Crystal structure, tunable luminescence and energy transfer properties of Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ phosphors

Abstract

A series of Tb³⁺ and/or Eu³⁺ doped Na₃La(PO₄)₂ phosphors were successfully synthesized and their crystal structure and photoluminescence (PL) properties were investigated in detail. Double phosphates with the compositions Na₃Tb(PO₄)₂ and Na₃Eu(PO₄)₂ were obtained by the substitution of Tb or Eu for La in the Na₃La(PO₄)₂ host. XRD pattern analysis indicates that these obtained compounds crystallize in the orthorhombic system with the space group Pbc2₁. The crystal structure of the Na₃RE(PO₄)₂ (RE = Tb, Eu) is made up of isolated PO₄ tetrahedra and of sodium and RE atoms arranged in an ordered way. The REO_y polyhedra are isolated from one another, resulting in a high critical concentration of Tb³⁺ or Eu³⁺ activators. Under excitation of near-ultraviolet (NUV) irradiation, Tb³⁺ doped Na₃La(PO₄)₂ shows a blue-greenish emission with a predominant peak at 546 nm, while the emission spectra of Eu³⁺-doped Na₃La(PO₄)₂ exhibits a reddish orange emission due to the ⁵D₀ → ⁷F_J transitions of Eu³⁺ ions. The energy transfer from Tb³⁺ to Eu³⁺ in the Na₃La(PO₄)₂ host is demonstrated by the luminescence spectra and fluorescence decay dynamics. Meanwhile, the emission color of Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ can be tuned from green to red through tuning the Tb³⁺/Eu³⁺ ratio. These results indicate that the Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ phosphor exhibits broadband NUV absorption and green-reddish orange tunable emission, which might serve as a down-converting phosphor for NUV light-emitting diodes.

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Introduction

Rare earth (RE) ions play an irreplaceable role in the development of lighting and display fields due to their abundant emission colors based on 4f–4f or 5d–4f transitions.^1–3 Recently, RE³⁺ ion doped phosphors based on double phosphate hosts have drawn much attention because of their high luminous efficiency, low sintering temperature, high thermal and chemical stability, and low cost.^4,5 Double phosphates of mono- and trivalent cations with the general formula M^I₃N^III(PO₄)₂ (M^I = Na, K; N^III = Y, Sc, In, Fe, rare earth elements) have high thermal and chemical stability and their host absorption edge locates at a rather short wavelength (about 140–180 nm),⁶ making them excellent host materials for luminescent materials. M^I₃N^III(PO₄)₂ compounds crystallize in a trigonal, orthorhombic or monoclinic structure, depending on the type of M^I or N^III cations.⁷ Among them, Na₃RE(PO₄)₂ (RE = La–Tb) compounds crystallize orthorhombic with the glaserite-type structure. They are built up on isolated REO_y polyhedral and PO₄ tetrahedra.⁸ The presence of this particular structure suggests that the lattice can accommodate other cations with similar radii and charges without significant changes to the structural frame.⁹ Furthermore, this structure can weaken the concentration quenching effect and the critical concentration of activator ions is much higher than that of conventional inorganic phosphors. Therefore, the structure and optical properties of Na₃RE(PO₄)₂-related phosphors have been extensively studied. A great number of glaserite-type phosphors, such as Na₃Y(PO₄)₂:Ce³⁺,¹⁰ Na₃La(PO₄)₂:Er³⁺,¹¹ Na₃Gd(PO₄)₂:Ce³⁺,¹² and Na₃RE(PO₄)₂:Yb³⁺ (RE = Y, La, Gd)¹³ have been reported. Meanwhile, most of the phosphors are single-colored, and combining different phosphors is applied when a multicolor emission is needed. However, this combination suffers from the disadvantages of reabsorption among phosphors and different degradation rates.¹⁴ Therefore, great efforts have been devoted to develop single-host phosphors with a multicolor emission to meet the increasing demand of different illumination applications. In order to achieve color tunable emitting in single-phase hosts, several strategies are used, including controlling the temperature,¹⁵ band-gap modulation,¹⁶ crystal field adjustment,¹⁷ the combination of multiple rare ions with various color emissions,¹⁸ and codoping ion pairs based on the energy transfer mechanism. Codoping different rare earth ions as sensitizers and activators in a single matrix is one of the most popular methods to control the emission color via energy transfer processes.¹⁹ Additional, tunable multicolor emission can be realized in phosphors under a single excitation wavelength. The multicolor tuning of phosphors has been achieved by co-doping RE³⁺ ion into suitable host lattice, such as Eu³⁺–Bi³⁺,²⁰ Tm³⁺–Dy³⁺ (ref. 21) and Tb³⁺–Eu³⁺.^22–24

It has been reported that Na₃La(PO₄)₂ crystallizes in the orthorhombic structure.⁸ Eu³⁺ and Tb³⁺ ions are frequently used as red and green activators in luminescent materials.¹⁹ However, the luminescence properties of Tb³⁺ and/or Eu³⁺ ions in Na₃La(PO₄)₂ host under near ultraviolet (NUV) light excitation have not been reported, and so far the energy transfer phenomenon from Tb³⁺ to Eu³⁺. In this contribution, Na₃La(PO₄)₂ was chosen as the host material. The structure, luminescence properties and chromaticity stability of Tb³⁺ and/or Eu³⁺ activated Na₃La(PO₄)₂ samples are studied in detail. The energy transfer process between Tb³⁺ and Eu³⁺ ions as well as the potential luminescence mechanism has been analyzed in Na₃La(PO₄)₂ host upon the excitation wavelength of 378 nm irradiation.

