A novel Eu³⁺/Eu²⁺ co-doped MgSrLa₈(SiO₄)₆O₂ single-phase white light phosphor for white LEDs

Abstract

A novel MgSrLa_8−x(SiO₄)₆O₂:xEu (MSLSO:xEu) phosphor was synthesized through a high-temperature solid-state reaction. The crystal structures, luminescent properties, fluorescence decay time, and oxygen vacancies were investigated systemically. XRD analysis shows a typical oxyapatite structure with the space group P6₃/m. Europium can enter crystal matrices simultaneously in the form of Eu³⁺ and Eu²⁺ and occupy nonequivalent crystallographic positions in a lattice, thus forming various optical centers. Under ultraviolet light excitation, the phosphors simultaneously show the blue-green emission of Eu²⁺ and the green-yellow-red emission of Eu³⁺. The optimal doping content of Eu is 7.5 mol% (x = 0.075). White light can be realized with a CIE coordinate of (0.3664, 0.3260) by adjusting the concentration of Eu. The lifetimes of Eu³⁺ and Eu²⁺ in this study are considerably longer than those in other references. The results suggest that the MSLSO:Eu²⁺/Eu³⁺ (0.075) phosphor is a promising candidate for white LEDs.

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Introduction

White-light-emitting diodes (WLEDs) are regarded as eco-friendly light sources because of their small size, low energy consumption, high luminous efficiency, and long lifetime, and they are environmentally benign.^1,2 The most commonly used method to obtain white light is by combining a blue LED chip and a YAG:Ce³⁺ yellow-emitting phosphor.^3,4 However, this kind of WLED present numerous disadvantages, such as poor color-rendering index (CRI) and high color temperature because of the lack of a red component in the spectrum.⁵ Thus, this kind of WLED cannot satisfy illumination requirements. To solve this problem, we used an ultraviolet (UV)/n-UV LED chip coated with a single-phase white-light-emitting phosphor to fabricate WLEDs.⁶

In recent years, silicate-based luminescent materials have been given considerable attention owing to their favorable luminescence properties and high chemical stability. Apatite-structure silicates are regarded as effective matrixes for the activation by rare-earth (RE) ions.^7,8 Moreover, trivalent RE (RE³⁺) ions play an important role in producing highly efficient luminescent materials.^9–12

Previously reported co-doped white phosphors present several shortcomings in the spectral distribution.^13–30 For the Eu²⁺/Mn²⁺ or Ce³⁺/Mn²⁺ co-doped systems, the intensity of the peaks are relatively weak in the green region, whereas the Ce³⁺/Eu²⁺ or Ce³⁺/Tb³⁺ co-doped systems are lack the red component, which will affect the overall luminescent efficiency of materials. In general, the introduction of a variety of activated ions can compensate for the lack of spectrum, whereas this method may reduce the luminous intensity and the efficiency of energy transfer among the doped ions to a certain extent. Therefore, we focus on how to improve the loss of the doping system in the spectral distribution and how to obtain white phosphors to meet the requirements of different applications.

Among the various RE element species, europium has traditionally occupied a dominant role. Luminescence of the Eu³⁺ ion originates from its 4f⁶ → 4f⁶ transition consisting of sharp lines in the red region. The positions of its emission lines are independent of the host materials. Eu²⁺ presents broad emission from the parity-allowed transitions 4f⁶5d¹ → 4f⁷, which presents wide-emission range changing in the blue-green-yellow or red band. The emission band position can be tuned by selecting different host materials.^31–35 Nevertheless, only a few studies have been considered this topic, thus inspiring us to explore the luminescence properties of Eu²⁺/Eu³⁺ co-doped single-phase white-light-emitting phosphors.

