Synthesis, luminescence and electron–vibrational interactions of UV-excitable green phosphor α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺

Abstract

A series of UV-excitable α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺ green phosphors were synthesized by conventional solid-state reactions. The photoluminescence (PL) properties and concentration quenching of the as-prepared phosphors were investigated. All of the phosphors exhibited strong, broad absorption bands in the near-ultraviolet range and gave bright green emission upon excitation with 310 nm light. Eu²⁺ ions occupy 10-coordinated Ca(3) sites and 6-coordinated Ca(2) sites in α-Na₂Ca₄(PO₄)₂SiO₄ and therefore generate two emission sub-bands at 497 and 557 nm. The electron–vibrational interaction (EVI) parameters, such as the Huang–Rhys factor, effective phonon energy and zero phonon line position, were calculated. With increasing Eu²⁺ concentration, the emission peak wavelength red-shifted from 510 to 530 nm, and the color hue can be tuned from green to yellowish green. The concentration quenching mechanism in α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺ consists of electric multipole–multipole interactions; these are most likely to be dipole–dipole and dipole–quadrupole interactions. These results indicate that α-NCPS:Eu²⁺ phosphors are promising candidates for application in NUV LEDs.

---

1. Introduction

Phosphor-converted white light emitting diodes (pc-WLEDs) have received considerable attention owing to their potential applications in solid-state lighting and flat panel displays.¹ Currently, the commercial method to obtain white light in LEDs involves combining a blue chip with YAG:Ce³⁺ yellow phosphors. However, the deficiency of the “blue + yellow” method is the lack of a red component, which results in poor color rendering indices (CRI < 80) and a high correlated color temperature (CCT > 4500 K); this will restrict the application of these LEDs for indoor lighting in the future.² In recent years, to achieve high-quality white emission with high CRI and low CCT, alternative routes involving the employment of ultraviolet LEDs with blue, green and red phosphors or singly-phased white light-emitting phosphors have been advocated to generate WLEDs.³ As indispensable components of w-LED devices, phosphors with efficient NUV absorption and different emission colors, such as Ca₆Ba(PO₄)₄O:Ce³⁺, Tb³⁺,⁴ Sr₄Al₁₄O₂₅:Mn⁴⁺ (ref. 5) and (Y,Sc)(Nb,V)O₄:Bi³⁺,⁶ are being explored and developed.

Because the 4f–5d transition is an allowed electro–dipole transition, the absorption and emission of Eu²⁺ in many hosts are very efficient; therefore, Eu²⁺-doped phosphors are good candidates for potential LED applications.^7,8 Because Eu²⁺ contains 5d electrons that are unshielded from crystal fields in the excited state, the luminescence properties of Eu²⁺ are strongly affected by the crystal lattice environment of the host materials, such as the crystal symmetry, atom coordination, covalence, bond lengths, and crystal field strength. Thus, choosing a proper host is very important for obtaining Eu²⁺ phosphors with efficient absorption and emission.

Phosphate and silicate are appropriate hosts for luminescent materials due to their structural diversity, high thermal stability, visible light transparency, and relatively easy preparation;^9,10 therefore, it is proposed that a combination of these matrices, silicophosphate, would have similar advantages. Na₂Ca₄(PO₄)₂SiO₄ phase has two different crystal structures, called α and β, which are formed at high and low temperatures, respectively.¹¹ The luminescence properties of Ce³⁺/Tb³⁺-doped α-Na₂Ca₄(PO₄)₂SiO₄ ¹² and Eu²⁺, Dy³⁺, Ce³⁺/Tb³⁺-doped β-Na₂Ca₄(PO₄)₂SiO₄ ¹³ have been reported recently. However, Eu²⁺-activated α-Na₂Ca₄(PO₄)₂SiO₄ (α-NCPS) phosphors have not been reported. In this work, we successfully synthesized α-NCPS:Eu²⁺ green phosphors that are isostructural to α-CaNaPO₄ via high-temperature solid-state reactions. The crystallographic site occupancies of Eu²⁺ ions and the photoluminescence of the as-prepared phosphors have been investigated in detail.

