Crystal structure and luminescence properties of a novel single-phase orange-red emitting phosphor Ca₉La (PO₄)₇:Sm³⁺

Abstract

Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors (0.01 < x < 0.25) were synthesized via high temperature solid-state reaction. Their properties including crystal structure, decay performance, thermal stability and photoluminescence were examined in detail. Phase analysis indicated that the phosphors crystallized in a rhombohedral structure, and the emission spectra of the phosphors consisted of four peaks centered around 570, 607, 653, and 714 nm, attributed to Sm³⁺ ions transitions ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, and 11/2, respectively). The optimal Sm³⁺ doping content was 0.12 mol and the Ca₉La(PO₄)₇:Sm³⁺ phosphor also exhibited better thermal stability compared to other phosphors. In addition, the mechanism behind the energy transfer between Sm³⁺ ions was determined to be dipole–dipole interactions as described by Dexter theory. The results indicated that as orange-red phosphors, Ca₉La (PO₄)₇:Sm³⁺ phosphors were promising candidates for being integrated into white light emitting diodes.

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1. Introduction

As a new generation of illumination light sources, white light emitting diodes (w-LEDs) have received much attention. Their main advantages over conventional light sources are extended lifetime, better security and reliability as well as higher energy-efficiency.^1–6 In general, w-LEDs can be achieved with the following two approaches: in the first method a yellow phosphor such as Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) is integrated into a blue LED chip (as InGaN-based chip);⁷ in the second one, a near-UV (n-UV) LED chip, emitting in the 350 - 420 nm spectral range, is incorporated in a tri-color phosphor (i.e. blue, green, and red).⁸ However, the former method suffers from a rather high color temperature (>4500 K) and poor color-rendering index (CRI < 75) resulted from the weak emission in the visible red range.⁹ The second method can better control the w-LED colors, still, it is a great challenge to obtain orange or red phosphors with a high stability and brightness, so this issue is necessary to addressed.

Lanthanides are commonly used in displays and lighting devices thanks to the 5d → 4f or 4f → 4f transitions.⁴ Red and orange-red emitting phosphors can be realized by doping lanthanides with Eu³⁺ or Sm³⁺ ions and exploiting 4f → 4f transitions. However, Eu³⁺ doped phosphors display efficient luminescence only under n-UV excitation, which limits their application in w-LEDs.⁵ While Sm³⁺ doped phosphors can be more easily stimulated by a blue region radiation. Moreover, Sm³⁺ doped phosphors can act as orange-red emitters due to the ⁴G_5/2 → ⁶H_J (with J = 5/2, 7/2, 9/2, and 11/2) transitions.¹⁰ These features have motivated intense researches on compounds such as Sr₃Lu(PO₄)₃:Sm³⁺,¹¹ Ba₃Lu(PO₄)₃:Sm³⁺,¹² NaSrVO₄:Sm³⁺ (ref. 13) and KLaSr₃(PO₄)₃F:Sm³⁺.¹⁴

Recently, multiple examples of phosphates with whitlockite crystal structure fabricated based on M₉R(PO₄)₇ compounds, in which M = Ca or Sr and R = La, Y, Sc, Gd, or Bi, have demonstrated the feasibility for them to be applied as host materials for phosphors. Specifically, such phosphates compounds possess good thermal stability,¹⁵ and perfect charge stabilization allowed by the rigid tetrahedral matrix of the phosphate.^16,17 Besides, they can be synthesized in an easy and low-cost method.¹⁸ Currently, the luminescence of rare-earth ions doped whitlockite-type phosphate have been intensively studied in many compounds including Ca₉La(PO₄)₇:Eu²⁺, Mn²⁺,¹⁹ Ca₉Ln(PO₄)₇:Tb³⁺ (with Ln = Y, La or Gd),²⁰ Ca₉Bi(PO₄)₇:Ce³⁺, Tb³⁺, Mn²⁺,²¹ Ca₉LiGd(2/3) (PO₄)₇:Eu²⁺,²² Sr₈MgLn(PO₄)₇:Eu²⁺ (with Ln = Y or La)²³ and Sr₉Sc(PO₄)₇:Eu²⁺, Mn²⁺.²⁴ Yet, the crystal structure and luminescence properties of Ca₉La(PO₄)₇:Sm³⁺ were seldomly reported before.

