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Table 4 shows that E_gap values for our (Ba_1-xPr_2x/3)WO₄ crystals (x = 0, 0.01 and 0.02) differ slightly from the values reported in the literature for pure BaWO₄ crystals.^13,91,92 These small discrepancies can be attributed to the fact that E_gap is very dependent upon the synthesis method, different defect levels defects in the band gap, shape, average crystal size, orientation and distortions in the lattice. According to Tyagi et al.,⁹⁰ BaWO₄ crystals have a direct band gap with less dispersion between the valence band (VB) and conduction band (CB). Therefore, our direct energy band gap values for bonbon-like BaWO₄ microcrystals are very close to pure BaWO₄ monocrystals values reported in the literature in ref. 60,90 (see Table 4). This behavior can be related to similar electronic densities in these BaWO₄ crystals. (Ba_1-xPr_2x/3)WO₄ crystals (x = 0.01 and 0.02) have intermediate electronic levels which are related to the addition of (4f orbitals) of praseodymium that promote a consequent reduction in E_gap values. Moreover, another important point to considered is that the replacement of trivalent ions (Pr³⁺) of praseodymium into an A-site normally occupied by divalent ions (Ba²⁺) leads to a negative charge compensation in the BaWO₄ lattice. This characteristic can be explained by the [2Pr^•_Ba, Va^′′_Ba and 3O^x_O] species.

PL emission, energy diagram levels (Pr³⁺) and wide band model of (Ba_1-xPr_2x/3)WO₄ crystals absorption spectroscopy analyses

Fig. 9(a–c) illustrate PL emission spectra of (Ba_1-xPr_2x/3)WO₄ crystals with x = 0, 0.01 and 0.02 synthesized by the CP method. The insets shows: (a) digital photos of PL emission for these crystals excited by the laser (λ = 350 nm) wavelength at room temperature; (b) the energy level diagram for the f–f transitions arising from Pr³⁺ ions; and (c) a wide band model proposed to explain the PL behavior of the crystals, respectively.

Fig. 9 (a) PL emission of the (Ba_1-xPr_2x/3)WO₄ crystals with x = 0; 0.01, and = 0.02, (b) energy level scheme for all the observed emission f–f transitions of Pr³⁺ ions and (c) model proposed in order to explain the origin of the intense visible PL emission at room temperature in the (Ba_1-xPr_2x/3)WO₄ crystals with x = 0; 0.01, and 0.02; I—wavelength of laser employed in the excitation process of the crystals; II—presence of pair (distorted [WO]^x_d clusters and ordered [WO]^x_o clusters) into the lattice able to charge transference; III—wide band model with intermediary energy levels (oxygen (O)-2p, tungsten (W)-5d and praseodymium (Pr)-4f states) within the band gap; IV—transitions oxygen-2p → tungsten-5d and 4f–4f transitions before excitation, V—excitation process and formation of self-trapped excitons (STE's), internal f–f transitions of Pr³⁺ ions and recombination of e′–h˙ pair and VI—PL emission spectra of (Ba_1-xPr_2x/3)WO₄ crystals with x = 0; 0.01, and = 0.02 precipitated at room temperature.

Table 4 and Fig. 9(a) verify that (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0) exhibit an intense PL emission at room temperature with a maximum at 520 nm in the green region. However, with the replacement of Ba²⁺ by Pr³⁺ ions (x = 0.01), a considerable decrease in PL emission is verified (Fig. 9(a)). This behavior can be justified by the formation of barium vacancies (V^′′_Ba) and complex oxygen vacancies which are able to show three different charge forms: neutral (V^x_O), singly ionized (V^•_O) and double ionized (V^••_O) and can donate up to two electrons. The V^x_O are able to donate up to two electrons, the V^•_O are donors or capture only one electron and V^••_O are not able to donate electrons, but can receive up to two electrons. These complex oxygen vacancies act as defects in the lattice of (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and can be stabilized by means of charge compensation into the lattice in agreement with the following eqn (13):