Experimental

Powder samples of Na₃La_1−x(PO₄)₂:xEu³⁺ (x = 0–1.0), Na₃La_1−y(PO₄)₂:yTb³⁺ (y = 0–1.0), Na₃La_0.7−xTb_0.3(PO₄)₂:xEu³⁺ (x = 0–0.7), and Na₃La_0.95−yEu_0.05(PO₄)₂:yTb³⁺ (y = 0–0.95) were prepared as follows. Stoichiometric amounts of analytical reagents NaNO₃, NH₄H₂PO₄, and 99.99% pure La₂O₃ were mixed. An appropriate amount of CO(NH₂)₂ was added as fuel. 99.99% pure Eu₂O₃ and Tb₄O₇ were dissolved in HNO₃ to convert into nitrate completely. These reagents were dissolved in water and then introduced into a muffle furnace maintained at 600 °C for 5 min. The obtained processor was subsequently ground in an agate mortar and then reacted at 900 °C for 4 h in air atmosphere. Finally, the products were gradually cooled to room temperature and reground for further measurements.

The phase purity of the products was checked by powder X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer (Bruker Co. Ltd., Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5406 Å), operating at 40 kV and 40 mA. Structure refinements of XRD data were performed using the computer software General Structure Analysis System (GSAS) program.²⁵ The luminescence emission and excitation spectra of the samples were measured on a fluorescence spectrophotometer (F-7000, Hitachi, Japan) equipped with a 150 W Xe light source. The luminescence decay data were collected on an Edinburgh FLS920 combined fluorescence lifetime and steady state spectrometer with a 450 W xenon lamp and 60 μF flash lamp. For comparison, all measurements were conducted at room temperature with the identical instrumental parameters.

Results and discussion

Phase identification and crystal structure

The XRD patterns of Tb³⁺ and/or Eu³⁺ doped Na₃La(PO₄)₂ samples were measured at room temperature. Fig. 1 shows the powder XRD profiles of some representative samples. No records of Na₃La(PO₄)₂, Na₃Tb(PO₄)₂ and Na₃Eu(PO₄)₂ are available in Joint Committee on Powder Diffraction Standards (JCPDS) or Inorganic Crystal Structure Database (ICSD). As shown in Table 1, the radius Nd³⁺ is quite close to that of La³⁺, Eu³⁺ and Tb³⁺.²⁶ The compound Na₃Nd(PO₄)₂ is isostructural with Na₃RE(PO₄)₂ (RE = La, Eu and Tb). Therefore, the standard data of Na₃Nd(PO₄)₂ (ICSD no. 100593) serve as a certified reference.²⁷ As presented in Fig. 1a, most of the samples are consistent with the standard file of Na₃Nd(PO₄)₂, indicating that the obtained samples are single phase and heavily doping Tb and/or Eu ions do not change the crystal structure. This is attributed to that Tb³⁺ or Eu³⁺ occupies La³⁺ sites for their similar radii and identical valence. However, as an exceptional case, two additional weak diffraction peaks at 30.95° and 34.27° ascribed to Na₆La(PO₄)₃ as a second phase can be discerned for the Na₃La(PO₄)₂ sample. A small shift of the XRD peaks of the Na₃RE(PO₄)₂ (RE = La, Eu, Tb) samples in comparison to the standard data of Na₃Nd(PO₄)₂ can be observed in Fig. 1b. The characteristic peak (3 4 2) shifts to the higher angle as the RE³⁺ sites are substituted by the La³⁺ → Nd³⁺ → Eu³⁺ → Tb³⁺ with the decrease of ionic radii. According to Bragg's diffraction equation, 2d sin θ = nλ, in which n is an integer, λ is the X-ray wavelength, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering planes. The substitution of the La³⁺ ions in the crystallographic structure by the smaller Tb³⁺ or Eu³⁺ ions reduces the cell dimensions of the crystal, leading to the increase of the 2θ value.

Fig. 1 Representative XRD patterns of Na₃RE(PO₄)₂ (RE = La, Tb, Eu) samples and ICSD no. 100593 (a); magnified XRD patterns in the region from 31.5 to 34.5 deg (b).