In this article, orange-red single-phase Ca₉La(PO₄)₇:Sm³⁺ phosphors were fabricated via solid state reaction. Then their crystal structure and photoluminescence performance were discussed in detail. Beside that, thermal quenching, lifetime and concentration quenching were also investigated. The energy transfer in Ca₉La(PO₄)₇ doped with Sm³⁺ were also detailly examined, which demonstrated the feasibility of Ca₉La(PO₄)₇:Sm³⁺ phosphors applied in n-UV or blue light excited w-LEDs as orange-red emitters.

2. Experimental methods

2.1. Materials and synthesis

A series of Ca₉La_1−x(PO₄)₇:xSm³⁺ (x = 0.01, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18 and 0.25) were fabricated via high temperature solid-state reaction with following precursors: CaCO₃ (Aladdin, 99.9%), NH₄H₂PO₄ (Aladdin, 99.9%), La₂O₃ (Aladdin, 99.99%) and Sm₂O₃ (Aladdin, 99.99%). Each reactant of stoichiometric amount was fully grinded and preheated at 650 °C for 6 h. Then, the mixture was heated at 1300 °C for 4 h. After cooling down to room temperature and grinding, the products were obtained.

2.2. Characterizations

Powder X-ray diffraction (XRD) data was acquired from a Bruker D8 Advance X-ray powder diffractometer, using Cu Kα radiation (λ = 1.5418 Å) with operation voltage of 40 kV and current of 40 mA. Then, Rietveld analysis was performed on XRD data (step size: 0.02°, counting time: 2 s per step, and 2θ: 5° – 120°) collected from Ca₉La_0.99(PO₄)₇:0.01Sm³⁺ and Ca₉La_0.88(PO₄)₇:0.12Sm³⁺ phosphors. TOPAS program was used to perform crystal structure refinement.²⁵ And photoluminescence emission (PL) and excitation (PLE) spectra, as well as temperature-dependent luminescence properties were all obtained by a Hitachi F-4600 fluorescence spectrophotometer. In addition, the decay curves were detected by a Yvon TBXPS spectrofluorometer. ZEISS 6035 scanning electron microscope (SEM) equipped with an Inca X-Max energy dispersive spectroscope (EDS) operated at voltage of 20 kV was used to characterize the particle morphology and conduct elemental analysis.

3. Results and discussion

3.1. Crystal structure

XRD patterns for samples Ca₉La_1−x(PO₄)₇:xSm³⁺ (x = 0.01, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18 and 0.25) were shown in Fig. 1, and due to the isostructure, the polycrystalline standard data of compound Ca₉Sc(PO₄)₇ (JCPDS: 50-0340) was taken as a reference. As can be seen from the Fig. 1, all the diffraction peaks of the phosphors perfectly matched with the reference, suggesting that the prepared phosphors was a single phase and that the introduction of Sm³⁺ ions did not sensibly influence the crystal lattice of Ca₉La(PO₄)₇. Besides, considering the similar ionic radius and the same valence state, the Sm³⁺ ions were thought to take up the La sites. Moreover, the diffraction peaks position slightly shifted toward higher angles along with the raised Sm³⁺ doping concentration (Fig. 1(b)), which also confirmed the occupation of La³⁺ ions by smaller Sm³⁺ions. To further verify the purity of Ca₉La_1−x(PO₄)₇:xSm³⁺ compounds, Rietveld refinement was performed on x = 0.01 and x = 0.12 samples at room temperature. The initial structural model for Rietveld refinement was constructed based on isostructural single crystal crystallographic data of Ca₉Sc(PO₄)₇. And Fig. 2 showed Rietveld refinement results on x = 0.01 (Fig. 2(a)) and x = 0.12 (Fig. 2(b)) samples using TOPAS 4.2. The original experimental patterns were indicated by black circles and diamonds, the calculated patterns were represented by solid lines. The difference between them were shown in purple and blue curves, the calculated Bragg reflections positions of Ca₉Sc(PO₄)₇ was shown in olive vertical lines. As shown in Table 1 and Fig. 2, the refinements possessed low R-factors. The reliability parameters finally converged to R_wp = 5.77%, R_p = 3.90% and x² = 1.60 for x = 0.01 and R_wp = 5.43%, R_p = 3.86% and x² = 1.46 for x = 0.12 phosphors, indicating that both samples were pure single phase. Besides, the results listed in Table 1 showed that the samples were well crystallized in an rhombohedral lattice with the space group of R3c. Interestingly, the cell volume dropped with the increasing of doping x, which was because of the smaller ion radius of Sm³⁺ compared to La³⁺ (3572.32 ų for x = 0.01 and 3570.16 ų for x = 0.12).