In addition to the wide band PL emission in these crystals, two small peaks of PL emission (at 449 and 486 nm) can be verified which are related to 4f–4f transitions of Pr³⁺ ions. The visible luminescence of Pr³⁺ ion transitions requires appropriate host materials when excited with UV wavelengths corresponding to 4^fn-15d.^94–96 The ground-state of these kinds of ions is characterized by Xe–4f³6s² valence electrons and PL properties of Pr³⁺ ions are ascribed to a 4f¹5d¹ transition.⁹⁷ According to Raju et al.,⁹⁸ the first electronic transition is ascribed to (³P₂ → ³H₄), and the second transition corresponds to (³P₁ → ³H₄). The ³P₂ → ³H₄ transition is hypersensitive and is subject to experimental measurements with a dependence on the coordination environment, the vibronic effect and the host matrix.^99–101 Therefore, this transition is suitable for a study of vibronic lines due to a coupling with high-frequency vibrations.¹⁰² The ³P₁ → ³H₄ transition is assigned to zero-phonon lines at the A-site of the BaWO₄ lattice. These results are in agreement with other research reported in the literature for KSrMoO₄:Pr³⁺ and BaMoO₄:Pr³⁺.^103,104Fig. 9(a) shows that the increase in the replacement of Ba²⁺ by Pr³⁺ ions (x = 0.02) promotes enhanced wide band intensity PL emission. Moreover, four PL emission peaks are visible at 444, 469, 486 and 612 nm which are related to 4f–4f transitions of Pr³⁺ ions. We can verify that increasing the Pr³⁺ ion content in the BaWO₄ host matrix leads to an enhancement of ³P₂ → ³H₄ and ³P₁ → ³H₄ transitions at 444 and 486 nm, respectively. These two bands are sharper in relation to the same transitions observed in (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01. Based on this information, depending on the concentration of Pr³⁺ ions in the BaWO₄ lattice, a possible exchange of virtual phonons between coupled Pr³⁺ ion pairs occurs.^105,106 The other two (³I₆ → ³H₄ and ³P₀ → ³H₆) transitions are small and are detected at 466 and 612 nm, respectively. These transitions also have been observed in PL emission spectra of Pr³⁺-doped glass host matrices with reasonable agreement.^107,108 The energy level diagram corresponding to the f–f transitions ascribed to Pr³⁺ ions is shown in Fig. 9(b). However, in our (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02, we can observe only four such transitions because of the percentage of Pr³⁺ ions in the BaWO₄ host lattice. This same behavior also is reported in the literature for the Pr³⁺-doped CaMoO₄ and CaWO₄.^109,110 Another consideration is the low temperature of the CP method of crystal preparation, regarding vibrational progressions in the lattice that lead to displacement on the equilibrium position of f–f transitions (Pr³⁺ ions).¹⁰¹Fig. 9(c)-I shows the laser employed in the excitation process of the (Ba_1-xPr_2x/3)WO₄ crystals with x = 0, 0.01, and 0.02, synthesized by the CP method. The use of different wavelengths promotes the excitation of electrons localized in different energy levels within the band gap. The E_gap values for our crystals with x = 0, 0.01, and 0.02 are 4.9, 4.0 and 3.82 eV, respectively. Therefore, in this case, it was not possible to use a band-to-band emission process because the energy the laser (3.54 eV) is smaller than the crystal E_gap. Based on our previous research,^73,89,92 where we calculated the electronic structure band of some scheelites, we have verified that after promoted displacements or distortions of the W/Mo atoms into the [WO₄]/[MoO₄] clusters, the appearance and redistribution of energy levels within the band gap are promoted. Therefore, we can postulate that our BaWO₄ crystals prepared by CP method present some distortions on the disordered [WO^x_d] clusters that are able to promote charge transfer to ordered [WO^x_o] clusters. Moreover, [PrO₈] clusters in the (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0.01 and 0.02) have electronic intra-4f-shell transitions of rare earths (see Fig. 9(c)-II). This polarization in the electronic structure by clusters leads to a non-homogeneous charge distribution and the formation of intermediate energy levels between the valence band (VB) and conduction band (CB). (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 have a 4f ↔ 4f transition which is intrinsic with Pr³⁺ ions (see Fig. 9(c)-III). Therefore, during the excitation process at room temperature, some electrons (e′) localized in these intermediary energy levels (oxygen-O 2p orbitals) and near the VB absorb photon energies () at this wavelength. Then these energetic levels are promoted to higher intermediate energy levels (tungsten-W 5d orbitals) near the CB, and the formation of holes (h˙) near the VB occurs. This mechanism results in the formation of self-trapped excitons (STE); i.e., the trapping of e′ by h˙. We believe that this phenomenon occurs in pure BaWO₄ crystals. (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 have praseodymium-Pr 4f orbitals beyond these levels (see Fig. 9(c)-IV). As a consequence of this phenomenon, when electrons revert to lower energy levels (again via radiative return processes), the energies arising from this direct electronic transition are converted to photons () with less energy than the initial () (see Fig. 9(c)-V). Thus, wide band PL emission is due to the absorption and emission processes of photons between O-2p and W-5d levels. For (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0.01 and 0.02), wide band and narrow band PL emission have been observed. The wide bands are due to 2p–5d transitions while the narrow band are related to intra 4f–4f transitions [(³P₂ → ³H₄), (³P₁ → ³H₄), (³I₆ → ³H₄), and (³P₀ → ³H₆)] represented by trivalent Pr³⁺ ions (see Fig. 9(c)-VI). Moreover, normalized PL emission analyses show that a narrowing of wide bands occurs due to the influence of 4f–4f transitions in the blue region.

Photocatalytic activity of (Ba_1-xPr_2x/3)WO₄ crystals

Fig. 10(a–c) show the progress in the PC degradation of RhB by (Ba_1-xPr_2x/3)WO₄ crystals with x = 0, 0.01, and 0.02 by monitoring the temporal changes of UV-vis absorbance spectra of the aqueous dye solution. The insets illustrate digital photos of the RhB aqueous dye solution after different times on the illumination UV light in the presence of the catalyst. The degradation rate (C_n/C₀) of the RhB aqueous solution in the presence of different catalysts and without a catalyst is shown in Fig. 10(d).

Fig. 10 Evolution of UV-vis absorption spectra from after 25 min of illumination for photodegradation of RhB dye solution by the catalysts (Ba_1-xPr_2x/3)WO₄ crystals with (a) x = 0; (b) x = 0.0; (c) x = 0.02 and (d) kinetic of weight-based photocatalytic degradation of RhB dye solution by the catalysts (Ba_1-xPr_2x/3)WO₄ crystals with x = 0; 0.01, and = 0.02, TiO₂–P25 (Degussa) and without catalysts. Inset shows a photograph of the photodegradation of RhB dye solution after different times of illumination on UV-lamps.