2. Experimental

2.1 Sample preparation

The samples (Na₂Ca_4−x(PO₄)₂SiO₄:xEu²⁺) were synthesized by high-temperature solid-state reactions. The starting materials of Na₂CO₃ (A.R.), CaCO₃ (A.R.), (NH₄)₂HPO₄ (A.R.), SiO₂ (A.R.), and Eu₂O₃ (99.99%) were ground in a stoichiometric ratio, placed in crucibles and preheated at 900 °C for 3 h in a box furnace. The precursors were re-ground and then sintered at 1473 K for 6 h under a reductive atmosphere of 10% H₂ and 90% N₂ in a tube furnace. The as-synthesized samples were cooled to room temperature and then crushed into powders for measurement.

2.2 Material characterization

The phase purity of the prepared phosphors was examined using a Rigaku D-max 2200 X-ray diffractometer with Cu Kα radiation (λ = 1.5405 Å) at 30 kV and 30 mA. The photoluminescence (PL) spectra, PL excitation (PLE) spectra and temperature change measurements were acquired on an FLS 920 time-resolved steady state fluorescence spectrometer with a 450 W xenon light source. The fluorescence lifetime curves were measured using a 150 W nF900 light source on the same instrument.

3. Results and discussions

3.1 XRD and crystal structure

The phase purity of the as-prepared Eu²⁺-doped α-Na₂Ca₄(PO₄)₂SiO₄ phosphors with different doping concentrations was checked by XRD. As shown in Fig. 1, the XRD patterns of all the samples show similar profiles except for small differences in the intensities of some XRD peaks. All the diffraction peaks of these samples are in good agreement with JCPDS 33-1229 (α-Na₂Ca₄(PO₄)₂SiO₄), indicating that the obtained α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺ samples are single phase and that Eu²⁺ doping does not cause any detectable change in the host structure. As reported by I. Kapralik and F. Hanic,¹¹ α-Na₂Ca₄(PO₄)₂SiO₄ crystallizes in the trigonal space group P3̄m1(164) with cell parameters of a = b = 5.365 Å, c = 7.158 Å, and V = 178.4 ų. Unfortunately, the numbers in the chemical formula of the unit cell Z are unknown; therefore, the specific structure cannot be obtained.

Fig. 1 XRD patterns of representative α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺ samples and JCPDS cards no. 33-1229 and 03-0751.

It is found that α-CaNaPO₄ (JCPDS 03-0751) has very similar XRD patterns and crystal lattice parameters (trigonal, P3̄m1(164), a = b = 5.23 Å, c = 7.04 Å, and V = 166.8 ų) to α-Na₂Ca₄(PO₄)₂SiO₄. Thus, it is speculated that the α-Na₂Ca₄(PO₄)₂SiO₄ phase is a solid solution of α-CaNaPO₄ with a coupled substitution of Na and P by Ca and Si. Therefore, the structure of α-CaNaPO₄ (Fig. 2) can serve as an analog of that of α-Na₂Ca₄(PO₄)₂SiO₄, and the status of the cation sites can be expressed as follows: The Ca(1) site at (0, 0, 0) and the Ca(2) site at (0, 0, 1/2) have 6-fold coordination, with Ca–O distances of 2.2945 and 2.7862 Å, respectively. Meanwhile, the Ca(3) site lying at the trigonal axis is 10-fold coordinated, with average Ca–O distances of 3.534 Å. According to I. Kapralik, substitution of Na⁺ for Ca²⁺ and P⁵⁺ for Si⁴⁺ occurs on the trigonal axis, because the Ca(3) site is larger and more open to substitution.¹¹ The Eu²⁺ ions were incorporated into the hosts by replacing Ca²⁺ ions based on their similar ionic radii, i.e. Ca²⁺ (r = 1.00 Å, CN = 6; r = 1.28 Å, CN = 10) and Eu²⁺ (r = 1.17 Å, CN = 6; r = 1.35 Å, CN = 10).¹⁴

Fig. 2 The structure of α-CaNaPO₄ and the coordination of Ca²⁺ ions in the α-CaNaPO₄ lattice.