In this study, in Eu³⁺-doped material, part of Eu³⁺ is reduced to Eu²⁺. The crystal structures, luminescent properties, fluorescence decay time, and oxygen vacancies were investigated systemically. Finally, the phosphors simultaneously show blue emission of the Eu²⁺ ion and green-yellow-red emission of the Eu³⁺ ion under UV light excitation. Surprisingly, high energy level emission peaks of Eu³⁺ can be observed, i.e., {⁵D₁–⁷F_J (J = 0, 1, 2) and ⁵D₂–⁷F_J (J = 0, 1, 2, 3)}, which are our biggest innovation. White light can be realized by adjusting the concentration of Eu. Results show that MSLSO:Eu²⁺/Eu³⁺ is a promising white-emitting phosphor for WLEDs.

Experimental

Sample preparation

MSLSO doped with Eu was synthesized with a high-temperature solid phase method by calcining a mixture of La₂O₃ (99.9%), MgCO₃·3H₂O (99.99%), SrCO₃, Eu₂O₃ (99.99%), SiO₂ (99.9%), and H₃BO₃, where H₃BO₃ acted as flux. All calculations were performed assuming that Eu replaces La³⁺ exclusively and formally yielding MSLSO:xEu (x = 0, 0.05, 0.075, 0.09, 0.10, and 0.125). That is to say, n(Mg) : n(Sr) : n(La) : n(Si) : n(Eu) = 1 : 1 : (8 − x) : 6 : x. Subsequently, the mixture was prefired at 1000 °C for 3 h in a weak reducing atmosphere with using carbon powder. Then, the products were cooled to room temperature naturally. After grinding for second time, and the samples were calcined at 1100 °C for 4 h under the same conditions.

Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 Advance diffractometer with Cu Kα₁ radiation (λ = 1.5406 Å) in the range of 2θ = 10–90° with the step of Δ2θ = 0.02° operating at 40 mA and 40 kV. The fluorescent properties of the as-obtained samples were recorded by FluoroMax-4 (HORIBA Jobin Yvon) fluorescence spectrometer with a Xe-arc lamp of 150 W power, and the lifetime was measured with a phosphorimeter attached to the main system with a Xe-flash lamp (25 W power).

Results and discussion

X-ray phase analysis

The solid solutions MSLSO:xEu (x = 0, 0.05, 0.075, 0.09, 0.10, and 0.125) belong to the oxyapatite structure type. According to ref. 36 the site-symmetry of atoms in Sr₂La₈(SiO₄)₆O₂ crystals (space group P6₃/m, z = 1) is La1-4f, Sr-4f, La2-6h, Si-6h, O-6h, O-6h, O-12i, O-2a; z = 1. The structure contains isolated SiO₄ tetrahedra. Oxygen O(4) in the 2a position does not enter into the composition of the tetrahedra and forms chains that are parallel to the hexagonal crystal axis.³⁷ On this basis, we assumed that the Mg atoms also occupy the 4f position. Europium replacing the La atoms in Sr₂La₈(SiO₄)₆O₂ can also be located in these two positions. In the unit cell, the six 6h sites are occupied exclusively by 6La³⁺, whereas the four 4f sites are randomly shared by 1Sr²⁺, 1Mg²⁺, and 2La³⁺.³⁸

Fig. 1(a) shows the characteristic diffraction patterns of the synthesized solid solutions MSLSO:xEu (x = 0, 0.05, 0.075, 0.09, 0.10, and 0.125). All diffraction peaks of the samples can be indexed in the hexagonal unit cell, and no second phase was detected at these doping levels, indicating that all as-prepared samples matched well with that of the standard Sr₂La₈Si₆O₂₆ phase (ICSD card no. 155625) and all samples have hexagonal apatite-like structure with the space group of P6₃/m. Therefore, the doped Eu³⁺ ions present no evident influence on the crystal line structure of the host MSLSO at various Eu³⁺ doping concentrations. Fig. 1(b) shows the energy dispersive X-ray spectroscopy (EDS) spectrum of the MSLSO, which confirmed the presence of Sr, La, Mg, Si, and O elements. In addition, n(Mg) : n(Sr) : n(La) : n(Si) : n(O) = 1 : 1 : 8 : 13 : 26.

