Effects of changing the M²⁺ cation on the crystal structure and optical properties of divalent samarium-doped MAl₂Si₂O₈ (M = Ca, Sr, Ba)

Abstract

In order to investigate the effects of changing the cation on crystal structure and optical properties of divalent samarium-doped MAl₂Si₂O₈, the Sm²⁺ doped triclinic CaAl₂Si₂O₈ (CASO), monoclinic SrAl₂Si₂O₈ (SASO) and BaAl₂Si₂O₈ (M-BASO) as well as hexagonal BaAl₂Si₂O₈ (H-BASO) compounds have been synthesized using a polymerizable-complex technique under a reducing atmosphere. At room temperature, under N-UV or blue excitation, Sm²⁺ doped SASO, M-BASO and H-BASO showed strong red emission, which widens the group of novel red-emitting materials. The emission bands of Sm²⁺ consist of a broad 4f–5d transition band and sharp f–f transition lines. The correlation between the crystal structure and the 4f–4f transition emission intensity of Sm²⁺ as well as the excitation peaks of the 5d energy level is discussed in detail, and the theoretical analysis is in good agreement with our experimental results. These are essential to understand the relationship between the crystal structure and luminescence properties, which are fundamental to design luminescent materials of Sm systems.

---

1. Introduction

The alkaline-earth feldspars MAl₂Si₂O₈ (M = Ca, Sr, Ba) were reported to be very suitable host lattices for luminescent divalent rare earth ions, especially Eu²⁺ ions.^1–8 Among the various rare earth ions, Sm²⁺ doped inorganic materials have found important applications in lighting, display and persistent spectral hole-burning (PSHB).^9–13 Sm²⁺ ion doped inorganic materials have attracted more and more attention because they have the advantages of 4f–5d broad excitation and emission bands as well as the sharp f–f transition emission.^9–14 Sm²⁺ generally has a broad continuum excitation band in the range from 200 to 500 nm and its emission is composed of the 4f–5d broad band and deep red 4f⁶–4f⁶ sharp emission. However, optical properties of Sm²⁺ doped alkaline-earth feldspars MAl₂Si₂O₈ (M = Ca, Sr, Ba) have not been reported.

On the other hand, the luminescence of divalent samarium ions is sensitive to the crystalline environment of the lattice site that they occupy.¹⁵ The relative position between the lowest-lying 4f⁵5d¹ excited states and the 4f⁶ (⁵D₀) level governs whether the emission displays the 4f⁶ → 4f⁶ sharp line and/or 4f⁵5d¹ → 4f⁶ broad band. It has been reported that the 4f–5d excited broad band is dependent on many factors such as the crystal field strength, the coordination number and the covalence of the bonds.¹⁰ Understanding the relationship between these factors and the 4f–5d transition of Sm²⁺ is of great importance due to the potential technological applications as functional photonic materials. We will choose the systematic alkaline-earth feldspars MAl₂Si₂O₈ (M = Ca, Sr, Ba) as the host lattices to investigate the effects of the crystal structure on the 4f–5d excitation band due to their characteristic structural properties. In these host lattices, firstly, the number of the M²⁺ site occupied by Sm²⁺ is different. In CaAl₂Si₂O₈, Ca²⁺ includes four different 2i sites,¹⁶ but only one kind of M²⁺ (Sr²⁺ or Ba²⁺) site exists in the SASO,¹⁷ M-BASO¹⁸ and H-BASO.⁴ So the effects of the number of the occupied sites on the 4f–5d transition can be obtained. Secondly, the symmetry of the M²⁺ is different. The Ba²⁺ site of H-BASO is central symmetry while the others are non-central symmetry. Thus the effects of the symmetry on the photoluminescence properties of Sm²⁺ can be shown. Thirdly, the SASO is monoclinic and isostructural with the M-BASO. They are similar to each other except the different cation site. So the effects of the cation ions on the 4f–5d of Sm²⁺ can be obtained. Fourthly, the H-BASO and the M-BASO are allomorphism. They are different in crystalline form without change in chemical constitution. So the effects the crystal structure with the same chemical constitution on the f–d transition peak can be investigated.

In order to obtain more exact photoluminescence data and high-efficiency phosphor emission, it is necessary to utilize solution-based techniques for synthesis. We used pechini-type sol–gel process¹⁹ (PSG, also known and called as polymerizable-complex technique) to synthesize the Sm doped MAl₂Si₂O₈ (M = Ca, Sr, Ba). If precursor powders are prepared using PSG, all the component ions can be evenly dispersed in the precursors, leading to the formation of high-purity phosphors with a uniform dispersion of Sm ions activators.

