Incorporating Ce³⁺ into a high efficiency phosphor Ca₂PO₄Cl:Eu²⁺ and its luminescent properties

Abstract

A series of Ce³⁺, Eu²⁺ and Ce³⁺/Eu²⁺ doped Ca₂PO₄Cl phosphors are synthesized by a high temperature solid-state method. Not only is the emission intensity of Ca₂PO₄Cl:Eu²⁺ obviously enhanced by codoping Ce³⁺, but also the spectral profile of the excitation band is almost not influenced. The energy transfer from Ce³⁺ to Eu²⁺ in Ca₂PO₄Cl has been validated and proved to be a resonant type via a dipole–dipole interaction. Under the 400 nm radiation excitation, the luminescent intensity of Ca₂PO₄Cl:Ce³⁺, Eu²⁺ is found to be about 200% that of Ca₂PO₄Cl:Eu²⁺, and 300% that of BaMgAl₁₀O₁₇:Eu²⁺. The thermal quenching properties reveal that Ca₂PO₄Cl:Ce³⁺, Eu²⁺ has excellent characteristics. Therefore, Ca₂PO₄Cl:Ce³⁺, Eu²⁺ may have potential application as a blue-emitting phosphor for white LEDs.

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1 Introduction

In recent years, there has been growing importance focused on research on light emitting diodes (LEDs) because of their long operation lifetime, energy-saving features and high material stability.¹ Therefore, white LEDs are promising candidates to replace conventional incandescent and fluorescent lamps. At present, the most wide used method to obtain white emission is the combination of a blue LED chip and a yellow phosphor YAG:Ce. However, the white LEDs exhibit a high correlated color temperature (CCT ≈ 7750 K) and a poor color rendering index (CRI ≈ 70–80), because they are lack of red component.² During the past few years, white LEDs fabricated using a near ultraviolet (n-UV) LED (380–420 nm) coupled with red, green and blue phosphors have attracted much attention.³ The most frequently used blue phosphor for n-UV LED is BaMgAl₁₀O₁₇:Eu²⁺ (BAM).⁴ However, the absorption of BAM for wavelength above 400 nm is quite poor. Accordingly, it is urgent to develop new blue phosphors that could be effectively excited in the n-UV range especially for wavelength of 400 nm.^5–7 For example, Song et al. investigated the luminescent property of RbBaPO₄:Eu²⁺;⁸ Liu et al. synthesized a high efficiency and high color purity blue-emitting phosphor NaSrBO₃:Ce³⁺.⁹ Moreover, in order to effectively improve the luminescence of phosphor, the interest in this field arises because the energy transfer from a donor to an acceptor. Actually, the energy transfer from the donor (sensitizer) Ce³⁺ to the acceptor (activator) Eu²⁺ has been already reported. For example, the energy transfer from Ce³⁺ to Eu²⁺ in LiSr₄(BO₃)₃ is studied, and the emission color of phosphor can be tuned by appropriate adjustment of the relative proportion of Ce³⁺/Eu²⁺, and the energy transfer mechanism of Ce³⁺ → Eu²⁺ in LiSr₄(BO₃)₃ is the dipole–dipole interaction.¹⁰ Actually, for many compounds, such as SrSi₂O₂N₂, CaSi₂O₂N₂, Ba_1.3Ca_0.7SiO₄, Sr₃Gd(PO₄)₃, Ca₉Y(PO₄)₇, NaMg₄(PO₄)₃, NaMgPO₄, Ca₈Gd₂(PO₄)₆O₂ and BaMg₂(PO₄)₂, the similar types of Ce³⁺ → Eu²⁺ energy transfer are also observed.^11–19 Obviously, the Eu²⁺ emission can be efficiently enhanced by Ce³⁺ → Eu²⁺ energy transfer. Hence, the exploration of material can effectively support our demand and guarantee the continuation of progress. Ca₂PO₄Cl:Eu²⁺ which has the excellent internal and external quantum efficiencies, the intense emission, the high quantum efficiency and excellent thermal stability,²⁰ and may serve as a potential candidate for n-UV LED. Therefore, in the present work, we tentatively introduced Ce³⁺ into Ca₂PO₄Cl:Eu²⁺ to enhance its emission intensity. The energy transfer from Ce³⁺ to Eu²⁺ is systematically studied using the photoluminescence excitation and emission spectra, and lifetimes, and the thermal quenching property of Ca₂PO₄Cl:Ce³⁺, Eu²⁺ is also investigated.