Fig. 1 X-ray diffraction patterns of Ca₉La_1−x(PO₄)₇:xSm³⁺ (x = 0.01, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18 and 0.25) phosphors and the polycrystalline standard card of Ca₉Sc(PO₄)₇ (JCPDS no. 50-0340) as a reference (a), the zoomed area 2θ = 30–32° of the patterns(b).

Fig. 2 Rietveld refinement of the XRD profile of x = 0.01 (a) and x = 0.12 (b).

Table 1 Main parameters of processing and refinement of the x = 0.01 and 0.12 sample

Compound	Ca9La0.99(PO4)7:0.01Sm^3+	Ca9La0.88(PO4)7:0.12Sm^3+
2θ-interval	5–120 (°)	5–120 (°)
Space group	R3c (161)	R3c (161)
Symmetry	Rhombohedral	Rhombohedral
Cell parameters	a (Å) = 10.48233 (9);	a (Å) = 10.48020 (9);
	c (Å) = 37.5409 (4);	c (Å) = 37.5334 (4);
	V (Å^3) = 3572.32 (7)	V (Å^3) = 3570.16 (7)
	Z = 6	Z = 6
Reliability factors	Rp = 3.90	Rp = 3.861
	Rwp = 5.77	Rwp = 5.43
	Rexp = 1.44	Rexp = 1.42
	RBragg = 1.79	RBragg = 1.59
	x^2 = 1.60	x^2 = 1.46

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Fig. 3 showed the details of the reference Ca₉La(PO₄)₇ crystal structure based on β-Ca₃(PO₄)₂,²⁶ and it was obvious that Ca₉La(PO₄)₇ was isostructural to Ca₉Sc(PO₄)₇ and Ca₉Fe(PO₄)₇.^27,28 In particular, three different Ca²⁺ ionic sites and only one kind of La³⁺ site were observed. More specifically, the three kinds of Ca²⁺ ionic sites were all coordinated with eight oxygen atoms, while the La³⁺ site was coordinated with six oxygen atoms. This could further explain why the Sm³⁺ (r(Sm³⁺) = 0.958 Å, CN = 6) ions occupied the sites of the La³⁺ (r(La³⁺) = 1.032 Å CN = 6), as described in Fig. 1.

Fig. 3 Simulated crystal structure of Ca₉La(PO₄)₇ compound.

Fig. 4 displayed the SEM image of the x = 0.12 phosphor including the EDS elemental analysis in the red rectangle area. The irregular particle size and the agglomeration occurred during the heating process were the typical morphology from high temperature solid-state reaction synthesis.⁵ Meanwhile, the EDS analysis showed that the atom ratio of each element Ca, La, O, P and Sm were 9.11, 0.83, 28.37, 7.06 and 0.14 at%, respectively. The results indicated the actual atom ratio of each element was very close to the intended atom ratio, and also confirmed the success of Sm doping.

Fig. 4 SEM image of x = 0.12 sample, the EDS element analysis of the surface points within the rectangle area is shown in the inset.

3.2. Luminescence properties

For the purpose of investigation on the luminescence properties of Sm³⁺ in Ca₉La(PO₄)₇, the PLE spectra at different emission wavelengths (λ_em = 570, 607, 653 nm) and PL spectra under different excitation wavelengths (λ_ex = 377, 406, 408 nm) of x = 0.12 sample were monitored under the same testing conditions as shown in the Fig. 5. All the excitation spectra as well as emission spectra possessed the same features except for the discrepancy in relative intensities. The PL spectra also indicated that Ca₉La(PO₄)₇:Sm³⁺ compounds had potential to be applied as near-UV light (350–420 nm) excited phosphor, since the emission intensities were maximal under the excitation at 406 nm.

Fig. 5 PLE spectrum (λ_em = 570, 607, 653 nm) included transitions: ⁶H_5/2 → ²L_15/2, ⁴H_9/2, ⁴D_3/2, ⁴D_1/2, ⁴F_7/2 and ⁴I_11/2 (a), PL spectrum (λ_ex = 377, 406, 478 nm) included transitions: ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2, ⁶H_9/2 and ⁶H_11/2 of x = 0.12 sample (b) and the energy level diagram of Sm³⁺ in Ca₉La(PO₄)₇ (c).