In our paper, the Sm²⁺ doped triclinic CaAl₂Si₂O₈ (CASO), monoclinic SrAl₂Si₂O₈ (SASO) and BaAl₂Si₂O₈ (M-BASO) as well as hexagonal BaAl₂Si₂O₈ (H-BASO) have been synthesized using polymerizable-complex technique under reducing atmosphere. The photoluminescence properties of Sm²⁺ in the systematic host lattices have been reported. Furthermore, the important chemical bonds such as the environmental factor (h_e) and parameter (F_C) were calculated using the dielectric theory of complex crystals. The effects of the crystal structure on the 4f–5d transition and f–f emission intensity of Sm²⁺ were discussed.

2. Experimental section

2.1. Sample preparation

Unless otherwise stated, all reagents were purchased from Sigma-Aldrich. The MAl₂Si₂O₈ (M = Ca, Sr, Ba) powder samples were prepared by a pechini-type sol–gel process (PSG, also known and called as polymerizable-complex technique), which can give the highest possible homogeneity for the material.¹⁹ The doping level of Sm ions was 5.0 mol% in MAl₂Si₂O₈ : Sm. The mixtures were heated in a hydrogen–nitrogen (5% H₂ + 95% N₂) atmosphere.

The raw materials were Sm₂O₃, Ca(NO₃)₂·4H₂O, Ba(NO₃)₂, Sr(NO₃)₂, Al(NO₃)₃·9H₂O, tetraethyl orthosilicate (Si(OC₂H₅)₄, TEOS) and citric acid as the chelating agent. The 0.05 M Sm(NO₃)₃ solution was prepared by dissolving Sm₂O₃ in the diluted HNO₃ and adding appropriate volume of de-ionized water. The 0.25 M M(NO₃)₂ (M = Ca, Sr, Ba), Al(NO₃)₃ solutions and 1 M citric acid solution were prepared using the de-ionized water. Briefly, a stoichiometric volumes of M(NO₃)₂ (M = Ca, Sr, Ba) and Al(NO₃)₃ solutions were dissolved in an aqueous solution of citric acid (citric acid–M²⁺ = 2 : 1, molar ratio) under vigorous stirring to form solution “A”. Then TEOS was dissolved using some ethanol under stirring to form solution “B”. Then A and B were mixed together and stirred for several hours at room temperature. The resultant mixture was heated up to 120 °C and kept at the temperature for 4 h to produce solid gels. The solid gels were prefired at 1200 °C for 4 h. After being fully ground, Sm : M-BASO (M = Ca, Sr) samples were annealed at 1400 °C in the reducing atmosphere for 6 h and single hexagonal and monoclinic phases of Sm : BASO were obtained between 1350 to 1650 °C in the reducing atmosphere for 6 h.

2.2. Characterizations

The phase purity of the as prepared phosphor was checked by powder X-ray diffraction (XRD) analysis by using a D/MAX 2500 instrument (Rigaku) with a R_int 2000 wide angle goniometer and Cu Kα1 radiation (λ = 1.54056 Å) at 40 kV and 100 mA. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed by using a fluorescence spectrophotometer (Photon Technology International) equipped with a 60 W Xe-arc lamp as the excitation light source. All the measurements were taken at room temperature.

3. Theoretical method

The dielectric theory of complex crystals was used to calculate the chemical bond parameters of inorganic compounds quantitatively.^20–24 In this theory, the complex crystals with crystal formula A_a1¹A_a2²⋯A_aiⁱB_b1¹B_b2²⋯B_bj^j can be written as a linear combination of the sub-formula of various binary crystals when the crystal structure is known. The sub-formula of any type of chemical bond A–B in the multi-bond crystal A_a1¹A_a2²⋯A_aiⁱB_b1¹B_b2²⋯B_bj^j… can be expressed by the following formula:whereand the bond sub-formula equation is given by:where A_aiⁱ, B_bj^j stands for the different constituent elements or different sites of the same element in the crystal formula, and a_i, b_j represents the number of the corresponding element. N_(Bj−Ai) is the number of B_j ions in the coordination group of a Aⁱ ion, and N_CAⁱ represents the nearest coordination number of Aⁱ ion. This means that the complex crystal is decomposed into the sum of different binary crystals like A_miⁱB_nj^j.

Once the different binary crystals are obtained, the covalency of any μ type binary bond can be defined as:²⁵

where

(E^μ_g)² = (E^μ_h)² + (C^μ)² (5)

and

E^μ_h = 39.74/(d^μ)^2.48 (6)

here, d^μ is the bond distance (in Å) and r^μ₀ is half of the bond distance. k^μ_s is the Thomas–Fermis screening wavenumber of valence electrons and α_B is the Bohr radius. b^μ is proportional to the square of the average coordination number. (Z^μ_A)^* and (Z^μ_B)^* are the number of effective valence electrons of the cation and anion in the μ type bond, respectively.

For the chemical bond of type μ, the polarizable coefficient α^μ₀ can be obtained from the Lorentz–Lorenz equation

(ε^μ − 1)/(ε^μ − 2) = (4π/3)α^μ₀ (9)

Hence, the polarizability of the chemical bond volume (ų) is given by:

α^μ_b = α^μ₀ν^μ_b (10)

More detail calculation process can be referred to the ref. 25.