3.2 Photoluminescence properties

The PL and PLE spectra of the representative sample α-NCPS: 0.02Eu²⁺ are depicted in Fig. 3a. Monitored at 510 nm, the PLE spectrum consists of a broad band from 200 to 420 nm with peak at 310 nm, which originates from the Eu²⁺ 5d excited state; this indicates that it can be pumped well by UV light. The PL spectrum (λ_ex = 310 nm) exhibits an emission peak around 510 nm and a long tail ranging from 510 to 800 nm, ascribed to the Eu²⁺ 4f⁶5d¹–4f⁷ allowed transition. No sharp f–f transition lines from Eu³⁺ ions are observed in the PL spectrum, implying that the Eu³⁺ ions in the raw material are completely reduced to Eu²⁺ during the sintering process. The shape of the PL spectrum is broad and asymmetric, indicating the existence of multiple Eu²⁺ emission centers in α-NCPS.

Fig. 3 PLE and PL spectra of α-Na₂Ca₄(PO₄)₂SiO₄:0.02Eu²⁺ (a); Gaussian fitting of the PL spectra and corresponding PLE spectra (b).

The energy of the Eu²⁺ 4f⁷–4f⁶5d¹ transition is lower than the free ion value when Eu²⁺ is brought into a crystal environment.¹⁵ For α-NCPS:Eu²⁺, the energy of the zero-phonon line, E₀, which is at the intersection of the PLE and PL spectra, was found to be 2.89 eV (429 nm). The energy E_abs of the f–d transition from the ground state to the lowest level of the 4f⁶5d¹ excited states corresponds to the ⁷F₀ level. By using the mirror-image relationship between the PLE and PL spectra, it is found that E_abs = 3.56 eV (348 nm). The relationship of the 4f → 5d absorption and the 5d → 4f emission can be expressed as:^16,17

E_abs = E_free − D

E_emi = E_free − D − ΔS

where E_free is the energy difference between the ground state and the lowest level of the 4f⁶5d¹ excited states for the free ion; D is the red-shift, which depends on the crystal environment (centroid shift or nephelauxetic effect), concerning the shift of the average of five 5d levels relative to the free ion value; ΔS is the Stokes shift; and E_emi is the energy of the emission peak. In the present case, E_free is equal to 4.19 eV for Eu²⁺, and E_emi is 2.43 eV (510 nm). Thus, the Stokes shift is ΔS = E_abs − E_emi = 1.13 eV, and the red-shift is D = E_free − E_emi = 0.63 eV.

As mentioned in Section 3.1, there are three Ca sites in the α-NCPS host, i.e. the 6-coordinated Ca(1) and Ca(2) sites and the 10-coordinated Ca(3) site, with average Ca–O distances of 2.2945 Å, 2.7862 Å and 3.5340 Å, respectively. Due to its compact coordinated polyhedron, it may be very difficult for Eu²⁺ to occupy the compact Ca(1)–O₆ sites in α-Na₂Ca₄(PO₄)₂SiO₄. Similar phenomena have been reported in Li₄SrCa(SiO₄)₂:Eu^2+ 18 and β-Ca₃(PO₄)₂:Eu²⁺.¹⁹ It is possible for substitution of Eu²⁺ in the compact CaO₆ site to occur when the doping level is high (>10 mol%) and the occupation percentage is low. Thus, it is reasonable to suppose that Eu²⁺ occupies the Ca(2) and Ca(3) sites in α-Na₂Ca₄(PO₄)₂SiO₄.

As depicted in Fig. 3b, the asymmetric emission band can be decomposed into two well-separated Gaussian components with maxima at 498 and 557 nm. Also, by monitoring the PLE spectrum at 498 and 557 nm (Fig. 3b), it is found that the spectral profiles are somewhat different; this indicates that the two emission peaks should be ascribed to two different Eu²⁺ emission centers.