Fig. 10(a) verifies a significant reduction in maximum absorption spectra of RhB aqueous solutions during the photodegradation process by the catalyst (Ba_1-xPr_2x/3)WO₄ crystals with x = 0. Before irradiation, the N,N,N',N'-tetraethylated rhodamine molecule RhB dye has one band with a maximum absorption centered at λ_max = 552 nm. The photodecoloration of RhB dye occurs due to an oxidative attack by one of the active oxygen species on the N-ethyl group.¹¹¹ We have not noted displacements of the maximum absorption of RhB dye to other wavelength positions of its major absorption band which moved toward the N,N,N'-tri-ethylated rhodamine (λ_max = 539 nm), N,N'-di-ethylated rhodamine, (λ_max = 522 nm); N-ethylated rhodamine, (λ_max = 510 nm) and rhodamine (λ_max = 498 nm) species.¹¹² We assumed that a high percentage of RhB dye was destroyed or photodegraded after 25 min under UV-illumination (see inset of Fig. 10(a)). Moreover, we have verified that our catalyst BaWO₄ crystals are more efficient for the degradation of RhB dye than other scheelite-type crystals, such as: PbMoO₄, SrMoO₄, SrWO₄, CdMoO₄, PbWO₄, and CdWO₄ under UV-illumination.^113–118 However, our catalyst (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 do not have a high photodegradation efficiency of RhB dye as a function of UV-irradiation time in comparison with the pure catalyst (see Fig. 10(b,c)). We believe that this behavior is due to the formation of V^′′_Ba and the low capability of [PrO₈] clusters to trap photogenerated electrons. According to the literature,^119–122 the main problem and/or factor responsible for the low photocatalytic efficiency of catalyst crystals is the high recombination rate between photogenerated electrons and holes on the crystal surface. Therefore, we can attribute that the h˙ generated by [PrO₈] clusters is not as effective in the reduction of the time and the rate of recombination of electron–hole pairs. Fig. 10(d) verifies that after 25 min under UV-illumination, the RhB dye undergoes no degradation. As an efficiency experiment, we compared our catalyst (Ba_1-xPr_2x/3)WO₄ crystals (x = 0, 0.01 and 0.02) with commercially available TiO₂ photocatalyst (Degussa P25, Brazil) under photocatalytic reaction conditions. Based on the results obtained by kinetic weight-based (C_n/C₀) photocatalytic degradation of RhB dye solutions for the catalysts, we found that pure BaWO₄ crystals have a higher efficiency than TiO₂–P25, and the degradation is complete after 25 min under UV-illumination.

Rate constants of catalyst (Ba_1-xPr_2x/3)WO₄ crystals for the degradation of RhB dye

The photocatalytic efficiency of our catalyst (Ba_1-xPr_2x/3)WO₄ crystals (x = 0, 0.01 and 0.02) were calculated and compared with the commercially available TiO₂–P25 photocatalyst. The rate constant (k) obtained for degradation of RhB aqueous solutions is illustrated in Fig. 11(a–d).

Fig. 11 First-order kinetics (a) without catalysts, (b) TiO₂–P25, (c–e) (Ba_1-xPr_2x/3)WO₄ crystals [x = 0 (c); 0.01 (d), and = 0.02 (e)]. The insets illustrate the normalized kinetics constant (k) values obtained by linear regression.

In general, a catalyst crystal should be evaluated to verify that its photocatalytic activity is better than the TiO₂–P25 (Degussa) photocatalyst.^123,124 This photocatalytic test was conducted in this work to prove the efficiency of our crystals with the possibility for future industrial applications as a photocatalyst due to easy preparation by CP at room temperature. Therefore, in order to quantitatively understand the reaction kinetics for the degradation of the RhB dye by catalyst crystals, we applied the pseudo-first order model as expressed by eqn (14) to obtain the rate constant (k) of catalyst crystals.¹²⁵

where C₀ is the initial concentration (0 min) of the RhB aqueous solution and C_n is the concentration of the RhB aqueous solution for different times (n.min; n = 5, 10, 15…) of UV-illuminations, t is the time and k is the pseudo-first order, respectively. This equation is generally used for the photocatalytic degradation process if the initial concentration of pollutant is low (1 × 10⁻⁵ mol L⁻¹).¹²⁶ According to eqn (14), if −ln(C_n/C₀) is plotted as a function of t, a straight line should, be obtained, whose slope is k (in min⁻¹). All the results shown in Fig. 11(a–e) are absolute and are not normalized by the specific surface area (S_BET) of the crystals. Fig. 11(a) verifies that the value (k_{wc} = 7.3781 × 10⁻⁵ min⁻¹) of the rate constant for crystals without catalyst for the degradation of RhB is very small which indicates that the dye practically does not degrade after 25 min of UV-illumination. Fig. 11(b) lists rate constant values (k_{P25} = 0.075 min⁻¹) of P25–TiO₂ for the degradation of a RhB aqueous solution. We have noted that this value is smaller than the value for our pure (Ba_1-xPr_2x/3)WO₄ crystals with x = 0 of k_{x=0} = 0.1889 min⁻¹ (see Fig. 11(b)). Based on previous results shown in (Fig. 1(b), Fig. 3(a) and Fig. 8(a)), we can attribute this behavior to the following factors: considerable crystallographic orientation in the (004) plane, hole [BaO₈]^•_d clusters associated with defects on the crystal surfaces and optical band gap values near the energy (4.88 eV) of UV-illumination. However, values for the correlation coefficient (R) and standard deviation (SD) for pure BaWO₄ crystals are higher due to the high degradation rate of the RhB aqueous solution in short time intervals (5 min). (Ba_1-xPr_2x/3)WO₄ crystals (x = 0.01, and 0.02) have the following values of k_{x=0.01} = 0.0275 min⁻¹ and k_{x=0.02} = 0.0359 min⁻¹, respectively. These results indicate that Pr³⁺-doped crystals lead to the formation of hole [PrO₈]^•_d clusters in the tetragonal lattice, which cannot decrease the high recombination rate between photogenerated electrons and holes on spindle-, rice-, and flake-like crystal surfaces. Therefore, catalyst crystal rate constants obey the following ascending order k_{x=0} > k_{P25} > k_{x=0.02} > k_{x=0.01} > k_{wc}.

A comparative study of the kinetic parameters (k_{absolute}/k_{normalized}) of catalyst crystals for RhB aqueous solution degradation reactions and S_BET are presented in Table 5.