4. Result and discussion

4.1. Crystal phase formation

Fig. 1 shows X-ray diffraction (XRD) patterns of Sm-doped MAl₂Si₂O₈ samples prepared in reducing atmosphere. It is clear that all the diffraction peaks of four Sm-doped compounds can be readily indexed to the corresponding standard data for triclinic phase of CaAl₂Si₂O₈ (JCPDS 48-1454), monoclinic phase of SrAl₂Si₂O₈ (JCPDS 38-1454) and BaAl₂Si₂O₈ (JCPDS 38-1450) as well as the hexagonal phase of BaAl₂Si₂O₈ (JCPDS 28-0124), respectively. No other impurities can be detected. In addition, the refined crystallographic unit cell parameters of the calcined products were calculated using the software Jade 5.0 and listed in Table 1.

Fig. 1 XRD patterns of Sm-doped MAl₂Si₂O₈ samples prepared in reducing atmosphere. (a) CaAl₂Si₂O₈ : Sm; (b) SrAl₂Si₂O₈ : Sm; (c) monoclinic BaAl₂Si₂O₈ : Sm; (d) hexagonal BaAl₂Si₂O₈ : Sm.

Table 1 Unit cell parameters of Sm-doped MAl₂Si₂O₈ samples

Compound	a (Å)	b (Å)	c (Å)	α (°)	β (°)	γ (°)
a Ref. 4.b Ref. 18.c Ref. 17.d Ref. 16.
H-BASO : 5% Sm	5.2942	5.2942	7.7836	90	90	120
Pure H-BASO^a	5.2955	5.2955	7.7817	90	90	120
M-BASO : 5% Sm	8.6316	13.0444	14.4053	90	115.11	90
Pure M-BASO^b	8.6268	13.045	14.408	90	115.22	90
SASO : 5% Sm	8.3705	12.9671	14.2559	90	115.25	90
Pure SASO^c	8.392	12.967	14.260	90	115.43	90
CASO : 5% Sm	8.16785	12.87079	14.17677	93.2473	115.731	91.1248
Pure CASO^d	8.173	12.869	14.165	93.113	115.913	91.261

---

4.2. Photoluminescence properties

The photoluminescence properties of Sm²⁺ in different host lattices are quite different because of their different environmental factor. Divalent Sm²⁺ has the 4f⁶ electron configuration. Under irradiation with UV to blue light, it can be excited into the 4f⁵5d¹ continuum, from which the ions rapidly relax to the lowest excited state.^9–13 Just as the Eu²⁺ or Ce³⁺ ions, the f–d luminescence wavelength of phosphors employing Sm²⁺ changes greatly with the type of the host crystal.^1,26

4.2.1. Characteristic luminescence of Sm²⁺ in MAl₂Si₂O₈ (M = Ca, Sr, Ba) : Sm. Fig. 2 shows the excitation and emission spectra of Sm doped CASO, SASO, M-BASO and H-BASO samples at room temperature with the initial samarium concentration of 5 mol%. Different excitation wavelength produces different photoluminescence spectra. For CASO : Sm, Fig. 2(a) shows the broad excitation band monitored by 689 nm with the peak at 375 nm. In the emission spectra under the excitation at 375 nm, a weaker ⁵D₀ → ⁷F₀ transition of Sm²⁺ at 698.5 nm can be found (Fig. 2(b)). Unusual luminescence of Sm³⁺ in CASO can be monitored. As can be shown in Fig. 2(b), the sharp emission lines at 565, 601, 648 nm come from the f–f transitions of Sm³⁺, which can be assigned to ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2 → ⁶H_9/2, respectively. It indicates the existence of Sm³⁺ in CASO.

Fig. 2 Photoluminescence emission and excitation spectra of Sm-doped MAl₂Si₂O₈ (M = Ba, Sr, Ca) samples related to the luminescence centre ions Sm²⁺ at room temperature.

Fig. 2(d), (f) and 2(h) show that Sm doped SASO, M-BASO and H-BASO samples exhibit efficient deep red emission under irradiation with UV light. The excitation and emission peak positions are listed in Table 2. For SASO : Sm, the excitation spectrum (Fig. 2(c), λ_ex = 687 nm) consists of a broad band from 250 to 500 nm with a maximum at 359.5 nm. This is assigned to 4f–5d transition of Sm²⁺. As is shown in Fig. 2(d), under the excitation into the 4f⁵5d¹ states with the wavelength at 360 nm, the obtained emission spectra of SASO : Sm show a broad band emission from 600 to 750 nm overlapped with the f–f transitions of Sm²⁺. The peak of the broad band emission is 662 nm assigned to f–d transition of Sm²⁺. The sharp emission bands consists of three groups of lines at 687, 702 and 727 nm corresponding to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Sm²⁺, respectively. The dominant line is about 687 nm (⁵D₀ → ⁷F₀ transition of Sm²⁺ ions), which shows that Sm²⁺ ions occupy the crystallographic sites without central symmetry in the host.