2 Experimental

2.1 Sample preparation

A series of Ca_2−x−yPO₄Cl:xCe³⁺, yEu²⁺ (x, y molar concentration) samples are synthesized by a high temperature solid-state method. The initial materials, including CaCO₃ (A.R.), CaCl₂·6H₂O (A.R.), NH₄H₂PO₄ (A.R.), CeO₂ (99.99%) and Eu₂O₃ (99.99%), are weighed in stoichiometric proportion, thoroughly mixed and ground by an agate mortar and pestle for more than 30 min till they are uniformly distributed. The obtained mixtures are heated at 1000 °C for 2 h in crucibles along with a reducing atmosphere (5% H₂/95% N₂), and then are naturally cooled to room temperature. In order to measure the characteristics of the phosphor, the samples are ground into powder.

2.2 Materials characterization

The phase formation is determined by X-ray diffraction (XRD) in a Bruker AXS D8 advanced automatic diffractometer (Bruker Co., German) with Ni-filtered Cu Kα1 radiation (λ = 0.15405 nm), and a scan rate of 0.02° s⁻¹ is applied to record the patterns in the 2θ range from 10° to 70°. The steady time resolved photoluminescence spectra are detected by a FLS920 fluorescence spectrometer, and the exciting source is a 450 W Xe lamp. The curve fittings are performed on the luminescence decay curves to confirm the decay time. The Commission International de I'Eclairage (CIE) chromaticity coordinates of sample are measured by a PMS-80 spectra analysis system. All measurements are carried out at room temperature.

3 Results and discussion

3.1 Phase formation

The XRD patterns of Ca₂PO₄Cl:Ce³⁺, Ca₂PO₄Cl:Eu²⁺ and Ca₂PO₄Cl:Ce³⁺, Eu²⁺ are measured and a similar diffraction patterns are observed for each sample. As a representative, Fig. 1 shows the XRD patterns of Ca₂PO₄Cl:0.03Ce³⁺, Ca₂PO₄Cl:0.07Eu²⁺, Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺. Compared the diffraction data with the standard JCPDS card (no. 19-0247), and there has no difference between the doped impurity Ca₂PO₄Cl and the pure Ca₂PO₄Cl. The uniform diffraction patterns indicate that the phase formation of Ca₂PO₄Cl is not influenced by a little amounts of Ce³⁺, Eu²⁺, or Ce³⁺/Eu²⁺. Ca₂PO₄Cl which crystallizes in the orthorhombic system with space group of Pbcm(57) and with four formula units per unit cell (N = 4), and the dimensions of unit cell are a = 0.6185 nm, b = 0.6983 nm and c = 1.082 nm.^20–25

Fig. 1 XRD patterns of Ca₂PO₄Cl:0.03Ce³⁺, Ca₂PO₄Cl:0.07Eu²⁺ and Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺. The standard data of Ca₂PO₄Cl (JCPDS no. 19-0247) is shown as reference.

3.2 Luminescent properties of Ce³⁺, Eu²⁺ and Ce³⁺/Eu²⁺ in Ca₂PO₄Cl

Fig. 2a presents Ca₂PO₄Cl:0.07Eu²⁺ has a broad emission band under the 370 nm radiation excitation, and the peak locates at 450 nm which is typically attributed to the 4f⁶5d¹ → 4f⁷ electronic dipole allowed transition of Eu²⁺ ion. Because two different crystallographic sites are available for the divalent Ca²⁺ ions, one with site symmetry C₂ and the other with site symmetry C_S. Therefore, Eu²⁺ ions may occupy two different Ca²⁺ sites.²⁰ The excitation band is observed to mainly consist of unresolved band due to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ion. When the Eu²⁺ ions occupy the lattice sites with C₂ or C_S symmetry in Ca₂PO₄Cl, the fivefold degeneracy of the 5d levels is expected in the excitation spectrum. Nevertheless, the dominating bands in the excitation spectrum is difficult to resolve because of serious overlap between 5d levels. The broad excitation band is ascribed to the high covalency of Ca_Eu–Cl bonding and large crystal-field splitting. The inset of Fig. 2a shows the emission intensity of Ca₂PO₄Cl:Eu²⁺ as function of Eu²⁺ doping content, and the optimal doping content is 0.07 mol.