Table 5 Comparative results between the E_gap values experimental of (Ba_1-xPr_2x/3)WO₄ crystals with x = 0; 0.01, and 0.02 obtained in this work with those reported in the literature^a

Samples	k{ }/min^−1	S BET /m^2 g^−1	k[ ]/min^−1 m^−2 g^−1	K{ }/K{P25}	K[ ]/K[P25]
a wc = without catalyst, P25 = TiO2-Degussa; (Ba1-xPr2x/3)WO4 crystals with (x = 0, 0.01, and 0.02); { } = absolute and [ ] normalized by SBET.
wc	7.3781.10^−5	—	—	9.8374. 10^−4	—
P25	0.075	50	0.0015	1	1
x = 0	0.1889	1.52	0.1242	2.52	82.8
x = 0.01	0.0106	9.95	0.001065	0.1413	0.71
x = 0.02	0.0359	22.77	0.001576	0.4786	1.05

---

The results presented in this table indicate that k_normalized values are smaller than k_{absolute} values; i.e., each catalyst crystal has a specific surface area (S_BET). Therefore, it is necessary normalize the k_{absolute} values obtained. These k_{normalized} values were obtained by dividing k_{absolute} by the specific area surface (S_BET) of each catalyst (see ESI,† Fig. SD-3(a–c)). After normalization, rate constants of the catalyst crystals obey the following ascending order: k_[x=0] > k_[x=0.02] > k_[P25] > k_[x=0.01] > k_[wc]. Moreover, it can be observed that in this table, after comparing several relationships between k values of catalyst (Ba_1-xPr_2x/3)WO₄ crystals (x = 0, 0.01, and 0.02) with the values of k values of the commercial catalyst (TiO₂-P25, Degussa), it was found that normalized k_[x=0] values are approximately 83 times higher than the normalized k_[P25].

Rate constants of catalyst (Ba_1-xPr_2x/3)WO₄ crystals for the degradation of RhB dye

Fig. 12(a–c) illustrate the schematic representation of all stages involved in experimental tests of photocatalysis, interactions of crystal surfaces, the RhB dye and a possible photocatalytic mechanism of the crystals with the species generated for the degradation of a RhB aqueous solution.

Fig. 12 Proposal of photocatalytic reaction mechanism for the degradation of the RhB dye solution by the catalysts (Ba_1-xPr_2x/3)WO₄ crystals with (x = 0; 0.01, and = 0.02): (a) Adsorption–desorption equilibrium of the RhB dye solution on the crystals surface, (b) Activation of the RhB* dye solution by defect on the crystal surface and distorted [WO₄]^x_d and [WO₄]^x_o clusters, and (c) distortions/defects in the electronic structure promoting the formation of intermediate energy levels within the E_gap and electron (e′) transference between clusters. The [WO₄]^·_o clusters adsorb on the (RhB*) and water (H₂O) in valence band, while the [WO₄]^′_o clusters adsorb reacts with oxygen molecule (O₂) to form oxoanion (O^′₂) in conduction band.

In our photocatalytic testing, the initial stage is extremely important for the optimization of this process by heterogeneous photocatalysis which is a very efficient technique for the degradation of organic pollutants such as “RhB dyes”.¹²⁷Fig. 12(a), illustrates that before UV-light in suspension of crystals and RhB dyes, it is necessary to have an optimal dispersion of the system. Therefore, 50 mg of our catalyst crystals was added to 1 × 10⁻⁵ mol L⁻¹ of the RhB aqueous solution and was well dispersed through an ultrasonic process for 10 min. We assume that this step is of fundamental importance for the reproducibility of these results and that the system reaches a perfect adsorption–desorption equilibrium. Fig. 12(b) illustrates the second state where this well dispersed system was stirred for 5 min inside a dark box followed by the collection of the first 3 mL aliquot. Then the six UV lights were triggered to start photocatalysis. During all these stages, the RhB aqueous solution is always kept at 20 °C with a thermostatic bath which is aided by a cooler exhaust. After the illumination of this system by UV light, the excitation of catalyst (Ba,Pr)WO₄ crystals occurs, and the RhB dye molecules are adsorbed on the crystal surfaces¹²⁸ As noted by previous analyses, (Ba,Pr)WO₄ crystals which are obtained have different orientations, shapes and E_gap values (see Fig. 1(b), Fig. 6(a–f) and Fig. 8(a–c)). These associated factors may also positively or negatively affect the photocatalytic properties of catalyst crystals. Moreover, we believe that each defect on the crystal surface can act as active site for the degradation of RhB dyes which is in agreement with recent research reported in the literature.¹²⁹Fig. 12(c) illustrates a proposed a model as a possible explanation for photocatalytic mechanisms of our catalyst crystals for the photooxidation of a RhB aqueous solution. In this model, we assume that before the excitation processes (i.e., before the arrival of the UV-light in the system), our catalyst (Ba,Pr)WO₄ crystals already have the ability to generate pairs of electrons (e′) and holes (h^•). This characteristic is due to intrinsic defects in the lattice of materials with scheelite-type tetragonal structures. Therefore, these defects which are caused by distorted [WO₄]_d clusters can polarize in the lattice and lead to possible electronic transitions between disordered [[WO₄]^x_d] and ordered [WO₄]^x_o clusters. When the UV light is absorbed by the crystals, the following processes can occur as expressed in eqn (15) and (16) below:

where [WO₄]^•_d clusters are located in intermediate levels near the VB while the [WO₄]^'_o clusters are located in intermediate levels below the CB. Moreover, the RhB dye is also excited by UV light as shown in eqn (17).

In the following step, due to an intermediary energy level between the VC and the BC of these crystals, electrons near them can be excited when illuminated by UV-light (λ = 254 nm ≈ 4.88 eV). This UV light energy can promote these excited electrons from the VB to the CB of the crystals. This process leads the formation of pairs (h^•–e′) within the crystal band gap (see Fig. 12(c)). During photooxidation processes, the species generated [WO₄]^•_d cluster can interact with the RhB* and H₂O molecules^130–133 as shown in eqn (18) and (19), which is a possible process that occurs in the VB:

The [BaO₈]^•_o clusters can react with the H^* to generate (H⁺) while, the species generated [WO₄]^′_o clusters react with oxygen molecules in an aqueous solution,^134,135 as shown in eqn (20) which is a possible process that occurs in the CB:

These cycles occur continuously while the system is exposed to the UV light. Finally, after several cycles of photooxidation, the degradation of RhB dye by the formed oxidant species occurs as indicated by eqn (21).