Table 2 The excitation and emission peak positions of Sm²⁺ in the samples MAl₂Si₂O₈ : 5% Sm (M = Ba, Sr, Ca) (“*” indicates the strongest peak position of the excitation and emission spectra.)

Sample	Excitation (f → d) (nm)	d → f	Emission (nm) f–f transition (^5D0 → ^7FJ)
			J = 0	J = 1	J = 2
CASO : Sm	375*	—	689.5*	—	—
SASO : Sm	359.5*	662	687*	702	727
M-BASO : Sm	365*	660	685*	700.5	727.5
H-BASO : Sm	329*	603	683.5	698*	725.5
	423*

---

The excitation and emission spectra of M-BASO : Sm show in Fig. 2(e) and (f). The broad excitation band with the peak at 365 nm extends from 250 to 500 nm. And the broad f–d transition emission of Sm²⁺ ranges from 600 to 750 nm with the peak at 660 nm. The sharp emission bands consists of three groups of lines at 685, 700.5 and 727.5 nm corresponding to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Sm²⁺, respectively. The optical properties of M-BASO : Sm are similar to those of SASO : Sm. This is because that the crystal structure of M-BASO is close to that of SASO : Sm, thus crystal environment of Sm²⁺ in M-BASO is near to SASO : Sm. However, compared with that of SASO : Sm, a red-shift of the excitation for M-BASO : Sm can be found and the reason will be discussed in the discussion part.

As can be shown in Fig. 2(g) and (h), the sample H-BASO : Sm shows different excitation and emission spectra. The spectra are quite different with the photoluminescence properties of Sm doped SASO and M-BASO samples. Two dominate excitation peaks at 329 nm and 423 nm can be found, which can be assigned to the 4f–5d transition of Sm²⁺. The emission bands consist of a broad band emission from 550 to 650 nm with the peak at 603 nm. The sharp emission bands consists of three groups of lines at 683.5, 698 and 725.5 nm correspond to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Sm²⁺, respectively. The strongest emission is at 698 (⁵D₀ → ⁷F₁ of Sm²⁺), which indicates that the Sm²⁺ ions occupy the crystallographic sites with central symmetry in the host.

Fig. 3 shows the CIE (commission International de I'Ecairage 1931) chromaticity coordinates of Sm doped CASO, SASO, M-BASO and H-BASO. For Sm-doped CASO, the emission light excited at 375 nm shows orange-red because it mainly comes from the emission of Sm³⁺. For Sm doped SASO, M-BASO and H-BASO, as can be shown in Fig. 2 and Table 3, excited at N-UV light, their CIE values are B (0.680, 0.319), C (0.635, 0.350) and D (0.616, 0.357), respectively. All of them yield strong deep red emission, which may be used as the red phosphors in the phosphors-converted light-emitting diodes.

Fig. 3 CIE (commission International de I'Ecairage 1931) chromaticity coordinates of Sm doped CASO, SASO, M-BASO and H-BASO.

Table 3 CIE values of Sm doped CASO, SASO, M-BASO and H-BASO

Compound	CIE (excitation wavelength)
CASO : Sm	(0.573, 0.407)
	(374 nm)
SASO : Sm	(0.680, 0.319)
	(360 nm)
M-BASO : Sm	(0.635, 0.350)
	(365 nm)
H-BASO : Sm	(0.616, 0.357)
	(330 nm)

---

4.2.2. Crystal structure and spectra of Sm doped MAl₂Si₂O₈ (M = Ca, Sr, Ba).
4.2.2.1. The relationship between the crystal structure and the f–f transition intensity of Sm²⁺. From the above analysis, it has been clearly shown that the f–d transition energy of Sm doped SASO, M-BASO and H-BASO can produce f–d transition emission accompanied by some strong f–f transitions of Sm²⁺, however, the f–f emission intensity are quite different. Their comparison of the emission intensity under the excitation of 360 or 460 nm can be shown in Fig. 4. The clear order of the strongest f–f emission intensity of Sm²⁺ is H-BASO ≫ M-BASO > SASO > CASO. We will make our focus on the spectra analysis of Sm doped H-BASO, M-BASO and SASO to try to find out the relationship between the crystal structure and their PL properties because their strong f–d and f–f transition of Sm²⁺ at room temperature.

Fig. 4 Comparison emission intensity of Sm doped CASO, SASO, M-BASO and H-BASO under the f–d transition energy excitation.