Table 1 The effective ionic radii of the different coordination sites of RE³⁺ in REO_y (RE = La, Nd, Eu, Tb; y = 6, 7, 8)

y	Ionic radius (Å)
	La^3+	Nd^3+	Eu^3+	Tb^3+
6	1.032	0.983	0.947	0.923
7	1.10	—	1.01	0.98
8	1.16	1.109	1.066	1.04

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Reported by Salmon et al.,²⁷ Na₃Nd(PO₄)₂ crystallizes in Pbc2₁ (no. 29) space group and orthorhombic crystal system (ICSD no. 100593). As shown in Fig. 1, solid solutions of Na₃RE(PO₄)₂ (RE = La, Eu and/or Tb) may exist due the same valence and similar radii of these ions.²⁸ Meanwhile, there are a lot of evidence about the iso-structural of orthorhombic Na₃Nd(PO₄)₂ with other sodium and rare-earth double orthophosphates Na₃RE(PO₄)₂ (RE = Y, La–Er).^10,12,29,30 Here the crystal structure data of Na₃Nd(PO₄)₂ is used as a starting model to refine the crystal structure. Fig. 2a and b exhibit the experimental, calculated and difference results from the Rietveld refinement of the two end components Na₃Tb(PO₄)₂ and Na₃Eu(PO₄)₂, respectively. All of the observed peaks can be indexed to the corresponding data. We can conclude that the desired single-phase phosphors with a glaserite-type structure have been synthesized and the patterns have not changed by doping Tb³⁺ and/or Eu³⁺ ions. No other phase or impurity can be detected, confirming the formation of a single phase. The low values of R_wp, R_p and χ² shown in Table 2 indicate that the refined crystal structure data are reliable. Both Na₃Tb(PO₄)₂ and Na₃Eu(PO₄)₂ crystallize in the orthorhombic crystal system with space group Pbc2₁ and N = 24. Their unit cell parameters differ from that of Na₃Nd(PO₄)₂ (a = 15.874 Å, b = 13.952 Å, c = 18.470 Å, V = 4090.63 ų), resulted from the substitution of Nd³⁺ by Tb³⁺ or Eu³⁺. The Rietveld analysis shows that the samples are in crystalline phase and no phase mixture was observed. Rietveld plots of Na₃Tb_0.95Eu_0.5(PO₄)₂ and Na₃Tb_0.3Eu_0.7(PO₄)₂ are presented in Fig. S1 and S2 (in the ESI†), respectively. All of the observed peaks satisfy the reflection conditions, confirming the formation of a single phase with no impurities. The remarkable good fit between the experimental data and calculated line confirm the phase purity of the as-prepared samples.

Fig. 2 Rietveld analysis patterns for X-ray powder diffraction data of Na₃Tb(PO₄)₂ (a) and Na₃Eu(PO₄)₂ (b) compounds.

Table 2 Crystallographic data and details in the data collection and refinement parameters for Na₃RE(PO₄)₂ (RE = Eu and Tb)

Sample	Na3Tb(PO4)2	Na3Eu(PO4)2
Space group	Pbc21	Pbc21
Symmetry	Orthorhombic	Orthorhombic
a/Å	15.899	15.920
b/Å	13.950	13.936
c/Å	18.405	18.425
V/Å^3	4082.1	4087.8
α = β = γ, deg	90	90
Rwp, %	5.74	5.26
Rp, %	4.74	4.07
χ^2	2.472	1.390

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Fig. 3a depicts the crystal structure of the Na₃RE(PO₄)₂ (RE = La, Eu, Tb) unit cell viewed along a-direction from the parallel projection, the coordination environment of RE³⁺ sites, and the ideal glaserite structure. The Na₃RE(PO₄)₂ framework is made up of isolated PO₄ tetrahedron and [REO_y] (y = 6, 7, 8) polyhedron that arranged in an ordered way which results in the tunnel. The basic structure units are helical ribbons [REO_y] formed by six corner sharing [PO₄] tetrahedron that alternate “up” and “down.”. The pinwheels are linked through [PO₄] tetrahedra to form layers with alkali atoms located between the layers. Fig. 3b and c demonstrate the six kinds of RE sites in a unit cell along b-direction. The ribbons run along some directions of unit-cell with a period of four or eight tetrahedral. The coordination of the RE atoms is 6-folded for RE1 and RE3, 7-folded for RE2, RE4, RE5 and 8-folded for RE6. The variety in coordination numbers is due to a rotation of the certain [PO₄] tetrahedron around one of their edges. The REO_y polyhedral are isolated because they do not share any O atom. In addition, the shortest RE–RE distances in Na₃Tb(PO₄)₂ and Na₃Eu(PO₄)₂ are about 4.641 and 4.646 Å (RE2–RE5), respectively, which is a long distance that energy migration of the doped rare earth ions is difficult, similar observation has also been witnessed in some systems such as Na₃Gd_1−xEu_x(PO₄)₂ (ref. 31) and Na₃Gd(PO₄)₂:Ce³⁺.¹²

Fig. 3 Crystal structure of the Na₃RE(PO₄)₂ (RE = La, Eu, Tb) unit cell viewed along a-direction from the parallel projection (a); the coordination environment of different Nd³⁺ sites in a unit cell (b); the expansion particular six kinds of Nd³⁺ (c); the ideal glaserite structure of the K₃Na(SO₄)₂ (d).