Fig. 2 Emission and excitation spectra of (a) Ca₂PO₄Cl:0.07Eu²⁺ and (b) Ca₂PO₄Cl:0.03Ce³⁺. The inset: (a) the emission intensity of Ca₂PO₄Cl:0.07Eu²⁺ as function of Eu²⁺ doping content (λ_ex = 370 nm); (b) the emission spectrum of Ca₂PO₄Cl:xCe³⁺ (λ_ex = 334 nm).

Fig. 2b depicts Ca₂PO₄Cl:0.03Ce³⁺ exhibits a strong broad emission band between 320–500 nm under the 334 nm radiation excitation, and the emission peak locates at 367 nm due to the 4f⁰5d¹ → 4f¹ transition of Ce³⁺ ion. The excitation spectrum shows broad band from 200 to 350 nm, and the peaks locate at 268 nm and 334 nm, respectively, which correspond to the 4f¹ → 4f⁰5d¹ transition of Ce³⁺ ion.^20–22 The inset of Fig. 2b presents the emission spectra of Ca₂PO₄Cl:Ce³⁺ with the different Ce³⁺ doping content. The emission spectra have no obvious change in the wavelength and band shape with increasing the Ce³⁺ doping content, which indicated that the crystal-field strength experienced by the activator does not change. In other words, doping into the lattice site does not cause the expansion or shrinkage of the unit cell, as revealed by the XRD data. However, the emission intensity of Ca₂PO₄Cl:Ce³⁺ is influenced by the Ce³⁺ doping content, and the optimal doping content is 0.03 mol Ce³⁺, and the concentration quenching effect is also observed.

Generally, the trivalent rare earths ions, such as Ce³⁺, substitute for the bivalent alkaline earth, such as Ca²⁺, in the compound, there has the charge unbalance which may result in the formation of defects in the phosphor. Hence the incorporation of alkali metal ions might offset the charge unbalance generated by Ce³⁺ ion substitution for Ca²⁺ ion, reduced the lattice distort, and affected the luminescent properties.^26–28 Among the radii of Li⁺, Na⁺ and K⁺, there has the slightest difference between Na⁺ and Ca²⁺ ions, Therefore, Na⁺ ion may lead to the slightest negative influence on the luminescence of Ce³⁺ in Ca₂PO₄Cl. Under the 334 nm radiation excitation, the emission spectra of Ca₂PO₄Cl:0.03Ce³⁺, and Ca₂PO₄Cl:0.03Ce³⁺, 0.03Na⁺ are shown in Fig. S1 (ESI).† The results present that the two phosphors have the same spectral profile, however, the emission intensity of Ca₂PO₄Cl:0.03Ce³⁺, 0.03Na⁺ is appreciably stronger than that of Ca₂PO₄Cl:0.03Ce³⁺. The results indicate that incorporating Na⁺ into Ca₂PO₄Cl:Ce³⁺ has no negative influence on the luminescence of Ca₂PO₄Cl:Ce³⁺, to some extent, can enhance its emission intensity. In the present work, the research focuses on the luminescence and energy transfer of Ce³⁺ → Eu²⁺ in Ca₂PO₄Cl. Therefore, the problem of charge compensation is no longer discussed.