As is well known, the relative intensity of f–f transitions is influenced by the crystalline environment due to the different symmetry of the occupied sites.^9,10,27–30 The crystal structure of MASO (M = Ca and Ba) has been shown in Fig. 5. In the Sm doped MASO (M = Ca and Ba) systematic samples, the Sm ions occupy the site of M²⁺ ions, therefore, the f–f transition intensity is mainly influenced by the symmetry of the M²⁺ sites. Just as the Eu³⁺ emission, the intensities of different ⁵D₀–⁷F_J transitions of Sm²⁺ depend on the local symmetry of the crystal field of the Sm²⁺ ions.^9–13 The ⁵D₀–⁷F₀ transition is hypersensitive, while the ⁵D₀–⁷F₁ transition is insensitive to the crystal field environment. For instance, in a site with inversion symmetry, the ⁵D₀–⁷F₁ transition is dominant, while in a site without inversion symmetry, the ⁵D₀–⁷F₀ electronic transition becomes the strongest one. The intensity of ⁵D₀–⁷F₀ transition is much higher than that of ⁵D₀–⁷F₁, which is strong evidence that Sm²⁺ ions mainly occupy the lattice site without inversion symmetry.⁹ From the crystal structure and the skeleton construction of MASO (Fig. 5), we can see that only in the Sm-doped H-BASO sample, Sm occupy the centered site with six identity oxygens (O₂), which is inversion symmetry, so the intensity of ⁵D₀–⁷F₁ transition of Sm²⁺ in H-BASO is the strongest among the four samples.

Fig. 5 Crystal structure of Sm doped-CASO, M-BASO and H-BASO. (a), (c) and (e): shape of the unit cell; (b), (d) and (f): Sm/Ca, Sm/Ba-centred skeleton construction with nearest neighbor atoms, respectively. The detail structure data of CASO, M-BASO, SASO and H-BASO can be referred to ref. 4 and 16–18, respectively.

The space groups of M-BASO and SASO are I12/c1 (no. 15).^17,18 Both of the occupied sites of Ba²⁺ in M-BASO and Sr²⁺ in SASO are 8f. The symmetry of Ba²⁺ in M-BASO crystal is similar to that of Sr²⁺ in SASO crystal, but the intensity of ⁵D₀–⁷F₀ transition of Sm²⁺ of M-BASO : Sm is about 2 time stronger than that of SASO : Sm, which can be shown in Fig. 4(b) and d. In their crystal structure, as can be shown in Fig. 5(c) and (d), there is one 8f site of Ba with seven different nearest neighbors atoms including two types of Ba–O1, one Ba–O2, one Ba–O3, one Ba–O4, one Ba–O7 and one Ba–O8 bonds. This type of symmetry produces the strong emission of the ⁵D₀–⁷F₀ transition of Sm²⁺. It has been found that the photoluminescence intensity ratio between ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ emission of Eu³⁺ (I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁)) increases with increasing of the distortion degree of LnO₆ or LnO₇ polyhedron from that of an ideal octahedron. So, just as the Eu³⁺, the transition intensity ratio between ⁵D₀–⁷F₀ and ⁵D₀–⁷F₁ emission of Sm²⁺ (I(⁵D₀ → ⁷F₀)/I(⁵D₀ → ⁷F₁)) is influenced by the distortion degree of LnO_n polyhedron from that of an ideal polyhedron.^27,28 Their ratio values of I(⁵D₀ → ⁷F₀)/I(⁵D₀ → ⁷F₁) for SASO : Sm, M-BASO and H-BASO are about 2.21, 3.34 and 0.95, respectively. The distortion degree can be calculated using the standard deviation of environmental factor of the individual bond (EFSD) σ(h_ei), which can be expressed as below:

where

h_ei = (f^μ_cα^μ_b)^1/2Q^μ_B (12)

and

The related chemical parameters, such as the covalency f_c, the polarizability of the chemical bond volume α_b, and the present charge of the ligand in the binary crystals, are listed in Table 4. According to the eqn (11)–(13), their standard deviation of the seven M–O environmental factors (σ(h_ei)) of MO₇ or MO₆ polyhedron in the SASO, M-BASO and H-BASO can be calculated to be 0.03379, 0.07348 and 0 respectively. The σ(h_ei) value of M-BASO is 2.2 times larger than that of SASO. Generally, the I(⁵D₀ → ⁷F₀)/I(⁵D₀ → ⁷F₁) value of Sm²⁺ increases with increasing of σ(h_ei).^27,28 Therefore, the I(⁵D₀ → ⁷F₀)/I(⁵D₀ → ⁷F₁) of Sm²⁺ in the crystal M-BASO is stronger than that in the crystal SASO or H-BASO, and I(⁵D₀ → ⁷F₀)/I(⁵D₀ → ⁷F₁) of H-BASO : Sm is the lowest.