The PLE spectrum in Fig. 5(a) clearly was made up of multiple narrow bands (at 320, 347, 365, 377, 406, and 478 nm) in the wavelength range from 300 to 500 nm, which were corresponding to the following transitions of Sm³⁺ ions: ⁶H_5/2 → ²L_15/2, ⁶H_5/2 → ⁴H_9/2, ⁶H_5/2 → ⁴D_3/2, ⁶H_5/2 → ⁴D_1/2, ⁶H_5/2 → ⁴F_7/2 and ⁶H_5/2 → ⁴I_11/2.^4,29 Among them, by analyzing the integrated areas of every narrow peaks, it was manifested that the ⁴F_7/2 was the most populated level under the excitation at 406 nm. The PL bands maxima in Fig. 5(b) were centered at 570, 607, 653, and 714 nm, corresponding to the ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2 → ⁶H_9/2 and ⁴G_5/2 → ⁶H_11/2 transitions of Sm³⁺,^5,30 respectively. Likewise, among these transitions, it was indicated the ⁴G_5/2 → ⁶H_7/2 (at 607 nm) transition was the most intense one by the integrated areas analysis. And according to the selection rules, this was a partially magnetic and partially electric dipole transition,³¹ while ⁴G_5/2 → ⁶H_9/2 as well as ⁴G_5/2 → ⁶H_11/2 was purely electric dipole transitions and ⁴G_5/2 → ⁶H_5/2 was purely magnetic dipole transition.⁴

For a better understanding of the luminescence behaviors of Sm³⁺ in Ca₉La(PO₄)₇, the energy level diagram of Sm³⁺ was shown in Fig. 5(c). According to the diagram, Sm³⁺ ions can be excited to the ⁴F_7/2, ⁴D_1/2 and ⁴I_11/2 states from the ⁶H_5/2, and the states were relaxed to the ⁴G_5/2 emitting state through non-radiative (NR) transitions. The efficient multi-channel transition emissions of Sm³⁺ ions occurred when the ⁴G_5/2 state was populated. As a result, Sm³⁺ exhibited visible emitting colors.

In order to identify the optimal Sm³⁺ doping content and clarify the dependence of the emission intensities on the Sm³⁺ concentration, the optical properties of a series of Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors (x = 0.01, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18 and 0.25) were investigated. And Fig. 6 showed the emission spectra under 406 nm excitation. The intensities of emission peaks initially increased with the enhanced Sm³⁺ doping content and reached a maximum value at the Sm³⁺ content of 0.12 mol, and then decreased due to the concentration quenching. According to ref. 32 and 33, the critical energy transfer distance R_C between Sm³⁺ ions can be calculated according to the concentration quench formula:

where N represented for the value of available sites in the unit cell which could be occupied by Sm³⁺ ions, x_c for the critical concentration of Sm³⁺ ions and V for the volume of the unit cell. For Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors, N = 54, V = 3570.16 ų and x_c = 0.12. As a result, the approximate value of R_C was calculated as 10.17 Å, which was more than 0.5 nm. Thus, the mechanism behind energy transfer between Sm³⁺ ions in Ca₉La(PO₄)₇:Sm³⁺ phosphors was determined as electric multipolar interaction.

Fig. 6 PL spectrum (λ_ex = 406 nm) of Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors (a), The relationship between log(I/x) versus log x in Sm³⁺ doped Ca₉La(PO₄)₇ phosphors (b).

According to Dexter theory, this kind of interaction between sensitizer and activator can be further analyzed based on the calculation from the following equation:^34,35

where θ stands for an indicator of the electric multipolar character, x for the activator content, K and β for the constants in the same excitation conditions for a given host lattice and I for the integral intensity of the emission spectra. Most importantly, θ values of 6, 8 and 10 referred to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively. Additionally, the relationship between log(I/x) and log x obtained in Sm³⁺ doped Ca₉La(PO₄)₇ phosphors at 406 nm exicitation was displayed in Fig. 6(b). The slope (equal to θ/3) was −1.422, −1.409, −1.410, and −1.410, indicated that the θ values were 4.266, 4.227, 4.230 and 4.230, respectively. These values close to 6 revealed that the principal concentration quenching mechanism behind the luminescence quenching of Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors was the dipole–dipole interaction.

To further investigate luminescence properties, Fig. 7 showed the fluorescence decay curves of Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors (x = 0.03, 0.12, and 0.25) under 406 nm excitation and monitored at 570, 607 and 653 nm. Each curve exhibited a single – exponential decay and could be calculated by the following equation:⁵

I(t) = I₀ + A₁ exp(−t/τ) (3)

where I(t) and I₀ represent the intensity at time t and 0, A₁ is a constant, t stands for time, τ is the decay time for an exponential component. The lifetimes of Sm³⁺ monitored at 607 nm resulting from eqn (3), were 1.52, 1.19 and 0.75 ms. That monitored at 653 nm were 1.50, 1.18 and 0.75 ms. Meanwhile, the lifetimes monitored at 570 nm were 1.49, 1.17 and 0.74 ms. All the results indicating a shorten lifetime with the increasing Sm³⁺ concentration, which further proved the energy transfer happened between Sm³⁺ ions in Ca₉La(PO₄)₇.