After 25 min of UV-illumination:

where CCO = colorless compounds organic.¹³⁵

Based on our photocatalytic mechanisms, we assume that the defects on the crystal surface and electronic structure ([WO₄]^•_d and [WO₄]^'_o clusters) play an important role in producing OH^• and O^'₂ radicals, which are the most oxidizing species in this process. Moreover, we believe that other generated species such as [BaO₈]^•_d and [PrO₈]^•_d clusters can participate in the mechanism of photocatalysis, but the [PrO₈]^•_d clusters do not act effectively to produce oxidizing radicals (see Fig. 12(c)).

Conclusions

In summary, we have reported the easy production of (Ba_1-xPr_2x/3)WO₄ crystals with x = 0, 0.01, and 0.02 by the CP method at room temperature. XRD patterns, Rietveld refinement data and FT-Raman spectra indicate that these crystals have a scheelite-type tetragonal structure without deleterious phases with the addition of Pr³⁺ ions. Normalized XRD patterns and structural refinement analyses indicate that BaWO₄ crystals are polycrystalline with a crystallographic orientation in the (004) plane while (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 do not show any crystallographic orientation which is in agreement with the standard ICSD card. Structural refinement data were employed to model [BaO₈], [PrO₈] and [WO₄] clusters by lattice parameters and atomic positions. FT-Raman spectra exhibited 13 modes which indicate that all crystals are structurally ordered at short range. FT-IR spectra showed that Pr³⁺ ions promote a widening in the vibrational modes due to [O–Pr–O] bonds. FE-SEM images revealed that Pr³⁺ ions affect the shape and cause a reduction in the crystal size. The growth process proposed for these crystals indicate that growth occurs by the self-assembly of small nanocrystals, and the further growth of intermediate superstructures and nanostructures leads to the formation of bonbon-, spindle-, rice- and flake-like crystals. UV-vis absorption spectra indicate that the replacement of Ba²⁺ by Pr³⁺ ions promotes a decrease in optical band gap values due to appearance of intermediary energy levels within the band gap. These levels are basically composed of oxygen 2p orbitals (above the VB) and tungsten 5d orbitals (below the CB). A wide band model was used to explain the wide and narrow PL behavior of (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 crystals synthesized by the CP method due to p–d transitions (defects at medium range in the lattice) and f–f transitions (Pr³⁺ ions), respectively. A high efficiency in the degradation of RhB dye was observed for our BaWO₄ catalyst crystals compared to a commercially available TiO₂-P25 photocatalyst while (Ba_1-xPr_2x/3)WO₄ crystals with x = 0.01 and 0.02 showed a low efficiency for the degradation of RhB dyes under UV light as well as a reduction in PL intensity. Kinetic parameters which were obtained indicate that normalized k_[x=0] values are 83 times higher than normalized k_[P25] values. Finally, we propose photocatalytic mechanisms to explain the fast degradation of RhB dye by our crystals under UV light. Based in resulted obtained, we assumed that the following factors: considerable crystallographic orientation in the (004) plane, optical band gap values near the energy (4.88 eV) of UV-illumination, defects on the crystal surface and electronic structure ([WO₄]^•_d, [BaO₈]^•_d, [BaO₈]^'_o and [WO₄]^'_o clusters) plays an important role to produce OH^• and O^'₂ radicals. Therefore, these factors associated are responsible by the fast degradation of RhB dyes by catalyst BaWO₄ crystals after 25 min of illumination under UV-light.

Acknowledgements

The authors acknowledge the financial support of the Brazilian research financing institutions: FAPESP (No.2009/53189-8), CNPq (No.159710/2011-1), CAPES, FAPEPI-GERATEC (No.01.08.0506.00) and LIMAV-UFPI.