Table 4 The environmental factor of any individual bond between the centre-M²⁺ atom and its nearest coordination atom (h_ei) in the monoclinic crystals M-BASO and SASO, their standard deviation of the seven M–O environmental factors (σ(h_ei)) as well as the emission intensity ratio between ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₁ in the hexagonal crystal H-BASO and monoclinic crystals M-BASO and SASO

Samples	Central ion	Bond type	fc	αb	QB	C.N.	hei	σ(hei)
SASO	Sr^2+	Sr–O1	0.0318	0.1810	1.1429	2	0.08671	0.03379	2.21
		Sr–O2	0.0696	0.3123	0.8571	1	0.12636
		Sr–O3	0.0650	0.5346	0.8571	1	0.15977
		Sr–O4	0.0639	0.6209	0.8571	1	0.17072
		Sr–O7	0.0646	0.5603	0.8571	1	0.16306
		Sr–O8	0.0650	0.5306	0.8571	1	0.15917
M-BASO	Ba^2+	Ba–O1	0.0304	0.2402	1.1429	2	0.09766	0.07348	3.34
		Ba–O2	0.0655	0.4451	0.8571	1	0.14635
		Ba–O3	0.0626	0.6692	0.8571	1	0.20222
		Ba–O4	0.0625	0.6809	0.8571	1	0.17681
		Ba–O7	0.0628	0.6497	0.8571	1	0.17313
		Ba–O8	0.0629	0.6437	0.8571	1	0.17246
H-BASO	Ba^2+	Ba–O2	0.1047	1.1695	1.000	6	—	0	0.951

---

4.2.2.2. The relationship between the crystal structure and the f–d transition energy of Sm²⁺. Fig. 6 shows the comparisons of 4f–5d transition excitation spectra in Sm²⁺ in H-BASO, M-BASO and SASO. (The intensity of H-BASO is reduced 6 times; and that of SASO is enlarged 1.5 times.) The lowest band comes from the Sm doped H-BASO (red line). The broad bands of Sm doped M-BASO and SASO have a blue-shift comparing with that of H-BASO. The 4f–5d transition band of Sm-SASO (blue line) has a blue-shift comparing with that of M-BASO (black line).

Fig. 6 The 4f–5d transition excitation spectra of Sm²⁺ in H-BASO, M-BASO and SASO. (The intensity of H-BASO is reduced 6 times; and that of SASO is enlarged 1.5 times.) The lowest band comes from the Sm doped H-BASO (red line). The broad bands of Sm doped M-BASO and SASO have a blue-shift comparing with that of H-BASO. The 4f–5d transition band of Sm-SASO (blue line) has a blue shift comparing with that of M-BASO (black line).

As is well known, the 4f–5d transition energy is greatly influenced by the crystalline environmental.^21,22,26,31 This is because that the excited states such as 5d are not shielded from the surrounding electronic shell, so the 5d electron has a strong interaction with the neighboring anions in the compound. The interactions for the 4f⁵5d configuration, which may be important for optical spectrum simulations, are as follows:³²

H = H₀ + H_Coul(ff) + H_so(f) + H_cf(f) + H_cf(d) + H_so(d) + H_Coul(fd) (14)

where H₀ is the central-field interaction which includes the kinetic energy of electron and the Coulomb interaction between the 4f⁵ core and the electrons; H_Coul(ff), H_so(f) and H_cf(f) are Coulomb, spin–orbit and crystal–field interactions for the 4f⁵ core; H_cf(d) and H_so(d) are crystal field and spin orbit interactions for the single 5d electron; H_Coul(fd) is the Coulomb interaction between the 4f⁵ core and the 5d electron. The order of magnitude of the energy splitting caused by these interactions is as follow:

H₀〉[H_Coul(ff),H_cf(d)]〉H_Coul(fd)〉H_so(f)〉H_so(d)〉H_cf(f) (15)

The interactions H_so(d) and H_cf(f) can be generally ignored. In our systematic samples the central ion is Sm²⁺ with the electron 4f⁶ configuration, the lowest 4f⁶ → 4f⁵5d energy (E_exp(4f⁶ − 4f⁵5d)) is determined by five parts: the energy centroid of the 5d orbital (E_C), the Coulomb, spin–orbital interactions between 4f and 4f electrons (E_Coul(ff),E_so(f)), the Coulomb interactions between 4f and 5d electrons (E_Coul(fd)) and the effect of the crystal field (E_cf(d)). Fig. 7 shows a schematic energy diagram of the of 4f⁶ → 4f⁵5d configuration of Sm²⁺ in any host lattice.

Fig. 7 A schematic energy diagram of 4f⁶ → 4f⁵5d configuration of Sm²⁺ in any host lattice.