Luminescence properties of Tb³⁺ and/or Eu³⁺ doped Na₃La(PO₄)₂ phosphors

Fig. 4 illustrates the UV-vis excitation (PLE, λ_em = 546 nm) and emission (PL, λ_em = 378 nm) spectra of Na₃La_1−x(PO₄)₂:xTb³⁺ with x = 0.01–1.0. The PLE spectrum of Na₃Tb(PO₄)₂ involves several sharp lines in the 280–420 nm range. The sharp f–f excitation lines at about 302, 317, 340, 351, 358, 368, 378 and 486 nm are assigned to ⁷F₆–⁵H₆, ⁷F₆–⁵H₇, ⁷F₆–⁵G₂, ⁷F₆–⁵D₂, ⁷F₆–⁵L₁₀, ⁷F₆–⁵G₆ and ⁷F₆–⁵D₄, respectively.³¹ Under 378 nm NUV excitation, the PL spectrum of Na₃La_0.99(PO₄)₂:0.01Tb³⁺ presents a group of ⁵D_3,4 → ⁷F_J transitions: ⁵D₃ → ⁷F₅ (415 nm), ⁵D₃ → ⁷F₄ (437 nm), ⁵D₄ → ⁷F₆ (490 nm), ⁵D₄ → ⁷F₅ (546 nm), ⁵D₄ → ⁷F₄ (586 nm) and ⁵D₄ → ⁷F₃ (623 nm). With the increase of Tb³⁺ concentration (x), the blue emissions from the ⁵D₃ → ⁷F_5,4 transitions are quenched gradually, while the green emissions from the ⁵D₄ → ⁷F_6,5,4,3 transitions increase continuously. For the Tb³⁺ ion, the energy gap between the ⁵D₃ and ⁵D₄ levels is about 5915 cm⁻¹, which is quite close to that between ⁷F₆ and ⁷F₀ levels (6000 cm⁻¹).³² Hence, if the Tb³⁺ concentration (y) is high enough, the emission from the ⁵D₃ level of Tb³⁺ is much weaker than that from the ⁵D₄ level due to the cross relaxation via the resonant energy transfer process: Tb³⁺ (⁵D₃) + Tb³⁺ (⁷F₆) → Tb³⁺ (⁵D₄) + Tb³⁺ (⁷F₀) and the green emission of the ⁵D₄ → ⁷F₅ (546 nm) becomes predominant.³³ The PL intensity of Tb^{3+ 5}D₄ → ⁷F₅ transition increases gradually with its concentration (x) increasing, and reaches a maximum at x = 1. This result indicates that no concentration quenching exists in the Na₃La(PO₄)₂ host among the Tb³⁺ ions. The Na₃Tb(PO₄)₂ sample shows strong green emission under 378 nm NUV irradiation excitation, which makes it be a potential green phosphor for NUV LED application.

Fig. 4 The PLE spectrum of Na₃Tb(PO₄)₂ and the PL spectra of Na₃La_1−x(PO₄)₂:xTb³⁺ (x = 0.01–1.0) samples.

The PLE spectrum of Na₃Eu(PO₄)₂ and PL spectra of Na₃La_1−y(PO₄)₂:yEu³⁺ (y = 0.01–1.0) are shown in Fig. 5. The PLE spectrum consists of a weak broad band assigned to the charge-transfer transition (CTB) between Eu³⁺ and O²⁻, some narrow lines in the range of 230–320 nm (the strongest peak located at about 306 nm is due to Rayleigh scattering), and several sharp lines from 360–480 nm. These sharp lines correspond to the characteristic f → f transitions of Eu³⁺ ions within its 4f⁶ configuration. They are ascribed to ⁷F₀ → ⁵D₄ (360 nm), ⁷F₀ → ⁵G_J,⁵L₇ (381 nm), ⁷F₀ → ⁵L₆ (394 nm), ⁷F₀ → ⁵D₃ (414 nm), and ⁷F₀ → ⁵D₂ (464 nm) transitions of Eu³⁺ ion, respectively. Excitation into the ⁷F₀ → ⁵L₆ transition of Eu³⁺ at 394 nm yields some characteristic emission lines from the ⁵D_0,1 excited states to the ⁷F_J ground states, i.e., ⁵D₁ → ⁷F₁ (536 nm), ⁵D₁ → ⁷F₂ (556 nm), ⁵D₀ → ⁷F₁ (594 nm), ⁵D₀ → ⁷F₂ (620 nm), ⁵D₀ → ⁷F₃ (655 nm), and ⁵D₀ → ⁷F₄ (703 nm), respectively.³⁴ However, the ⁵D₀ → ⁷F₀ transition (about 580 nm) is very weak and can hardly be detected.

Fig. 5 The PLE spectrum of Na₃Eu(PO₄)₂ and the PL spectra of Na₃La_1−y(PO₄)₂:yEu³⁺ (y = 0–1) samples.