Fig. 7 Decay curves of Sm³⁺ in x = 0.03, 0.12, and 0.25 phosphors excited at 406 nm and monitored at 607 (a), 653 (b) and 570 (c) nm.

As one of the most important parameters for technological utilization of phosphors, thermal stability was also investigated.⁵ The PL spectra (λ_ex = 406 nm) of x = 0.12 phosphor within 25–300 °C were showed in Fig. 8(a) and the relationship between relative emission intensities and the temperature was showed in the inset. Each emission spectrum included the several characteristic peaks of Sm³⁺ within 500–800 nm. However, the emission intensities declined gradually along with the rising temperature, which was because of the thermal quenching effect.^2,5 As shown in the inset, the integrated emission intensities of the x = 0.12 phosphor at 150 °C was about 79% of the one at 25 °C, which indicated rather good stability of the phosphor.

Fig. 8 The PL spectra (λ_ex = 406 nm) of x = 0.12 phosphor under different temperatures ranging from 30 to 300 °C, and the temperature – intensities curve is shown in the inset (a); The Arrhenius fitting of the calculated activation energy for thermal quenching and the emission intensity of x = 0.12 (b).

To detailly investigate the thermal quenching effect and estimate the activation energy during the quenching process, the variation tendency of the luminescence intensity of x = 0.12 sample along with changing temperature could be depicted by the Arrhenius equation:³⁶

where I₀ represented for the initial intensity, I_T for the intensity at a given temperature T, c for a constant, ΔE for the activation energy for thermal quenching, and k for Boltzmann's constant (8.617 × 10⁻⁵ eV K⁻¹). In Fig. 8(b), ln[(I₀/I_T) − 1] versus 1/(kT) for the x = 0.12 sample was plotted, and according to the linear regression, the thermal activation energy ΔE for quenching process was determined as 0.167 eV.

The CIE (Commission Internationale de L′ Eclairage) color chromaticity coordinates (x, y) of the x = 0.12 phosphor acquired from the PL spectra under 406 nm excitation were labelled with a cross symbol in the CIE chromaticity diagram, as showed in Fig. 9. The resulting coordinates (0.5548, 0.4358) indicated the emission in the orange-red region. For exhibition purpose, the inset in Fig. 9 showed the digital image of the phosphor. Above results suggested that this orange-red phosphors could be promising candidate in w-LEDs excited by n-UV chips.

Fig. 9 CIE chromaticity coordinates (λ_ex = 406 nm) and the digital image of x = 0.12 phosphor.

4. Conclusions

In summary, a series of Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors were successfully fabricated via conventional solid-state reaction. The crystal structure, photoluminescence performance, decay lifetime and thermal quenching behaviors were studied in detail. The crystallographic analysis indicated that the doped Ca₉La(PO₄)₇:Sm³⁺ compounds crystallized in rhombohedral crystal structure (space group: R3c (161)). The PL spectra of Ca₉La_1−x(PO₄)₇:xSm³⁺ samples were made up of four emission bands around 570, 607, 653, and 714 nm, corresponding to the ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) transitions of Sm³⁺ ions, respectively. Among them, the ⁶H_7/2 was the most populated level. The optimal Sm³⁺ doping content turned out to be x = 0.12 mol, and the critical transfer distance R_C of Sm³⁺ in Ca₉La(PO₄)₇ was determined as 10.17 Å. Besides, On the basis of the Dexter theory, the electric dipole–dipole interaction was determined to be the principal concentration quenching mechanism behind the energy transfer between Sm³⁺ in Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors. From the life time decay curves that the energy transfer between Sm³⁺ ions in Ca₉La(PO₄)₇ was also observed. Moreover, Ca₉La(PO₄)₇:Sm³⁺ possessed fine thermal and color stability. All the results pointed out that the Ca₉La_1−x(PO₄)₇:xSm³⁺ phosphors have great potential as orange-red phosphors for developing w-LEDs excited by n-UV radiation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Qinghai Provincial Natural Science Foundation for Young Scholars (No. 2017-ZJ-929Q) and the Qinghai Provincial Innovation Platform Program (No. 2017-ZJ-Y17).

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