From the diagram (Fig. 7), the lowest 4f⁶ → 4f⁵5d transition energy of Sm²⁺ can be obtained as follows:

E_exp(4f⁶ − 4f⁵5d) = E_C − E_Coul(ff) − E_so(f) − E_Coul(fd) − E_cf(d) (16)

In our case, all of the luminescence centre ions are Sm²⁺, it is expected that E_Coul(ff) and E_so(f) are unchanged in the MASO (M = Sr and Ba) crystals. The energy difference among Sm-doped SASO, H-BASO and M-BASO mainly comes from the difference of the other three parts of eqn (16): E_C, E_Coul(fd) and E_cf(d). E_C and E_Coul(fd) are related to the environmental factor h_e (ref. 31) and E_cf(d) is concerned with the environmental parameter F_C.²² The values of h_e can be calculated using the below equations:

where Q^μ_B stands for the presented charge of the nearest anion in the chemical bond, and α^μ_b is the polarizability of the chemical bond volume in the μ type of chemical bonds.³¹ Also, the relationship between the environmental factor (h_e) and the environmental factor of any individual bond between the centre atom and their nearest coordination atom (h_ei) can be expressed by

The values of the environmental parameter F_C can be obtained as follows:

where Ē_h is the average homopolar part of the energy gap. Q̄ is the average present charge of the anions, and f̄_i is the average bond ionicity between the central ion and the nearest neighbors.

According to the eqn (18) and (19), the values of the environmental factor h_e and the environmental parameter F_C as well as their related chemical bond parameters are calculated and listed in Table 5. The h_e values of Sm doped H-BASO, M-BASO and SASO are 0.8573, 0.4028 and 0.3710, respectively. Generally, an increase of h_e leads to an decrease of the energy centroid of the 5d orbital (E_C).³¹ Thus the order of their E_C values is E_C(H-BASO)〈E_C(M-BASO)〈E_C(SASO), and the order of the E_Coul(fd) value is just reverse.²⁶ It has been shown that the E_cf(d) increases with increasing of F_C, so the order of the E_cf(d) value is E_cf(H-BASO)〈E_cf(SASO)〈E_cf(M-BASO). Finally, according to the eqn (16), it can be judged that among the three samples the lowest 4f–5d transition energy of Sm doped H-BASO is the lowest, which coincides with the experimental data (423 nm). For the other two crystal M-BASO and SASO, their difference between the environmental factors (h_e) (Δ(h_e) = 0.0318) is near to the difference between F_C(Δ(h_e) = 0.0553). On the other hand, the magnitude of H₀ is larger than others in eqn (14). Thus the difference of the E_exp(4f⁶ − 4f⁵5d) value is mainly demined by the difference of E_C, so the lowest 4f–5d energy of Sm-doped SASO is a little larger than that of Sm-doped M-BASO, which is consistent with the experimental result.

Table 5 The values of the corresponding crystal factors (F_C) and environmental factor (h_e) of the central ions M²⁺ for H-BASO, SASO and M-BASO samples

Samples	Central ion	Bond type	Eh	fi	QB	C.N.	FC	he
SASO	Sr^2+	Sr–O1	3.4944	0.9682	1.1429	2	0.4278	0.3710
		Sr–O2	4.3145	0.9304	0.8571	1
		Sr–O3	3.1692	0.9350	0.8571	1
		Sr–O4	2.9126	0.9361	0.8571	1
		Sr–O7	3.0862	0.9354	0.8571	1
		Sr–O8	3.1828	0.9350	0.8571	1
M-BASO	Ba^2+	Ba–O1	2.9564	0.9696	1.1429	2	0.3725	0.4028
		Ba–O2	3.4860	0.9345	0.8571	1
		Ba–O3	2.7685	0.9374	0.8571	1
		Ba–O4	2.7417	0.9375	0.8571	1
		Ba–O7	2.8147	0.9372	0.8571	1
		Ba–O8	2.8292	0.9371	0.8571	1
		Ba–O2	2.9377	0.8953	1.000	6
H-BASO	Ba^2+	Ba–O2	2.9377	0.8953	1.000	6	0.4384	0.8573

---

5. Conclusion

The Sm²⁺ doped triclinic CaAl₂Si₂O₈ (CASO), monoclinic SrAl₂Si₂O₈ (SASO) and BaAl₂Si₂O₈ (M-BASO) as well as hexagonal BaAl₂Si₂O₈ (H-BASO) have been synthesized using polymerizable-complex technique under reducing atmosphere. At room temperature, under the N-UV or blue excitation, Sm²⁺ doped SASO, M-BASO and H-BASO showed strong red emission. Their strongest emission spectra mainly come from the ⁵D₀ → ⁷F₀ transition of Sm²⁺ in the H-BASO crystal and the ⁵D₀ → ⁷F₁ transition of Sm²⁺ in SASO and M-BASO, respectively. The calculated standard deviation of environmental factor of the individual bond of Sm disclosed the reason why the emission intensity ratio (I(⁵D₀ → ⁷F₀)/I(⁵D₀ → ⁷F₁)) of Sm²⁺ in SASO is stronger than that in M-BASO and that of H-BASO is the lowest. The lowest excitation energy of 5d energy levels of Sm²⁺ have a blue-shift in the order H-BASO < M-BASO < SASO. The physical reason for spectral and energy level changes is analyzed in detail to be a comprehensive result from the shift of the energy centroid of the 5d orbital, the Coulomb interaction between 4f and 5d electrons, and the crystal-field splitting of 5d energy level. The theoretical analysis is in good agreement with our experimental result.