Fig. 1 (a) Powder XRD patterns of Sr₂La₈Si₆O₂₆ (ICSD 15-5625) and MSLSO:xEu phosphors. (b) EDS spectrum.

Photoluminescence

Fig. 2(a) illustrates that, in the PLE spectrum of 614.5 nm luminescence for the unreduced sample, two bands are centered at about 284 and 304 nm by using Gaussian fitting corresponding to the allowed O²⁻ → Eu³⁺ charge transfer state (CTS1 and CTS2) of Eu³⁺ ion. The sharp excitation peaks between 350 and 600 nm are attributed to the intra-4f transitions from the Eu³⁺ ground state ⁷F₀. Meanwhile, for the reduced samples, three bands are centered at about 267, 304, and 330 nm by using Gaussian fitting corresponding to the allowed O²⁻ → Eu³⁺ charge transfer state (CTS1 and CTS2) of Eu³⁺ ion and the 4f⁷ → 4f⁶5d¹ from Eu²⁺, respectively, as shown in Fig. 2(c). CTS1 and CTS2 correspond to the 6h sites and the 4f sites, as is consistent with the reported literature.³⁹ Weak sharp excitation peaks are found between 350 and 600 nm, meaning that for the emission of the Eu³⁺ ions when Eu²⁺ is excited and the CTS majorly contribute, and the contribution of f–f transition is smaller. An inflection around 290 nm of the band is centered at 330 nm, suggesting that Eu²⁺ in this site can exchange the excitation energy with Eu³⁺ probably located at the same site-type. For the reduced samples, by comparing the intensity of the peaks of the excitation spectrum, x = 0.075 is a more favorable condition. Next, in Fig. 2(b), we will research the emission spectra of the reduced and unreduced samples about MSLSO:0.075Eu excited at 267 and 290 nm, respectively. Compared with the unreduced sample, the reduced sample possess several sharp peaks, the intense emission peaks from transitions ⁵D₁–⁷F_J (J = 0, 1, 2) and weak peaks from ⁵D₂–⁷F_J (J = 0, 1, 2, 3) of Eu³⁺ appear to the emission spectra besides the peaks from ⁵D₀–⁷F_J (J = 0, 1, 2, 3, 4) of Eu³⁺. According to our previous experimental results, given the energy transfer from Eu²⁺ ions to the Eu³⁺ ions, high-energy level emission of Eu³⁺. The emission spectra of the unreduced sample excited at 290 nm, where high energy level emission lines are not observed. The levels with J = 0 are normally not degenerated, so the ⁵D₀–⁷F₀ transition shows no more than one band at the spectrum. Two lines for the transition ⁵D₀–⁷F₀ indicate that the Eu³⁺ ions occupies the 4f and 6h positions.⁴⁰ According to G. Blasse,⁴¹ for the unreduced sample, the Eu^{3+ 5}D₀–⁷F₀ transition is observed as the strongest peak at 577 nm and the ⁵D₀–⁷F₀ emission of Eu³⁺ is rather intense, indicating a strong linear crystal-field component at the Eu³⁺ ion. This case is evident in the (6h) site. The site symmetry is C_s, with one free oxygen ion in the symmetry plane. For the reduced sample, we found two bands with peaks at 577 and 581 nm (see the Table 1) for the ⁵D₀–⁷F₀ transition, indicating that Eu³⁺ occupies two sites (6h and 4f, respectively) confirming the occurrence of two optical centers generated by Eu³⁺. The transitions of emission spectra were identified in Table 1.^42–46

Fig. 2 (a) Excitation spectra of phosphors MSLSO:Eu²⁺/Eu³⁺ monitored at 623 nm and MSLSO:0.075 Eu³⁺ monitored at 614.5 nm. (b) Emission spectra of the phosphors MSLSO:Eu²⁺/Eu³⁺ (0.075) excited at 267 and 290 nm. (c) is the insert figure from (a).