References

1. R. C. Hughes, P. P. Coppola and H. T. Evans, J. Appl. Phys., 1952, 23, 635.
2. Q. Guo and O. J. Kleppa, Thermochim. Acta, 1997, 303, 183.
3. V. I. Smirnov and V. V. Aleksandrov, J. Eng. Phys. Thermophys., 1993, 65, 1117.
4. A. R. Patel and S. K. Arora, J. Cryst. Growth, 1974, 23, 95.
5. S. Lin, J. L. Chen, N. F. Zhuang, B. Zhao and J. Z. Chen, J. Cryst. Growth, 2005, 277, 223.
6. D. Ran, H. Xia, S. Sun, Z. Ling, W. Ge and H. Zhang, Mater. Sci. Eng., B, 2006, 130, 206.
7. A. K. Chauhan, J. Cryst. Growth, 2003, 254, 481.
8. W. Ge, H. Zhang, J. Wang, J. Liu, X. Xu, X. Hu, J. Li and M. Jiang, J. Cryst. Growth, 2004, 270, 582.
9. J. Wang, H. Zhang, Z. Wang, W. Ge, J. Zhang and M. Jiang, J. Cryst. Growth, 2006, 292, 377.
10. Y. K. Voronko and A. A. Sobol, Inorg. Mater., 2005, 41, 420.
11. A. Phuruangrat, T. Thongtem and S. Thongtem, J. Phys. Chem. Solids, 2009, 70, 955.
12. N. Li, F. Gao, L. Hou and D. Gao, J. Phys. Chem. C, 2010, 114, 16114.
13. P. Afanasiev, Mater. Lett., 2007, 61, 4622.
14. L. S. Cavalcante, J. C. Sczancoski, L. F. Lima Jr, J. W. M. Espinosa, J. A. Varela, P. S. Pizani and E. Longo, Cryst. Growth Des., 2009, 9, 1002.
15. Y. Liu and Y. Chu, Mater. Chem. Phys., 2005, 92, 59.
16. S. Mann, Nat. Mater., 2009, 8, 781.
17. X. Wang, H. Xu, H. Wang and H. Yan, J. Cryst. Growth, 2005, 284, 254.
18. S. Hou, Y. Xing, H. Ding, X. Liu, B. Liu and X. Sun, Mater. Lett., 2010, 64, 1503.
19. Z. Song, J. Ma, X. Li, Y. Sun, J. Fang, Z. Liu and C. Gao, J. Am. Ceram. Soc., 2009, 92, 1354.
20. H. An, Z. Yang, D. Xiao, P. Yu, Z. Liu and R. Xie, Ferroelectrics, 2009, 385, 61.
21. F. Q. Dong, Q. S. Wu and Y. P. Ding, J. Alloys Compd., 2010, 476, 571.
22. T. Thongtem, A. Phuruangrat and S. Thongtem, Appl. Surf. Sci., 2008, 254, 7581.
23. J. Liao, B. Qiu, H. Wen, W. You and Y. Xiao, J. Lumin., 2010, 130, 762.
24. J. Liao, B. Qiu, H. R. Wen, Y. Li, R. Hong and H. You, J. Mater. Sci., 2011, 46, 1184.
25. F. Zhang, S. P. Yang, H. M. Chen, Z. H. Wang and X. B. Yu, J. Cryst. Growth, 2004, 267, 569.
26. C. Zhang, E. Shen, E. Wang, Z. Kang, L. Gao, C. Hu and L. Xu, Mater. Chem. Phys., 2006, 96, 240.
27. B. A. Hernandez-Sanchez, T. J. Boyle, H. D. Pratt, M. A. Rodriguez, L. N. Brewer and D. R. Dunphy, Chem. Mater., 2008, 20, 6643.
28. T. Thongtem, S. Kaowphong and S. Thongtem, Solid State Phenom., 2007, 124–126, 315.
29. K. P. F. Siqueira, R.L. Moreira, M. Valadares and A. Dias, J. Mater. Sci., 2010, 45, 6083.
30. L. S. Cavalcante, J. C. Sczancoski, J. W. M. Espinosa, J. A. Varela, P. S. Pizani and E. Longo, J. Alloys Compd., 2009, 474, 195.
31. A. Phuruangrat, T. Thongtem and S. Thongtem, J. Ceram. Soc. Jpn., 2008, 116, 605.
32. D. Ye, D. Li, W. Zhang, M. Sun, Y. Hu, Y. Zhang and X. Fu, J. Phys. Chem. C, 2008, 112, 17351.
33. Y. Yin, F. Yang, Y. Yang, Z. Gan, Z. Qin, S. Gao, B. Zhou and X. Li, Superlattices Microstruct., 2001, 49, 599.
34. P. Siriwong, T. Thongtem, A. Phuruangrat and S. Thongtem, Cryst. Eng. Comm, 2011, 13, 1564.
35. G. Tian and S. Sun, Cryst. Res. Technol., 2001, 46, 389.
36. J. Huang and L. Gao, J. Am. Ceram. Soc., 2006, 89, 3877.
37. S. B. Kim, B. G. Kum, H. M. Jang, A. Lakshmanan and B. K. Kang, J. Lumin., 2011, 131, 1625.
38. L. D. Feng, X. B. Chen and C. J. Mao, Mater. Lett., 2010, 64, 2420.
39. L. Ma, Y. Sun, P. Gao, Y. Yin, Z. Qin and B. Zhou, Mater. Lett., 2010, 64, 1235.
40. G. Wan and G. Wang, Mod. Phys. Lett. B, 2010, 24, 3081.
41. T. T. Basiev, A. Y. Karasik, A. A. Sobol, D. S. Chunaev and V. E. Shukshin, Quantum Electron., 2011, 41, 370.
42. T. D. Nguyen, D. Mrabet, T. T. D. Vu, C. T. Dinh and T. O. Do, Cryst. Eng. Comm, 2011, 13, 1450.
43. S. Rajagopal, D. Nataraj, O. Y. Khyzhun, Y. Djaoued, J. Robichaud and D. Mangalaraj, J. Alloys Compd., 2010, 493, 340.
44. Z. Song, J. Ma, H. Sun, W. Wang, Y. Sun, L. Sun, Z. Liu and C. Gao, Ceram. Int., 2009, 35, 2675.
45. L. Ratker and P. W. Voorhees, Growth and coarsening: Ostwald ripening in material processing, Springer, 2002, 117–118.
46. R. L. Penn and J. F. Banfield, Am. Mineral, 1998, 83, 1077.
47. J. C. Sczancoski, M. D. R. Bomio, L. S. Cavalcante, M. R. Joya, P. S. Pizani, J. A. Varela, E. Longo, M. S. Li and J. A. Andrés, J. Phys. Chem. C, 2009, 113, 5812.
48. J. Liao, B. Qiu, H. Wen, J. Chen, W. You and L. Liu, J. Alloys Compd., 2009, 487, 758.
49. T. Thongtem, S. Kaowphong and S. Thongtem, Appl. Surf. Sci., 2008, 254, 7765.
50. M. Nikl, P. Bohacek, E. Mihokova, M. Kobayashi, M. Ishii, Y. Usuki, V. Babin, A. Stolovich, S. Zazubovich and M. Bacci, J. Lumin., 2000, 87–89, 1136.