It is known that the energy of the Eu²⁺ excited 5d level is influenced by both crystal field splitting (ε_cfs) and centroid shift (ε_c). The influence of the polyhedron on the crystal field splitting, as concluded by P. Dorenbos, can be expressed as:¹⁶

ε_cfs = β_av^QR_av⁻²

where R_av is close to the average distance between the anions and the cations that are replaced by Eu²⁺. β_av^Q is a constant that depends on the type of the coordinating polyhedron; Q is 2+ for Eu²⁺. The ratio of the different coordinated polyhedrons is β_octa(6)²⁺ : β_cubal(8)²⁺ : β_cubo(12)²⁺ = 1 : 0.89 : 0.42 (the numbers in parentheses are the coordination numbers). The R_av values for Ca(2)–O and Ca(3)–O in α-Na₂Ca₄(PO₄)₂SiO₄ are 2.7826 and 3.534 Å, respectively. Therefore, the crystal field splitting for the 5d levels of Eu²⁺ on the Ca(3) site is smaller than that on the Ca(2) site.

Additionally, the 5d centroid shift for Eu²⁺ can be roughly estimated as:¹⁸

where R_i is the distance (pm) between Eu²⁺ and anion i in the undistorted lattice. The summation is over all N anions that coordinate Eu²⁺. 0.6ΔR is a correction for lattice relaxation around Eu²⁺, and ΔR is the difference between the radii of Eu²⁺ and the cation sites that the Eu²⁺ ions occupy. α_spⁱ (10⁻³⁰ m⁻³) is the spectroscopic polarizability of anion i. A is a constant. By introducing the values of N, R_i and ΔR, it can be found that the 5d centroid shift for Eu²⁺ on the Ca(3) site is smaller than that on the Ca(2) site.

By assessing the above results, it can be concluded that the emission peak at 498 nm belongs to the Eu²⁺ emission in the 10-coordinated Ca(3) site, and the emission peak at 557 nm belongs to the Eu²⁺ emission in the 6-coordinated Ca(2) site. Additionally, the emission intensity of the 498 nm band is higher than that of the 557 nm band, which indicates that Eu²⁺ occupies the loose Ca(3) sites with long Ca–O distances more easily than the relatively compact Ca(2) sites.

3.3 Electron–vibrational interaction parameters of α-NCPS:xEu²⁺

Fig. 4a shows the dependence of the PL spectra of α-NCPS:xEu²⁺ on the Eu²⁺ concentration. The PL intensity of α-NCPS:xEu²⁺ reaches its maximum at x = 0.02. Above that value, concentration quenching occurs. This concentration quenching is mainly caused by energy transfer between Eu²⁺ ions. When the Eu²⁺ content increases, the probability of energy transfer between Eu²⁺ ions is enhanced because the distance between them is decreased. Because the 4f → 5d transition of Eu²⁺ ions is allowed, non-radiative energy transfer between Eu²⁺ ions occurs as result of an electric multipolar interaction. The overlap of the PL and PLE spectra is negligible, which indicates that both radiation re-absorption and energy migration play small roles in the concentration quenching process.

Fig. 4 PL (λ_ex = 310 nm, a) and PLE spectra (λ_em = 510 nm, b) of α-NCPS:xEu²⁺ (x = 0.01 to 0.16).

A shift in the emission band toward longer wavelengths is observed upon increasing the Eu²⁺ concentration during excitation at 310 nm. There is a continuous red-shift in the emission wavelength from 510 to 530 nm as the Eu²⁺ content increases from x = 0.01 to x = 0.16. Several possible reasons for this red-shift phenomenon in Eu²⁺-doped phosphors have been reported, including crystal field strength, Stokes shift and re-absorption. In the present case, the substitution of larger Eu²⁺ for smaller Ca²⁺ creates a shorter distance between Eu²⁺ and O²⁻ ions, which enhances the crystal field strength and leads to red-shift.²⁰ The PLE spectra of α-NCPS:xEu²⁺ (Fig. 4b) exhibit similar red-shift behavior with increasing Eu²⁺ content. This behavior is in agreement with the effect of crystal field strength, which causes obvious changes in the position and shape of the PLE spectra. In addition, because multi-luminescent centers exist in α-NCPS:xEu²⁺, the changes in site occupancy may also cause a red-shift of the Eu²⁺ emission; i.e. the percentage of Eu²⁺ occupying Ca(2) sites may increase with increasing Eu²⁺ content, therefore increasing the intensity of emissions with longer wavelength and leading to red-shift.²¹

Because the 4f–5d transitions involve the outer 5d shells of Eu²⁺ ions, which interact strongly with the vibrating environment in the crystal lattice of the host, a detailed analysis of the electron–vibrational interaction (EVI) between the 5d states and crystal lattice vibrations and the influence of this interaction on the overall appearance of the optical spectra is an interesting undertaking that is of great importance.