Table 1 Transitions in the emission spectra of MSLSO:Eu²⁺/Eu³⁺

Wavelength (nm)	Transition	Wavelength (nm)	Transition
489	^5D3–^7F5	585.5	^5D1–^7F3
460	f^6d–f^7 (Eu^2+)	623	^5D1–^7F4
466	^5D2–^7F0	577, 581	^5D0–^7F0
473.5	^5D2–^7F1	587, 589.5, 594, 598	^5D0–^7F1
494.5	^5D2–^7F2	614.5, 619, 623	^5D0–^7F2
511	^5D2–^7F3	669	^5D0–^7F3
527	^5D1–^7F0	701.5, 704	^5D0–^7F4
538	^5D1–^7F1	—	—
555, 561	^5D1–^7F2	—	—

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The excitation spectra of MSLSO:Eu²⁺/Eu³⁺ (x = 0.05, 0.075, 0.09, 0.10, and 0.125) are shown in Fig. 3(a) and 4(a) monitored at 460 nm, the excitation spectra consists of two distinct broad excitation bands peaking at 330 nm and 365 nm related to 4f⁷ (⁸S_7/2) → 4f⁶5d¹ from Eu²⁺. The emission spectra of MSLSO:Eu²⁺/Eu³⁺ (x = 0.05, 0.075, 0.09, 0.10 and 0.125) are shown in Fig. 3(b) and (c) and 4(b), in addition, the insert Fig. 3(c) is from Fig. 3(b) between 415 and 520 nm, the whole emission spectra consists of a broad emission band peaking at 460 nm related to 4f⁶5d¹ → 4f⁷ (⁸S_7/2) luminescence of Eu²⁺ and several sharp lines between the 530 nm and 710 nm region, ascribed to the ⁵D_I → ⁷F_J (I = 0, 1, 2; J = 0, 1, 2, 3, 4) transitions in Eu³⁺. That is to say, part of the energy of Eu²⁺ transfer to Eu³⁺, another part for its own emission; in this case, it is possible to achieve white light emission. In the oxyapatite structure,³⁴ Sr²⁺ occupy the 4f sites, the ionic radii of Eu²⁺ is very close to that of Sr²⁺, moreover, the Eu²⁺ ion match the Sr²⁺ electrovalence, so it is reasonable that Eu²⁺ will substitute for Sr²⁺ to occupy the 4f sites. Thus, the energy transfer between Eu²⁺ and Eu³⁺ occurs to the 4f sites.

Fig. 3 (a) Excitation spectra of phosphors MSLSO:xEu²⁺/Eu³⁺ monitored at 460 nm (monitoring the emission of Eu²⁺). (b) The emission spectra of phosphors MSLSO:xEu²⁺/Eu³⁺ excited at 330 nm. (c) is the insert figure from (b).

Fig. 4 (a) The range from 200 nm to 430 nm is the excitation spectra of phosphors MSLSO:xEu²⁺/Eu³⁺ monitored at 460 nm (monitoring the emission of Eu²⁺). (b) The range from 380 nm to 710 nm is the emission spectra of phosphors MSLSO:xEu²⁺/Eu³⁺ excited at 365 nm.

The above results suggest that Eu²⁺ ions play a sensitizing agent role in the emission process and also acts as an activator.

To further understand the relationship plot between the intensity of ⁵D₁, we constructed ⁵D₀ emission and the Eu³⁺ ion concentration for the luminescence intensity of ⁵D₀ and ⁵D₁ under the excitation wavelength of 330 and 365 nm, as shown in Fig. 5. Evidently, the intensity of transition (⁵D₀ and ⁵D₁) increased with increasing Eu³⁺ ion concentration in the MSLSO host lattice from 5 mol% to 7.5 mol%; however, owing to concentration quenching, the emission intensity decreased when the Eu³⁺ ion concentration further increased over 7.5 mol%.

Fig. 5 Intensity of ⁵D₁, ⁵D₀ emission excited at 330 (a) and 365 nm (b) for phosphors MSLSO:xEu²⁺/Eu³⁺ (x = 0.05, 0.075, 0.09, 0.10, and 0.125).