As shown in Fig. 2, there has an obvious spectral-overlap between the emission spectrum of Ce³⁺ and the excitation spectrum of Eu²⁺, indicating the possibility of energy transfer from Ce³⁺ to Eu²⁺ in Ca₂PO₄Cl. Under the Ce³⁺ 334 nm excitation peak, the spectra of Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ (y = 0–0.1) are measured and shown in Fig. 3. With the lower Eu²⁺ doping content, there has both Ce³⁺ and Eu²⁺ emission bands, and the emission intensities of Ce³⁺ decrease with increase the Eu²⁺ doping content. With the higher Eu²⁺ doping content, such as 0.05 mol Eu²⁺, the Ce³⁺ emission disappears, only the Eu²⁺ emission exists in the spectra of Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺. The results mean that the energy transfer from Ce³⁺ to Eu²⁺ in Ca₂PO₄Cl is validated. Fig. S2 (ESI)† depicts the emission spectra of Ca₂PO₄Cl:0.07Eu²⁺, Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺ and BaMgAl₁₀O₁₇:Eu²⁺ under the 400 nm radiation excitation. The results obviously present that the luminescent intensity of Ca₂PO₄Cl:Ce³⁺, Eu²⁺ is found to be about 200% as high as that of Ca₂PO₄Cl:Eu²⁺, and 300% as high as that of BaMgAl₁₀O₁₇:Eu²⁺.

Fig. 3 Emission spectra of Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ (λ_ex = 334 nm). The inset: variation of η_T for Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ (y = 0–0.1) with the Eu²⁺ doping content.

Generally, the energy transfer efficiency (η_T) from a sensitizer to an activator can be expressed according to Paulose et al.²⁹

η_T = 1 − (I_S/I_S0) (1)

where I_S0 and I_S are the luminescent intensities of the sensitizer in the absence and presence of the activator. In fact, for Ce³⁺/Mn²⁺, Eu²⁺/Mn²⁺, Ce³⁺/Eu²⁺ and Ce³⁺/Tb³⁺ codoped phosphors, the energy transfer efficiency can be easily achieved using eqn (1),^30–33 generally, the energy transfer efficiency (η_T) from the sensitizer to the activator gradually increases with increase the activator doping content. In the present work, In order to well understand the energy transfer process, using eqn (1), the energy transfer efficiencies (η_T) Ce³⁺ → Eu²⁺ of Ca₂PO₄Cl:Ce³⁺, yEu²⁺ (y = 0–0.1) are calculated, and shown in the inset of Fig. 3. As a result, the η_T is found to gradually increase with enhancing the Eu²⁺ doping content. The results mean that the efficient energy transfer Ce³⁺ → Eu²⁺ exist in Ca₂PO₄Cl.

For the 450 nm emission of Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺, the corresponding excitation spectra are shown in Fig. 4. It can be seen from Fig. 2, Ca₂PO₄Cl:Eu²⁺ has an obvious absorption in the range of 300–355 nm, and Ca₂PO₄Cl:Ce³⁺ presents a strong excitation band from 300 to 340 nm. However, Ca₂PO₄Cl:Ce³⁺ has weaker absorption in the range of 345–355 nm. Therefore, though the intensity of excitation band (from 330 to 355 nm) increases with enhancing the amount of Eu²⁺ doping content. In fact, the corresponding excitation band which is common to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ion and the 4f¹ → 4f⁰5d¹ transition of Ce³⁺ ion. Hence the increase of intensity (330–355 nm) comes from the emission of Eu²⁺, not Ce³⁺. In other words, the results mean that the excitation characteristics of Ce³⁺ decreases, and that of Eu²⁺ increases. Especially, there has only the excitation characteristics of Eu²⁺ with further increase the Eu²⁺ doping content, viz., the efficient energy transfer from Ce³⁺ to Eu²⁺ ought to exist in Ca₂PO₄Cl.

Fig. 4 Excitation spectra of Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ (λ_em = 450 nm).

To further validate the process of energy transfer, the fluorescence lifetimes, τ, for Ce³⁺ with the different Eu²⁺ doping content are measured, as a representative, the results of Ca₂PO₄Cl:0.03Ce³⁺, Ca₂PO₄Cl:0.03Ce³⁺, 0.01Eu²⁺, Ca₂PO₄Cl:0.03Ce³⁺, 0.05Eu²⁺ and Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺ are shown in Fig. 5 (λ_ex = 334 nm, λ_em = 367 nm). The decay curves are well fitted with a second-order exponential decay mode by eqn (2)^34–36

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I is the luminescence intensity; A₁ and A₂ are constants; t is the time, and τ₁ and τ₂ are the lifetimes for the rapid and slow decays, respectively. The average lifetimes (τ*) can be calculated by the formula as follow^34–36

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (3)

Fig. 5 Decay curves of Ce³⁺ emission monitored at 367 nm for Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ (λ_ex = 334 nm).

For Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ (y = 0–0.1), the calculated average lifetimes (τ*) are 52.96, 43.69, 35.51, 29.68, 23.89, 17.98, 14.91, 10.02 and 6.87 ns, respectively. The energy transfer efficiency (η_T) can be calculated using the following equation^{29, 37–39}

η_T = 1 − (I_S/I_S0) ≈ 1 − (τ_S/τ_S0) (4)

where τ_S0 and τ_S are the decay lifetimes of the sensitizer Ce³⁺ in the absence and presence of the activator Eu²⁺. With increase the Eu²⁺ doping content, the η_T value is calculated and shown in Fig. 6. And the energy transfer efficiency from Ce³⁺ to Eu²⁺ gradually increases with enhancing the Eu²⁺ doping content. At 0.1 mol Eu²⁺, the energy transfer efficiency is 87.03%.

Fig. 6 Variation of η_T for Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ (y = 0–0.1) with the Eu²⁺ doping content.

On the basis of Dexter's energy transfer formula for exchange and multipolar interactions, the following relation can be obtained^40–44

ln(η₀/η) ∝ C (5)

(η₀/η) ∝ C^α/3 (6)

where η₀ and η are the luminescence quantum efficiency of Ce³⁺ in the absence and presence of Eu²⁺, respectively; C is the total doping content of the Ce³⁺ and Eu²⁺. Eqn (5) corresponds to the exchange interaction, and eqn (6) with α = 6, 8, 10 corresponds to the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Actually, for many codoped phosphors, the energy transfer mechanisms for exchange and multipolar interactions have already been discussed using eqn (5 and 6). For example, Guo et al. investigated the energy transfer mechanism of Eu²⁺/Mn²⁺ codoped Ca₉Lu(PO₄)₇,⁴¹ Shang et al. reported the luminescence and energy transfer of Ce³⁺/Eu²⁺ in Ca₈La₂(PO₄)₆O₂.³² Therefore, in the present work, the energy transfer mechanism of Ce³⁺/Eu²⁺ in Ca₂PO₄Cl can also be explored by eqn (5 and 6). Because the value of η₀/η can be approximately estimated from the correlated lifetime ratio (τ_S0/τ_S), hence eqn (5 and 6) can be changed as follows

ln(τ_S0/τ_S) ∝ C (7)

(τ_S0/τ_S) ∝ C^α/3 (8)

The relationships of ln(τ_S0/τ_S) ∝ C and (τ_S0/τ_S) ∝ C^α/3 are illustrated in Fig. 7. By consulting the fitting factor R, the relation (τ_S0/τ_S) ∝ C^6/3 has the best fitting, implying that the dipole–dipole interaction is applied for the energy transfer from Ce³⁺ to Eu²⁺.

Fig. 7 Dependence of ln(τ_S0/τ_S) of Ce³⁺ on (a) C and τ_S0/τ_S of Ce³⁺ (b) C^6/3, (c) C^8/3, and (d) C^10/3.

In general, the critical distance (R_c) can be calculated by the concentration quenching method. The critical distance R_Ce–Eu between Ce³⁺ and Eu²⁺ can be estimated by⁴⁵

R_Ce–Eu = 2[3V/(4πx_cN)]^1/3 (9)

where x is the sum concentration of Ce³⁺ and Eu²⁺, N is the number of Z ions in the unit cell (for Ca₂PO₄Cl, N = 4), and V is the volume of the unit cell (for Ca₂PO₄Cl, V = 0.46731 nm³).