The chief parameters of electron–vibrational interaction are the Stokes shift (ΔE_s), Huang–Rhys factor (S), effective phonon energy (hω) and zero phonon line position (ZPL). The Stokes shift (ΔE_s) represents the energy difference between the lowest absorption and emission peaks corresponding to the same electronic states, and it can be expressed as:²²

ΔE_s = (2S − 1)hω

The constant S is the Huang–Rhys coupling constant, which indicates the average number of phonons of energy ℏω. This constant measures the strength of electron lattice coupling, i.e. S < 1 indicates a weak coupling regime, 1 < S < 5 indicates an intermediate coupling regime and S > 5 indicates a strong coupling regime.²³ hω is the effective phonon energy. The relationship between S and hω is:²²

where Γ(T) is the full width at half maximum (FWHM) of the emission curve at an absolute temperature T. The easiest way to find the values of S and hω is to solve these equations graphically with the values of Γ(T) and ΔE_s extracted from the experimental emission and excitation spectra. Additionally, the point of intersection of the emission and excitation curves indicates the zero phonon line position.

Table 1 contains a summary of the experimental data required to estimate the EVI parameters, as well as the obtained values of the Stokes shift, Huang–Rhys factor, effective phonon energy, and experimental ZPL position. It is observed that the analyzed systems show rather large Stokes shifts, intermediate Huang–Rhys factors, and high effective phonon energies. It is also found that for the highly doped samples, the effective phonon energies decrease, which is due to decreases in the FWHM and Stokes shift. We attribute these variations to considerable changes in the phonon spectra of the hosts at high concentrations of Eu²⁺ and the appearance of some local modes. One possible reason for the non-monotonic variation of S with concentration may be the possible redistribution of Eu²⁺ ions in different Ca sites with increased concentration.¹⁹

Table 1 The main spectroscopic and EVI parameters of the α-NCPS:xEu²⁺ (x = 0.01 to 0.16) samples

Composition	1^st absorption max (cm^−1)	Emission max (cm^−1)	ΔEs (cm^−1)	FWHM (cm^−1)	S	hω (cm^−1)	Exp. ZPL (cm^−1)
x = 0.01	28 735	19 763	8972	5522	4.65	1081	23 866
x = 0.02	28 490	19 569	8921	5448	4.72	1058	23 310
x = 0.04	28 248	19 305	8943	5513	4.64	1080	23 256
x = 0.06	27 700	19 193	8507	5278	4.60	1038	22 883
x = 0.08	27 472	19 083	8359	5186	4.60	1019	22 883
x = 0.10	27 100	19 047	8053	5172	4.35	1046	22 472
x = 0.12	27 027	18 903	8124	5120	4.49	1018	22 223
x = 0.14	27 027	18 867	8160	5069	4.60	995	22 271
x = 0.16	27 027	18 832	8195	4988	4.77	960	22 223

---

3.4 Decay times and concentration quenching mechanism

To further validate the process of energy transfer, the fluorescence lifetimes, τ, of the α-NCPS:xEu²⁺ samples were measured. As a representative, the results for α-NCPS: 0.01Eu²⁺ are shown in Fig. 5a (λ_ex = 310 nm, λ_em = 510 nm). The decay curve can be well fitted to a second-order exponential decay model because of the two occupation sites (Ca(2) and Ca(3)):

I = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂)

where I is the luminescence intensity; A₁ and A₂ are constants; t is the time; and τ₁ and τ₂ are the lifetimes of the rapid and slow decays, respectively. Furthermore, the average lifetime (τ*) can be calculated as:²⁴
where I(t) is the fluorescence intensity at time t. The average fluorescence lifetimes of Eu²⁺ were calculated and are shown in Fig. 5b. In the α-NCPS:xEu²⁺ samples, as the value of x increases from 0.01 to 0.16, the fluorescence dynamics of the Eu²⁺ ions deviate from the second-order exponential function, and the corresponding average lifetime is found to decrease monotonically from 1.054 to 0.665 μs. This result strongly demonstrates the energy transfer between Eu²⁺ ions, which becomes more evident when x > 0.02. The measured lifetime is also related to the total relaxation rate by:
where t₀ is the radiative lifetime; A_nr presents the non-radiative rate ascribed to multi-phonon relaxation; and P_t is the energy transfer rate between Eu²⁺ ions. The distance between the Eu²⁺ ions decreases with increasing Eu²⁺ concentration, resulting in enhanced energy transfer rates between Eu²⁺ and the probability of energy transfer to luminescent ‘killer’ sites. Consequently, the lifetimes show a gradually decreasing trend with increasing Eu²⁺ concentration (Fig. 5b).

Fig. 5 Decay curve of the α-NCPS:0.01Eu²⁺ sample from a pulse excitation of 310 nm (a); decay curves of Eu²⁺ ions in samples of α-NCPS:xEu²⁺ (x = 0.01 to 0.16; λ_ex = 310 nm) at room temperature (b).

According to Dexter's energy transfer expression of multipolar interaction and Reisfeld's approximation,²⁵ the energy transfer mechanism between Eu²⁺ ions in this host should occur via electric multipole–multipole interactions. The following relation can be adopted:

where τ and τ₀ represent the Eu²⁺ decay lifetimes of the α-NCPS:xEu²⁺ samples at the critical concentration (x = 0.02) and above the critical concentration (x > 0.02), respectively. c is the doping concentration of the Eu²⁺ ions. In the above expression, α = 6, 8, and 10 correspond to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The least-squares fittings of τ₀/τ versus c^α/3 (α = 6, 8, 10 respectively) are exhibited in Fig. 6. It is found that the goodness of fit (R²), the corresponding variance (Chi²/DoF) and the standard error (SE) are R² = 0.9737, Chi²/DoF = 0.00098, SE = 0.01456 for dipole–dipole interaction, R² = 0.9629, Chi²/DoF = 0.00136, SE = 0.01599 for dipole–quadrupole interaction, and R² = 0.9380, Chi²/DoF = 0.00248, SE = 0.1968 for quadrupole–quadrupole interaction, respectively. By considering the goodness and variance of fit and the results of the error analysis, we believe that dipole–dipole and dipole–quadrupole interactions are more likely to be the concentration quenching mechanism of α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺.

Fig. 6 Dependence of τ₀/τ of Eu²⁺ on c^6/3 (a), c^8/3 (b), and c^10/3 (c) of α-NCPS:xEu²⁺.

All the samples show green color under excitation with near-UV light, as shown in the photos in the inset of Fig. 7; this can also be confirmed by the CIE (Commission International de l'Ecairage, 1931) coordinates calculated from the emission spectra. The CIE chromaticity coordinates of α-NCPS:xEu²⁺ (x = 0.01 to 0.16) are shown in Fig. 7; they are in the green region. With increasing Eu²⁺ doping, the color of α-NCPS:xEu²⁺ undergoes a shift from the green region (0.290, 0.401) to the yellowish green region (0.349, 0.483).

Fig. 7 CIE chromaticity coordinates of α-NCPS:xEu²⁺ (x = 0.01 to 0.16) phosphors. Inset shows images of selected phosphors under a 365 nm lamp.