In summary, whether excitation spectrum or emission spectrum, the intensity of the peaks gradually increase when x = 0.075, and the intensity of the peak reached the maximum and then gradually decreases with the increasing value of x. Overall, x = 0.075 is the optimal condition.

As seen from Fig. 3(b), 4(b), and 5, the ratio of the band intensity centered at 330 nm to that of centered at 365 nm in the excitation spectrum monitored at 460 nm is close to 1/2, whereas the ratio of the intensity of several emission peaks (538, 555, 585.5, 614.5, and 623 nm) of Eu³⁺ in the emission spectrum excited at 330 nm to the same emission peaks of Eu³⁺ in the emission spectrum excited at 365 nm is close to 2. This finding shows that the energy transfer from Eu²⁺ to Eu³⁺ mainly originated from the 330 nm excitation instead of the 365 nm excitation.

Moreover, according to the emission spectrum of the MSLSO:Eu²⁺/Eu³⁺ (0.075) excited at different wavelengths (λ_ex = 267, 330, and 365 nm), the chromaticity coordinate were calculated to be (0.5008, 0.4518), (0.3664, 0.3260), and (0.2300, 0.2153). The coordinates are respectively displayed in the yellow, white, and blue regions. Both spectra are fitted in the 1931CIE chromaticity diagram as shown in Fig. 6(a), indicating that the emission hue is tunable from yellow to white and eventually to blue by adjusting the excitation wavelength. Moreover, Fig. 6(b) shows the color emission spectra of sample MSLSO:Eu²⁺/Eu³⁺ (0.075) excited at 330 nm, containing the composition of the three primary colors of white light so that white light emission is exhibited.

Fig. 6 (a) CIE chromaticity diagram for MSLSO:Eu²⁺/Eu³⁺ (0.075) phosphor excited at 267, 330, and 365 nm; (b) the color emission spectra of sample MSLSO:Eu²⁺/Eu³⁺ (0.075) excited at 330 nm.

Fig. 7 illustrates the fluorescence decay curves of MSLSO:Eu²⁺/Eu³⁺ (0.075) phosphor at room temperature. The experimental decay curves of MSLSO:Eu²⁺/Eu³⁺ (0.075) can be best fitted with a double exponential eqn (1):

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) + y₀ (1)

Fig. 7 Fluorescence decay curves of Eu²⁺ and Eu³⁺ emission in MSLSO:Eu²⁺/Eu³⁺ (0.075) phosphor monitored at 623 nm (a), 614.5 nm (b), 585.5 nm (c), 555 nm (d), 538 nm (e), 460 nm (f) under excitation at 330 nm.

Suggesting the significance of the energy transfer between Eu³⁺ ions and Eu²⁺ ions, where I is the luminescence intensity, A₁ and A₂ are constants, t is the time, τ₁ and τ₂ are the decay times for the exponential components. The average decay times τ can be estimated by the following eqn (2):⁴⁷

τ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (2)

The fluorescence decay times of Eu²⁺ ions and Eu³⁺ ions are listed in Table 2. The lifetimes of ⁵D₁ and ⁵D₀ levels in the current study are considerably longer than those of the reports from ref. 48–51 and the lifetimes of ⁵D₁ level are longer than those of the ⁵D₀ level. We speculate that the energy transmission from ⁵D₁ to ⁵D₀ is interfered. As a result, the intense luminescence of ⁵D₁ level is achieved due to the enhanced probability of radiation transition.