The two dominant bands at 594 (⁵D₀ → ⁷F₁ transition) and 620 nm (⁵D₀ → ⁷F₂ transition) confer on the sample an orange-red luminescence upon excitation with 394 nm light. It is known that the magnetic-dipole transition ⁵D₀ → ⁷F₁ is insensitive to the symmetry of the Eu³⁺ site, while the forced electric dipole transition ⁵D₀ → ⁷F₂ is hypersensitive to the local environment.^35,36 Therefore, the intensity ratio (R) of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) gives a measure of the Eu³⁺ site symmetry in the lattice. A higher value of R (R > 1) suggests that Eu³⁺ locates at the site without inversion symmetry. Otherwise, Eu³⁺ ion locates at the site with inversion symmetry, leading to a lower value of R (1 > R > 0).³⁷ As shown in Fig. 3d, Na⁺ has an inversion symmetric environment in the ideal glaserite structure of K₃Na(SO₄)₂. However, upon substitution of Na⁺ sites by RE³⁺, there will be some distortion in NaO₆ octahedron. The Na₃RE(PO₄)₂ structure seems to be a distorted glaserite structure. In this structure six different types of REO_y polyhedral can be expected. Hence, the Eu³⁺ ion may occupy six sites in the Na₃RE(PO₄)₂ lattice, as shown in Fig. 3b and c. Here, the intensity of ⁵D₀ → ⁷F₁ is comparable with that of ⁵D₀ → ⁷F₂. The value of R calculated is about 1.02, indicating that the Eu³⁺ ions occupies a symmetric and a non-symmetric site almost equally. This agrees with the results of Eu³⁺ doped K₃Y(PO₄)₂ (ref. 34) and Na₃Y(PO₄)₂ (ref. 38) but challenges the results of Eu³⁺ doped Rb₃Y₂(PO₄)₃ and Rb₃La(PO₄)₂.³⁹

The PL intensity (⁵D₀ → ⁷F₂) of Na₃La_1−y(PO₄)₂:yEu³⁺ increases with increasing Eu³⁺ concentration (y) until a maximum intensity about y = 1.0 is reached. These observations confirm that the concentration quenching of Eu³⁺ does not occur in Na₃La(PO₄)₂ host, so highly doping concentration samples are performed here.

It is worthy to be noted that both Na₃La_1−x(PO₄)₂:xTb³⁺ and Na₃La_1−y(PO₄)₂:yEu³⁺ phosphors have a much high quenching concentration, actually the complete quenching would not occur even at x or y = 1.0, at which the La³⁺ sites are replaced by Tb³⁺ or Eu³⁺ completely. Taking into account the inherent structural feature of Na₃RE(PO₄)₂ host, in which the Tb³⁺ or Eu³⁺ ions occupy the RE³⁺ sites of polyhedral which isolate from each other by a large spatial distance and join by RE–O–P–O–RE. The large spatial distance between RE³⁺ ions and the shielding of PO₄ tetrahedrons hinders the long range energy transfer between Tb³⁺ or Eu³⁺ ions and consequently prevents the occurrence of concentration quenching. Therefore, the high concentration quenching was observed in Na₃La_1−x(PO₄)₂:xTb³⁺ and Na₃La_1−y(PO₄)₂:yEu³⁺ phosphors. The similar phenomenon was also reported by Chen et al. in K₃R(PO₄)₂:Tb³⁺ (R = Y and Gd) phosphors,⁴⁰ Ju et al. in Na₃Gd_1−xEu_x(PO₄)₂ phosphors,³⁰ and Jiang et al. in K₃Gd(PO₄)₂:Tb³⁺,Eu³⁺ phosphor.⁴¹

In order to obtain multicolor tunable luminescence of the Na₃La(PO₄)₂ phosphor, Tb³⁺ and Eu³⁺ ions with different relative concentration into the Na₃La(PO₄)₂ host lattice were codoped in our work. The PLE and PL spectra of the Na₃La_0.65(PO₄)₂:0.3Tb³⁺,0.05Eu³⁺ sample are shown in Fig. 6. For comparison, the PLE and PL spectra of Na₃Tb(PO₄)₂ and Na₃Eu(PO₄)₂ are also presented. The PLE spectrum (Fig. 6c) of the Na₃La_0.65(PO₄)₂:0.3Tb³⁺,0.05Eu³⁺ by monitoring the emission of Tb³⁺ at 546 nm is almost identical to that of Na₃Tb(PO₄)₂ (Fig. 6a) within the experimental error. The PLE band/line of Eu³⁺ is undetectable, implying that Eu³⁺ cannot transfer energy to Tb³⁺. The PLE spectrum recorded at the 620 nm of Eu³⁺ emission is dominated by Tb³⁺ bands/lines, which are similar to that of monitoring the Tb³⁺-emission, but shows large difference with that of Eu³⁺ (Fig. 6b). Only several f–f transition lines of Eu³⁺ are evidently observed (marked by stars in Fig. 6c). The presence of Tb³⁺-related PLE bands/lines in the PLE spectrum of Eu³⁺ emission clearly indicates the occurrence of energy transfer from Tb³⁺ to Eu³⁺. Upon 394 nm excitation (⁷F₀ → ⁵L₆ of Eu³⁺), only emission from Eu³⁺ is observed, and the positions of all emission peaks are identical to those in Fig. 6b of Na₃Eu(PO₄)₂. Excited at 378 nm UV irradiation (⁷F₆ → ⁵G₆ of Tb³⁺), the characteristic sharp emissions from both Eu³⁺ and Tb³⁺ can be detected, confirming that Tb³⁺ can partially transfer excitation energy to Eu³⁺ via its absorption of 4f state. Therefore, the relative intensities of these two emissions can be varied by adjusting the concentrations of the two activators through the principle of energy transfer.