The estimated distance (R_Ce–Eu) for Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺ phosphors (x_c = 0.03, 0.035, 0.04, 0.05, 0.06, 0.07, 0.08, 0.1 and 0.13) are 1.95, 1.85, 1.77, 1.65, 1.55, 1.47, 1.41, 1.31 and 1.20 nm, respectively. The distances between Ce³⁺ and Eu²⁺ become shorter with increasing the Eu²⁺ doping content. x is the critical concentration at which the emission intensity of the donor (Ce³⁺) in the presence of the acceptor (Eu²⁺) is half that in the absence of the acceptor (Eu²⁺). Therefore, the critical distance (R_c) of energy transfer is calculated to be about 1.77 nm for Ca₂PO₄Cl:0.03Ce³⁺, y Eu²⁺. R_Ce–Eu for various Eu²⁺ content levels is much larger than the typical critical distance for the exchange interaction (0.5 nm).⁴⁶ The results indicate that the exchange interaction plays no role in the energy transfer process for Ca₂PO₄Cl:0.03Ce³⁺, yEu²⁺. Therefore, the energy transfer between Ce³⁺ and Eu²⁺ exists in Ca₂PO₄Cl, and the emission intensities of Eu²⁺ are obviously enhanced by the efficient energy transfer from Ce³⁺ to Eu²⁺, which belongs to the multipolar interaction.

3.3 Thermal stability and CIE coordinates Ca₂PO₄Cl:Ce³⁺, Eu²⁺

For the application of high power LEDs, the thermal stability of phosphor is one of important issues to be considered. For Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺, the temperature dependence of emission spectra under the 370 nm radiation excitation is shown in Fig. 8. The activation energy (E_a) can be expressed by^20,47

ln(I₀/I) = ln A − E_a/kT (10)

where I₀ and I are the luminescence intensity of Ca₂PO₄Cl:Ce³⁺, Eu²⁺ at room temperature and the testing temperature, respectively. A is a constant; k is the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹). E_a is achieved to be 0.0114 eV K⁻¹. The inset depicts and compares the thermal quenching properties of Ca₂PO₄Cl:Ce³⁺, Eu²⁺ and BAM. We observed only 6% decay at 100 °C for Ca₂PO₄Cl:Ce³⁺, Eu²⁺, as shown in Fig. 8, Ca₂PO₄Cl:Ce³⁺, Eu²⁺ has thermal quenching as good as that of BAM.

Fig. 8 Temperature-dependent emission spectra of Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺ (λ_ex = 370 nm). Inset: normalized intensity of Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺ and BAM as function of temperature (λ_ex = 370 nm).

Color coordinates are one of the important factors for evaluating phosphors' performance, and the chromatic standard issued by the Commission Internationale de I'Eclairage in 1931 (CIE 1931). For Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺, the CIE color coordinates are (0.1454, 0.0448). It can be seen that the color coordinates of Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺ are about the blue region. The all results also indicate that Ca₂PO₄Cl:Ce³⁺, Eu²⁺ may be a potential blue phosphor for the high-LED application.

4 Conclusions

In summary, Ca₂PO₄Cl:Ce³⁺, Eu²⁺ phosphors are synthesized by the conventional solid state method. Ca₂PO₄Cl:Ce³⁺, Eu²⁺ can produce two emission bands, which correspond to the 4f⁰5d¹ → 4f¹ transition of Ce³⁺ ion and the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ion, respectively. The energy transfer Ce³⁺ → Eu²⁺ in Ca₂PO₄Cl has been validated and proved to be a resonant type via the dipole–dipole interaction. The thermal quenching property reveals that Ca₂PO₄Cl:Ce³⁺, Eu²⁺ has an excellent characteristics. Based on the results, we are currently evaluating the potential application of Ca₂PO₄Cl:Ce³⁺, Eu²⁺ as a blue-emitting n-UV convertible phosphor.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (no. 50902042), the Natural Science Foundation of Hebei Province, China (no. A2014201035; E2014201037) and the Education Office Research Foundation of Hebei Province, China (nos. ZD2014036; QN2014085).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1, emission spectra of Ca₂PO₄Cl:0.03Ce³⁺ and Ca₂PO₄Cl:0.03Ce³⁺, 0.03Na⁺. Fig. S2, emission spectra of Ca₂PO₄Cl:0.07Eu²⁺, Ca₂PO₄Cl:0.03Ce³⁺, 0.07Eu²⁺ and BaMgAl₁₀O₁₇:Eu²⁺. See DOI: 10.1039/c4ra03087g

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