51. G. Zhang, R. Jia and Q. Wu, Mater. Sci. Eng., B, 2006, 128, 254.
52. H. L. Li, Z. L. Wang and J. H. Hao, IOP Conf. Ser.: Mater. Sci. Eng., 2009, 1, 012010.
53. Z. Shan, Y. Wang, H. Ding and F. Huang, J. Mol. Catal. A: Chem., 2009, 302, 54.
54. I. K. Robinson, Phys. Rev. B, 1986, 33, 3830.
55. D. Errandonea, J. Pellicer-Porres, F. J. Manjón, A. Segura, C. Ferrer-Roca, R. S. Kumar, O. Tschauner, J. López-Solano, P. Rodríguez-Hernández, S. Radescu, A. Mujica, A. Muñoz and G. Aquilanti, Phys. Rev. B, 2006, 73, 224103.
56. H. M. Rietveld, J. Appl. Crystallogr., 1969, 2, 65.
57. A. C. Larson and R. B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 1994, 86, 748.
58. B. H. Toby, J. Appl. Crystallogr., 2001, 34, 210.
59. http://www.crystalimpact.com/diamond/download.htm .
60. D. Ran, H. Xia, S. Sun, P. Zhao, F. Liu, Z. Ling, W. Ge, H. Zhang and J. Wang, Cryst. Res. Technol., 2006, 41, 1189.
61. http://polyhedra.org/poly/show/0/tetrahedron .
62. http://en.wikipedia.org/wiki/Deltahedron .
63. http://polyhedra.org/poly/show/128/snub_disphenoid .
64. T. T. Basiev, A. A. Sobol, Y. K. Voronko and P. G. Zverev, Opt. Mater., 2000, 15, 205.
65. T. T. Basiev, A. A. Sobol, P. G. Zverev, L. I. Ivleva, V. V. Osiko and R. C. Powell, Opt. Mater, 1999, 11, 309.
66. A. Jayaraman, B. Batlogg and L. G. Vanuitert, Phys. Rev. B, 1983, 28, 4774.
67. S. Desgreniers, S. Jandl and C. Carlone, J. Phys. Chem. Solids, 1984, 45, 1105.
68. S. P. S. Porto and J. F. Scott, Phys. Rev., 1967, 157, 716.
69. J. C. Sczancoski, L. S. Cavalcante, M. R. Joya, J. W. M. Espinosa, P. S. Pizani, J. A. Varela and E. Longo, J. Colloid Interface Sci., 2009, 330, 227.
70. G. Jia, C. Wang and S. Xu, J. Phys. Chem. C, 2010, 114, 17905.
71. A. Golubović, R. Gajić, Z. Dohčević-Mitrović and S. Nikolić, J. Alloys Compd., 2006, 415, 16.
72. A.S. Barker Jr., Phys. Rev., 1964, 135, A742.
73. J. C. Sczancoski, L. S. Cavalcante, N. L. Marana, R. O. da Silva, R. L. Tranquilin, M. R. Joya, P. S. Pizani, J. A. Varela, J. R. Sambrano, M. S. Li, E. Longo and J. Andrés, Curr. Appl. Phys., 2010, 10, 614.
74. (a) F. J. Manjón, D. Errandonea, N. Garro, J. Pellicer-Porres, P. Rodríguez-Hernández, S. Radescu, J. López-Solano, A. Mujica and A. Muñnoz, Phys. Rev. B, 2006, 74, 144111.75; (b) R. C. Hughes, P. P. Coppola and H. T. Evans, J. Appl. Phys., 1952, 23, 635.
75. D. Rangappa, T. Fujiwara and M. Yoshimura, Solid State Sci., 2006, 8, 1074.
76. Z. C. Ling, H. R. Xia, D. G. Ran, F. Q. Liu, S. Q. Sun, J. D. Fan, H. J. Zhang, J. Y. Wang and L. L. Yu, Chem. Phys. Lett., 2006, 426, 85.
77. R. K. Khanna and E. R. Lippincott, Spectrochim. Acta, 1968, 21A, 905.
78. G. M. Clark and W. P. Doyle, Spectrochim. Acta, 1966, 22, 1441.
79. L. A. Al-Hajji, M. A. Hasan and M. I. Zaki, J. Therm. Anal. Calorim., 2010, 100, 43.
80. D. Gao, Y. Li, X. Lai, Y. Wei, J. Bi, Y. Li and M. Liu, Mater. Chem. Phys., 2011, 126, 391.
81. Y. Zhou, J. Liu, X. Yang, X. Yu and L. Wang, J. Electrochem. Soc., 2011, 158, K74.
82. J. Zheng, F. Huang, S. Yin, Y. Wang, Z. Lin, X. Wu and Y. Zhao, J. Am. Chem. Soc., 2010, 132, 9528.
83. C. S. Xavier, J. C. Sczancoski, L. S. Cavalcante, C. O. Paiva-Santos, J. A. Varela, E. Longo and M. S. Li, Solid State Sci., 2009, 11, 2173.
84. Y. Tian, B. Chen, H. Yu, R. Hu, X. Li, J. Sun, L. Cheng, H. Zhong, J. Zhang, Y. Zheng, T. Yu and L. Huang, J. Colloid Interface Sci., 2011, 360, 586.
85. P. Kubelka and F. Munk-Aussig, Zeit. Für. Tech. Physik, 1931, 12, 593.
86. A. E. Morales, E. S. Mora and U. Pal, Rev. Mex. Fis. S, 2007, 53, 18.
87. R. A. Smith, Semiconductors, 2nd ed. (Cambridge University Press, London, 1978, 434.
88. R. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea, D. Martínez-García and A. Segura, Europhys. Lett., 2008, 83, 37002.
89. V. S. Marques, L. S. Cavalcante, J. C. Sczancoski, A. F. P. Alcântara, M. O. Orlandi, E. Moraes, E. Longo, J. A. Varela, M. S. Li and M. R. M. C. Santos, Cryst. Growth Des., 2010, 10, 4752.
90. M. Tyagi, S. G. Singh, A. K. Chauhan and S. C. Gadkari, Phys. Rev. B, 2010, 405, 4530.
91. H. W. Eng, P. W. Barnes, B. M. Auer and P. M. Woodward, J. Solid State Chem., 2003, 175, 94.
92. M. Anicete-Santos, F. C. Picon, C. N. Alves, P. S. Pizani, J. A. Varela and E. Longo, J. Phys. Chem. C, 2011, 115, 12180.
93. P. Parhi, T. N. Karthik and V. Manivannan, J. Alloys Compd., 2008, 465, 380.