Fig. 6 PLE and PL spectra of Na₃Tb(PO₄)₂ (a), Na₃Eu(PO₄)₂ (b), and Na₃La_0.65(PO₄)₂:0.3Tb³⁺,0.05Eu³⁺ (c).

Fig. 7 illustrates the variations of PL spectra and corresponding intensities of Na₃La_0.95−x(PO₄)₂:xTb³⁺,0.05Eu³⁺ and Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ phosphors. The emission profile of all the Tb³⁺/Eu³⁺ codoped samples contain the characteristic sharp emission peaks of both Tb³⁺ and Eu³⁺ under excitation at 378 nm. The increasing concentrations of the Eu³⁺ or Tb³⁺ ions bring no obvious alteration in the intensity ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁), indicating that the degree of the local symmetry around Eu³⁺ ions keeps constant. As shown in Fig. 7a, the PL intensities of Eu³⁺ at 620 nm increase systematically with increasing the Tb³⁺ concentration (x), because the increase of Tb³⁺ concentration results in more sensitizers transferring the energy to Eu³⁺ ions. Meanwhile, the Tb³⁺ green emission intensity reaches its maximum at x = 0.3, and then decreases due to the concentration quenching effect with further increasing the Tb³⁺ concentration (x). The inset of Fig. 7a depicts the dependences of the PL intensities ((⁵D₄ → ⁷F₅ transition of Tb³⁺; ⁵D₀ → ⁷F₂ transition of Eu³⁺; λ_ex = 378 nm)) on the Tb³⁺ concentration (y). The Eu³⁺ PL intensity is enhanced about 11 times by codoping with Tb³⁺. In Fig. 7b, the PL intensity of Tb³⁺ decreases monotonously with the increase of Eu³⁺ concentration from y = 0 to 0.7, while the Eu³⁺ PL intensity increases to a maximum at y = 0.7. This observation indicates that the energy transfer from Tb³⁺ to Eu³⁺ ions can occur in current excitation condition in Tb³⁺/Eu³⁺ codoped Na₃La(PO₄)₂ phosphor. Therefore, the relative intensities of these two emissions can be varied by adjusting the concentrations of the two activators through the principle of energy transfer to realize the tunable emission color. The digital emission color photos were depicted in the inset of Fig. 7b, clearly indicating that the emission color can be tuned from green to reddish orange with increasing the Eu³⁺ concentration (y).

Fig. 7 PL spectra (λ_ex = 378 nm) of Na₃La_0.95−x(PO₄)₂:xTb³⁺,0.05Eu³⁺ (x = 0–0.95, a) and Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ (y = 0–0.7, b) phosphors together with their digital photographs under a 365 nm UV lamp.

Decay curves and energy transfer mechanism

It has been witnessed that an efficient energy transfer from Tb³⁺ to Eu³⁺ occurs in Na₃La(PO₄)₂ host. In order to further investigate the energy transfer between Tb³⁺ and Eu³⁺ in Na₃La(PO₄)₂, luminescent decay curves of Tb³⁺ emission and Eu³⁺ emission in Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ (y = 0–0.7) samples have been measured. The decay curves monitored at 546 nm (Tb^{3+ 5}D₄ → ⁷F₅ transition) and 620 nm (Eu^{3+ 5}D₀ → ⁷F₅ transition) with excitation of 378 nm irradiation are presented in Fig. 8a and b, respectively.

Fig. 8 Luminescence decay curves of Tb³⁺ 546 nm emission (⁵D₄ → ⁷F₅) (a), Eu³⁺ 620 nm emission (⁵D₀ → ⁷F₅) (b) and the corresponding simulation curves (c).

It is found that the decay curves of Tb³⁺ emission cannot be fitted in terms of a single-exponential function, but can be well fitted by a double-exponential function:

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (1)

where I is the luminous intensity at time t; A₁ and A₂ are the fitting parameters; and τ₁ and τ₂ are rapid and slow lifetimes for exponential components, respectively. The decay process of these samples is characterized by an effective lifetime τ, which can be calculated using eqn (2) as follows

τ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (2)

The values of τ_Tb are calculated to be 3.70, 2.54, 1.33, 0.93 and 0.38 ms for Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ phosphors with y = 0, 0.01, 0.05, 0.4 and 0.7. As shown in Fig. 9, the effective lifetime of Tb³⁺ ions decreases with the increase of Eu³⁺ due to the ET_Tb→Eu process.

Fig. 9 Dependence of Tb³⁺ decay time, P_Tb→Eu and η_Tb→Eu on Eu³⁺ concentration (y) in Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺.