Acknowledgements

This work is financially supported by the National Natural Science Foundations of China (Grant no. 21301053).

References

1. F. Clabau, A. Garcia, P. Bonville, D. Gonbeau, T. Le Mercier, P. Deniard and S. Jobic, J. Solid State Chem., 2008, 181, 1456.
2. W. B. Dai, J. Mater. Chem. C, 2014, 2, 3951.
3. H. Guo, X. Y. Liu, F. Li, R. F. Wei, Y. L. Wei and C. Ma, J. Electrochem. Soc., 2012, 159, J223.
4. W. B. Im, Y.-I. Kim and D. Y. Jeon, Chem. Mater., 2006, 18, 1190.
5. M. Ma, D. Zhu, C. Zhao, T. Han, S. Cao and M. Tu, Opt. Commun., 2012, 285, 665.
6. Y. Wang, Z. Wang, P. Zhang, Z. Hong, X. Fan and G. Qian, Mater. Lett., 2004, 58, 3308.
7. C. Zhang, J. Yang, C. Lin, C. Li and J. Lin, J. Solid State Chem., 2009, 182, 1673.
8. Q. Zhang, X. Liu, Y. Qiao, B. Qian, G. Dong, J. Ruan, Q. Zhou, J. Qiu and D. Chen, Opt. Mater., 2010, 32, 427.
9. Y. Huang, W. Kai, K. Jang, H. S. Lee, X. Wang, Y. Zhang, D. Qin and C. Jiang, Mater. Lett., 2008, 62, 1913.
10. P. Mikhail, J. Hulliger, M. Schnieper and H. Bill, J. Mater. Chem., 2000, 10, 987.
11. M. Nogami and Y. Abe, J. Appl. Phys., 1997, 81, 6351.
12. Z. Pei, Q. Su and J. Zhang, J. Alloys Compd., 1993, 198, 51.
13. X. Wang, H. Riesen, M. A. Stevens-Kalceff and R. P. Rajan, J. Phys. Chem. A, 2014, 118, 9445.
14. Q. Zeng, Z. Pei, S. Wang, Q. Su and S. Lu, Chem. Mater., 1999, 11, 605.
15. Q. Su, H. Liang, T. Hu, Y. Tao and T. Liu, J. Alloys Compd., 2002, 344, 132.
16. C. J. E. Kempster, H. D. Megaw and E. W. Radoslovich, Acta Crystallogr., 1962, 15, 1005.
17. P. Benna and E. Bruno, Am. Mineral., 2001, 86, 690.
18. R. E. Newnham and H. D. Megaw, Acta. Crystallogr. A, 1960, 13, 303.
19. J. Lin, M. Yu, C. Lin and X. Liu, J. Phys. Chem. C, 2007, 111, 5835.
20. F. M. Gao, J. L. He, E. D. Wu, S. m. Liu, D. l. Yu, D. c. Li, S. Y. Zhang and Y. J. Tian, Phys. Rev. Lett., 2003, 91, 015502.
21. J. Shi and S. Zhang, J. Phys.: Condens. Matter, 2003, 15, 4101.
22. J. S. Shi, Z. J. Wu, S. H. Zhou and S. Y. Zhang, Chem. Phys. Lett., 2003, 380, 245.
23. Z. J. Wu and S. Y. Zhang, J. Phys. Chem. A, 1999, 103, 4270.
24. S. Y. Zhang, Chemical Bond Theory of Complex Structure Crystals on Dielectric Description and Application, Science Publisher, Beijing, 2005.
25. L. Li and S. Y. Zhang, J. Phys. Chem. B, 2006, 110, 21438.
26. Z. Fu, S. Zhou and S. Zhang, J. Phys. Chem. B, 2005, 109, 14396.
27. L. Li, X. Liu, H. M. Noh, S. H. Park, J. H. Jeong and K. H. Kim, J. Alloys Compd., 2015, 620, 324.
28. L. Li, H. K. Yang, B. K. Moon, Z. Fu, C. Guo, J. H. Jeong, S. S. Yi, K. Jang and H. S. Le, J. Phys. Chem. C, 2009, 113, 610.
29. J. G. Muller, J. Karthikeyan, P. Murugan and N. Lakshminarasimhan, J. Phys. Chem. C, 2014, 118, 19308.
30. X. Zhang and H. J. Seo, J. Alloys Compd., 2011, 509, 2007.
31. J. S. Shi and S. Y. Zhang, J. Phys. Chem. B, 2004, 108, 18845.
32. S. Y. Zhang, Spectroscopy of Rare Earth Ions-Spectral property and Spectral Theory, Science Publisher, Beijing, 2008.

---