94. J. Cao, Y. Wang, X. Ma, J. Li, Z. Zhu, Z. You, F. Yang, C. Sun, T. Cao, Y. Ji and C. Tu, J. Alloys Compd., 2001, 509, 185.
95. P. S. Peijzel, A. Meijerink, R. T. Wegh, M. F. Reid and G. W. Burdick, J. Solid State Chem., 2005, 178, 448.
96. C. D. Cordero-Montalvo and N. Bloembergen, Phys. Rev. B, 1984, 30, 438.
97. A. Flórez, O. L. Malta, Y. Messaddeq and M. A. Aegerter, J. Non-Cryst. Solids, 1997, 213, 315.
98. G. S. R. Raju, J. Y. Park, H. C. Jung, R. Balakrishnaiah, B. K. Moon and J. H. Jeong, Curr. Appl. Phys., 2011, 11, S292.
99. R. Reisfeld, Radiative and nonradiative transition of rare earths in glasses, Struct. Bond, Springer-Verlag, New York, vol. 22, 1975,pp. 123–175.
100. R. D. Peacock, The intensities of lanthanide f-f transitions: Struct. Bond. , Springer-Verlag, New York, vol. 22, 1975, pp. 83–122.
101. H. Yersin, Transition metal and rare earth compounds III: excited states, transitions, interactions, Springer-Verlag, New York, 2004, 1edn, pp. 198–205.
102. C. M. Donegá and A. Meijerink, J. Lumin., 1993, 55, 315.
103. Q. Li, J. Huang and D. Chen, Luminescence, 2011, 26, 349.
104. X. Yang, J. Liu, H. Yang, X. Yu, Y. Guo, Y. Zhou and J. Liu, J. Mater. Chem., 2009, 19, 3771.
105. M. Galczynnski and W. Strek, J. Phys. Chem. Solids, 1991, 52, 681.
106. M. Galczynnski, M. Blazej and W. Strek, Mater. Chem. Phys., 1992, 31, 175.
107. G. Lakshminarayana and J. Qiu, J. Alloys Compd., 2009, 478, 630.
108. M. Olivier, P. Pirasteh, J. L. Doualan, P. Camy, H. Lhermite, J. L. Adam and V. Nazabal, Opt. Mater., 2011, 33, 980.
109. F. Zhu, Z. Xiao, F. Zhang, L. Yan and A. Huang, J. Lumin., 2011, 132, 22.
110. F. Zhu, Z. Xiao, L. Yan, F. Zhang and A. Huang, Appl. Phys. A: Mater. Sci. Process., 2010, 101, 689.
111. Y. Zhao, C. Li, X. Liu and F. Gu, J. Alloys Compd., 2007, 440, 281.
112. T. Wu, G. Liu, J. Zhao, H. Hidaka and N. Serpone, J. Phys. Chem. B, 1998, 102, 5845.
113. J. Bi, L. Wu, Y. Zhang, Z. Li, J. Li and X. Fu, Appl. Catal., B, 2009, 91, 135.
114. G. J. Xing, R. Liu, C. Zhao, Y.-L. Li, Y. Wang and G. M. Wu, Ceram. Int., 2011, 37, 2951.
115. J. Yu, L. Qi, B. Cheng and X. Zhao, J. Hazard. Mater., 2008, 160, 621.
116. L. Zhen, W. S. Wang, C. Y. Xu, W. Z. Shao, M. M. Ye and Z. L. Chen, Scr. Mater., 2008, 58, 461.
117. H. Fu, C. Pan, L. Zhang and Y. Zhu, Mater. Res. Bull., 2007, 42, 696.
118. T. Yan, L. Li, W. Tong, J. Zheng, Y. Wang and G. Li, J. Solid State Chem., 2011, 184, 357.
119. A. Zhang and J. Zhang, J. Mater. Sci., 2010, 45, 4040.
120. A. Zhang and J. Zhang, Chin. J. Chem. Phys., 2010, 23, 73.
121. C. J. Philippopoulos, M. D. Nikolaki, 2010; Photocatalytic processes on the oxidation of organic compounds in water, New trends in technologies, Blandna Ramov (Ed.), ISBN: 978-953-7619-62-6, InTech, p. 97.
122. A. Zhang and J. Zhang, J. Hazard. Mater., 2010, 173, 265.
123. E. Y. Kim, D. S. Kim and B. T. Ahn, Bull. Korean Chem. Soc., 2009, 30, 193.
124. D. A. H. Hanaor and C. C. Sorrell, J. Mater. Sci., 2011, 46, 855.
125. A. R. Malagutti, H. A. J. L. Mourão, J. R. Garbin and C. Ribeiro, Appl. Catal., B, 2009, 90, 205.
126. J. Bi, L. Wu, Z. Li, Z. Ding, X. Wang and X. Fu, J. Alloys Compd., 2009, 480, 684.
127. N. Barka, S. Qourzal, A. Assabbane, A. Nounah and Y. Ait-Ichou, J. Photochem. Photobiol., A, 2008, 195, 346.
128. A. L. Linsebigler, G. Lu and J. T. Yates Jr, Chem. Rev., 1995, 95, 735.
129. T. Saison, N. Chemin, C. Chanéac, O. Durupthy, V. Ruaux, L. Mariey, F. Maugé, P. Beaunier and J. P. Jolivet, J. Phys. Chem. C, 2011, 115, 5657.
130. J. M. Wu and T. W. Zhang, J. Photochem. Photobiol., A, 2004, 162, 171.
131. H. A. J. L. Mourão, V. R. de Mendonça, A. R. Malagutti and C. Ribeiro, Quim. Nova, 2009, 32, 2181.
132. K. Rajeshwar, M. E. Osugi, W. Chanmanee, C. R. Chenthamarakshan, M. V. B. Zanoni, P. Kajitvichyanukul and R. Krishnan-Ayer, J. Photochem. Photobiol., C, 2008, 9, 171.
133. T. Montini, V. Gombac, A. Hameed, L. Felisari, G. Adami and P. Fornasiero, Chem. Phys. Lett., 2010, 498, 113.
134. S. Banerjee, J. Gopal, P. Muraleedharan, A. K. Tyagi and B. Raj, Curr. Sci, 2006, 90, 1378.
135. K. Yu, S. Yang, H. He, C. Sun, C. Gu and Y. Ju, J. Phys. Chem. A, 2006, 113, 10024.

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Footnote

† Electronic supplementary information (ESI) available: Characterization details and figures for supplementary information. CCDC reference numbers 871280–871282, 873869, 873870, 873871. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra20266b

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