For the ET_Tb→Eu process, the transfer probability (P_Tb→Eu) can be expressed by eqn (3)⁴²

P_Tb→Eu = 1/τ − 1/τ₀ (3)

where P_Tb→Eu is the energy transfer probability and τ and τ₀ are the lifetimes for Tb³⁺ with and without the Eu³⁺ ions, respectively. In addition, the energy transfer efficiency (η_Tb→Eu) can be evaluated using eqn (4),

η_Tb→Eu = 1 − τ/τ₀ (4)

The values of P_Tb→Eu and η_Tb→Eu are calculated and are also shown in Fig. 9. Both the values of P_Tb→Eu and η_Tb→Eu increase obviously with increasing the Eu³⁺ concentration (y), indicating that the energy-transfer process become more efficient with high Eu³⁺ ion concentration.

Fig. 8b illustrates the decay curves of Eu³⁺ emission. All the decay curves of Eu³⁺ emission could be well fitted into singly exponential equation

I_t = I₀ exp(−t/τ) (5)

where I₀ and I_t are for the intensities of Eu³⁺ emission at time t₀ and t, respectively; τ denotes the lifetimes for Eu³⁺. The calculated values of τ_Eu are 5.02, 4.21, 3.32 and 3.27 ms for y = 0.01, 0.05, 0.4 and 0.7 in Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ phosphors.

It is obvious that the Eu³⁺ decay curves recorded at the ⁵D₀ → ⁷F₅ transition (620 nm) exhibit a rising step. The fluorescence intensity increases with increasing time, then reaching a maximum, and finally decreases until the decay process completes. Therefore, there are two different processes for the emission of Eu³⁺: decay process and build-up process. In the initial build-up process, the energy absorbed by the ⁷F₆ → ⁵G₆ transition of Tb³⁺ ions is transferred to Eu³⁺ ions. This process is significantly influenced by the Eu³⁺ concentration. As shown in Fig. 8b, with increasing the Eu³⁺ concentration (y), the initial build-up process becomes faster and faster, suggesting that the ET_Tb→Eu process becomes more efficient with higher Eu³⁺ concentration.⁴³ When the Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ samples are excited by 378 nm irradiation, the rate equations for the population densities in the ⁵D₄ level of Tb³⁺ and the ⁵D₀ of Eu³⁺ ion can be expressed as follows,⁴⁴

dN_Tb/dt = −N_Tb/τ_Tb − K_Tb–EuN_Tb (6)

dN_Eu/dt = −N_Eu/τ_Eu + K_Tb–EuN_Tb (7)

where the N_Tb and N_Eu are the population intensities of the ⁵D₄ level of Tb³⁺ and the ⁵D₀ of Eu³⁺, respectively. K_Tb–Eu is the non-radiative energy transfer rate from the ⁵D₄ state of Tb³⁺ to ⁵D₀ of Eu³⁺. Then the fluorescence intensity I(t) of Eu³⁺ ions at 620 nm excited at 378 nm irradiation can be given as following equation:

Using the measured values of τ_Tb and τ_Eu, the simulation curves for Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ samples are obtained as presented in Fig. 8c, which show two process for Eu³⁺ emission, being similar to the measured curves. That is to say, the theoretical results are consistent with the experimental observations.

In general, the energy transfer between the sensitizer and activator may take place via exchange interaction and multipolar interaction. The energy transfer mechanism can be determined using the following relationship:⁴⁵

ln(I₀/I) ∝ C and I_S0/I_S ∝ C^α/3 (9)

in which I_S0 and I_S are the luminescence intensities of Tb³⁺ in the absence and presence of Eu³⁺, respectively; C is the total concentration of Tb³⁺ and Eu³⁺; ln(I_S0/I_S) ∝ C is corresponding to the exchange interaction; α = 6, 8, and 10 are dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The relationships are illustrated in Fig. 10a–d, respectively for Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ phosphors. The linear behavior was observed only when α = 6, indicating that energy transfer from Tb³⁺ to Eu³⁺ in Na₃La(PO₄)₂ host occurs via a dipole–dipole interaction.

Fig. 10 Dependence of ln(I_S0/I_S) on C_Tb+Eu (a) and the dependence of I_S0/I_S on C_Tb+Eu^6/3 (b), C_Tb+Eu^8/3 (c) and C_Tb+Eu^10/3 (d).

The scheme of energy transfer from Tb³⁺ to Eu³⁺ in Na₃La(PO₄)₂ host is demonstrated in Fig. 11. Tb³⁺ ions absorb the energy from 378 nm irradiation and are excited from the ground state of (⁷F₆) to the excited states of ⁵D_J (J = 2, 3, 4). Some of the excited Tb³⁺ radiative transmit from ⁵D₃ to the ground state ⁷F₆ directly with relatively weaker blue light emission of 415 and 437 nm, and other excited Tb³⁺ relax to the lowest excited state ⁵D₄ through non-radiative transition, then radiative decay to the ground state (⁷F₆) with a strong green emission. When Eu³⁺ ions are codoped, part of the energy from ⁵D₄ to ⁷F_J transition of Tb³⁺ will be transferred to Eu³⁺ through cross-relaxation due the obvious overlap between the ⁵D₄ → ⁷F_J emission of Tb³⁺ and ⁷F_0,1 → ⁵D_0,1,2 absorption of Eu³⁺, then relax to the ground state ⁵D₀ of Eu³⁺ and finally radiative decay to the ground state (⁷F₀) with an reddish orange emission.⁴⁴

Fig. 11 Scheme of energy transfer from Tb³⁺ to Eu³⁺ in Na₃La(PO₄)₂ host.