4. Conclusion

An Eu²⁺-activated green-emitting α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺ phosphor was prepared by a conventional solid-state reaction. The luminescence spectra show that this phosphor exhibits strong absorption in the range of 240–450 nm, along with a green emission broadband with peaks varying from 510 to 530 nm, depending on the concentration of Eu²⁺. Dorenbos theory analysis suggests that the 497 and 557 emission bands belong to Eu²⁺ ions in the 10-coordinated Ca(3) site and the 6-coordinated Ca(2) site, respectively. The electron–vibrational interaction (EVI) parameters, such as Huang–Rhys factor, effective phonon energy and zero phonon line position, were estimated. It was found that dipole–dipole and dipole–quadrupole interactions are the likely concentration quenching mechanisms of α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺. These results indicate that α-Na₂Ca₄(PO₄)₂SiO₄:Eu²⁺ is a promising UV-converted green-emitting phosphor for use in UV-pumped pc-WLEDs.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 21601081). The authors thank Prof. Jilin Zhang (College of Chemistry and Chemical Engineering, Hunan Normal University) for helpful discussion.

References

1. Z. Xia, Z. Xu, M. Chen and Q. Liu, Dalton Trans., 2016, 11214–11232.
2. K. Li, M. Shang, H. Lian and J. Lin, J. Mater. Chem. C, 2016, 4, 5507–5530.
3. Z. Xia and Q. Liu, Prog. Mater. Sci., 2016, 84, 59–117.
4. M. Chen, Z. Xia and Q. Liu, J. Mater. Chem. C, 2015, 3, 4197–4204.
5. M. Peng, X. Yin, P. A. Tanner, M. G. Brik and P. Li, Chem. Mater., 2015, 27, 2938–2945.
6. F. Kang, H. Zhang, L. Wondraczek, X. Yang, Y. Zhang, D. Lei and M. Pang, Chem. Mater., 2016, 28, 2692–2703.
7. K. Lee and W. Im, J. Am. Ceram. Soc., 2013, 96, 503–508.
8. Z. Xia, Y. Zhang, M. Molokeev and V. Atuchin, J. Phys. Chem. C, 2013, 117, 20847–20854.
9. H. Yu, D. Deng, D. Zhou, W. Yuan, Q. Zhao, Y. Hua, S. Zhao, L. Huang and S. Xu, J. Mater. Chem. C, 2013, 1, 5577–5582.
10. Z. Xia, J. Zhou and Z. Mao, J. Mater. Chem. C, 2013, 1, 5917–5924.
11. I. Kapralik and F. Hanic, Trans. J. Br. Ceram. Soc., 1977, 76, 126–133.
12. D. Wen, G. Yang, H. Yang, J. Shi, M. Gong and M. Wu, Mater. Lett., 2014, 125, 63–66.
13. K. Li, M. Shang, D. Geng, H. Lian, Y. Zhang, J. Fan and J. Lin, Inorg. Chem., 2014, 53, 6743–6751.
14. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751–767.
15. X. Zhang, F. Mo, L. Zhou and M. Gong, J. Alloys Compd., 2013, 575, 314–318.
16. P. Dorenbos, J. Lumin., 2003, 104, 239–260.
17. M. Nazarov, B. Tsukerblat and D. Noh, Indian J. Eng. Mater. Sci., 2009, 16, 147–150.
18. J. Zhang, W. Zhang, Y. He, W. Zhou, L. Yu, S. Lian, Z. Li and M. Gong, Ceram. Int., 2014, 40, 9831–9834.
19. J. Han, W. Zhou, Z. Qiu, L. Yu, J. Zhang, Q. Xie, J. Wang and S. Lian, J. Phys. Chem. C, 2015, 119, 16853–16859.
20. D. Wen, Z. Dong, J. Shi, M. Gong and M. Wu, ECS J. Solid State Sci. Technol., 2013, 2, R178–R185.
21. Y. Sato, H. Kuwahara, H. Kato, M. Kobayashi, T. Masaki and M. Kakihana, Opt. Photonics J., 2015, 5, 326–333.
22. P. Bhoyar and S. J. Dhoble, J. Lumin., 2013, 139, 22–27.
23. G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer-Verlag, New York, 1994.
24. N. Guo, Y. Zheng, Y. Jia, H. Qiao and H. You, New J. Chem., 2012, 36, 168–172.
25. D. L. Dexter, J. Chem. Phys., 1953, 21, 836–850.

---