CIE chromaticity coordinates of Na₃La(PO₄)₂:Tb³⁺,Eu³⁺

The CIE chromaticity coordinates (X, Y) for Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ (y = 0–0.7) samples were calculated in the case of 378 nm excitation and the results are shown in Fig. 12. The chromaticity coordinates of Na₃Eu(PO₄)₂ phosphor is also presented. The CIE chromaticity coordinates (X, Y) changes from point 1 (0.2987, 0.5695) to point 7 (0.6203, 0.3505) with the increase of Eu³⁺ concentration (y). The as formed Na₃La_0.95−x(PO₄)₂:xTb³⁺,0.05Eu³⁺ (x = 0–0.95) phosphors show typical reddish-orange luminescence. However, their CIE coordinates are too close to be distinguished from each other in a chromaticity diagram with the changes of the Tb³⁺ ion concentration, so they are not presented.

Fig. 12 CIE chromaticity diagram of the selected Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ (y = 0–0.7) and Na₃Eu(PO₄)₂ phosphors under 378 nm UV irradiation excitation. The inset shows the PL spectrum of Na₃La_0.67(PO₄)₂:0.3Tb³⁺,0.03Eu³⁺ under 378 nm UV irradiation excitation.

The PL spectrum of Na₃La_0.67(PO₄)₂:0.3Tb³⁺,0.03Eu³⁺ phosphor shown as inset in Fig. 12 exhibits a green emission of the Tb³⁺ and the red emission of the Eu³⁺ ions. These emission lines of Tb³⁺ and Eu³⁺ cover the whole visible light region with tunable intensity, resulting in a color-tunable emission. Therefore, it can be expected to achieve color-tunable emission by regulating spectral composition induced by the energy transfer between Tb³⁺ and Eu³⁺ ions in the Na₃La(PO₄)₂ host under NUV irradiation excitation. According to Grassman's Laws of additive color mixture,^46,47 the emission color of Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ can be matched by a linear combination of the emission color of Na₃La(PO₄)₂:Tb³⁺ and that of Na₃La(PO₄)₂:Eu³⁺. It is obvious that the coordinate Y changes linearly with the coordinate X as shown in Fig. 12. Plotting y vs. x, a straight line is obtained. The values of intercept, slope, and linear regression coefficient are 0.7782, −0.7037, and 0.9902 respectively. The expression is shown as below:

Y = 0.7782 − 0.7037X (10)

The above equation should mathematically demonstrate the range of chromaticity coordinates that we can obtain by adjusting the Tb³⁺ and Eu³⁺ concentrations. It is obvious that the chromaticity coordinates for the Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ phosphor falls on a line connecting the two chromaticity coordinates of Na₃La(PO₄)₂:Tb³⁺ and Na₃La(PO₄)₂:Eu³⁺, respectively. With increasing the Eu³⁺ concentration (y), the chromaticity coordinate for Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ phosphors move from the chromaticity point 1 of Na₃La_0.7(PO₄)₂:0.3Tb³⁺ toward point 8 of Na₃Eu(PO₄)₂ along this straight line. The prepared Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ phosphor exhibits efficient tunable emission in the visible-light region upon excitation with NUV irradiation, and might find potential applications in multicolor displays and other optoelectronic devices.

Conclusions

In a conclusion, a series of Tb³⁺ and/or Eu³⁺ doped Na₃La(PO₄)₂ phosphors have been successfully synthesized and their luminescence properties have been investigated in detail. The glaserite-like orthorhombic structure provides the Na₃La(PO₄)₂ host the possibility of doping with Tb³⁺ or Eu³⁺ ions without substantial luminescence quenching. Tb³⁺ can efficiently sensitize Eu³⁺ emission under NUV excitation. The energy transfer mechanism (Tb³⁺ → Eu³⁺) was demonstrated to be dominated by a dipole–dipole interaction. The luminescence decay properties of Eu³⁺ in Na₃La(PO₄)₂:Tb³⁺,Eu³⁺ samples under ⁷F₆–⁵G₆ excitation (378 nm) within Tb³⁺ ions were simulated with the energy transfer theory. The emission color of Na₃La_0.7−y(PO₄)₂:0.3Tb³⁺,yEu³⁺ phosphors can be tunable from green through yellow and red region by adjusting the Eu³⁺ concentration. These results indicate that the as-synthesized phosphors may find potential applications as a color-tunable emitting material in solid state lighting.

Acknowledgements

This research was financially supported by “the Fundamental Research Funds for the Central Universities”, South-Central University for Nationalities (CZY15002).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: F-7000 instrumental parameters; Fig. S1 Rietveld refinement of the powder XRD pattern of Na₃Tb_0.95Eu_0.5(PO₄)₂; Fig. S2 Rietveld refinement of the powder XRD pattern of Na₃Tb_0.3Eu_0.7(PO₄)₂. See DOI: 10.1039/c6ra26164g

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