Table 2 Decay properties of MSLSO:Eu²⁺/Eu³⁺ (0.075) phosphor under excitation at 330 nm

Monitored wavelength (nm)	A1	A2	τ1	τ2	τ (ms)	Fitted function
460	0.20	0.83	0.0008	0.00003	0.0007	Double exponential
538	0.92	0.15	0.18	7.96	7.01	Double exponential
555	1.09	0.12	0.15	8.10	6.95	Double exponential
586	0.89	0.16	0.19	7.51	6.60	Double exponential
614.5	0.97	0.08	0.41	8.34	5.50	Double exponential
623	0.90	0.11	0.45	11.02	8.34	Double exponential

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Reduction of Eu³⁺ → Eu²⁺ was considered, which may be due to vacancy formation in the 4f crystal lattice position and the negative charge transfer by this vacancy to two types of Eu³⁺ ions.³⁹

Fig. 8(a) displays three series of sharp peaks in the excitation spectra monitored at different wavelengths ranging from 474 nm to 655.5 nm, 382.5 nm to 523 nm, and 377.5 nm to 438 nm, respectively. These peaks all describe a number of sharp peaks with decreasing intensity, which are brought about by absorption of radiation by oxygen vacancies. As the wavelength of detected signals increases, the luminescence maxima of the peaks are displaced to the long-wave region of the spectrum. A similar displacement of lines in the excitation spectra is described in work.^39,52,53 This displacement reflects an inhomogeneous character of oxygen vacancy distribution in the silicate matrix. Fig. 8(b) shows the peaks of oxygen vacancies vary from monitor wavelengths, the linear relationship formulas are as follows:

λ_vacancy-1 = 0.91λ_moni + 1.10 (3)

λ_vacancy-2 = 0.72λ_moni + 9.76 (4)

λ_vacancy-3 = 0.61λ_moni − 0.99 (5)

Fig. 8 (a) Excitation spectra monitored at various wavelengths and the peaks of oxygen vacancies for the MSLSO:Eu²⁺/Eu³⁺ (0.075) are marked in arrow direction. (b) The peaks of oxygen vacancies vary from the monitor wavelengths for phosphor MSLSO:Eu²⁺/Eu³⁺ (0.075).

Under normal circumstances, transfer of energy from the ⁵D₁ level to ⁵D₀ level rapidly occurs in phosphors; therefore, the lifetime of the ⁵D₁ level is relatively short and the lines of the intense ⁵D₁ emission cannot be observed. In this host, the transfer of energy from ⁵D₁ to ⁵D₀ was hindered by oxygen vacancies; thus, the energy cannot be transmitted from ⁵D₁ to ⁵D₀. Therefore, the lines of the intense ⁵D₁ emission can be observed and ⁵D₁ level lifetimes are extremely long.

We infer two possible routes to transmit the excitation energy: one route is direct transmission to levels of Eu³⁺, and another route is through the oxygen vacancies trapping the energy and then releasing the energy again. Direct transmission is fast, and the release process is slow. As the slow release process leading to the decay curves displayed double exponential form, we summarize these two processes as a slow process that determines ⁵D₁ level with extraordinary long lifetime and superior strength.

Conclusions

MSLSO doped with Eu phosphors were synthesized via high-temperature solid phase method. XRD analysis shows a typical oxyapatite structure with the space group of P6₃/m. The broad excitation spectra ranging from 220 nm to 430 nm, matched well with UV LED chips. Luminescence measurements indicate that the emission spectrum of as-obtained phosphors contain both the characteristic emissions of Eu²⁺ and Eu³⁺ ions, in which the broad band luminescence of Eu²⁺ and narrower 4f → 4f luminescent of Eu³⁺ can be observed upon 330 and 365 nm excitation. With increasing Eu contents, the relative intensity of the red component from Eu³⁺ become stronger gradually, whereas that of Eu²⁺ decreases, correspondingly. In addition, the optimal doping contents of Eu is confirmed to be 7.5 mol% (x = 0.075). Moreover, in the silicate lattice, inhomogeneously distributed oxygen vacancies are responsible for nonradiative transfer of excitation energy. The oxygen vacancies trap and release the energy, and the release process is slow. The slow release process leads to the decay curves displayed double exponential form. The CIE chromaticity diagram for MSLSO:Eu²⁺/Eu³⁺ (0.075) phosphor excited at 330 nm is displayed in the white region. In other words, by adjusting the concentration of Eu, the white light can be realized with the CIE coordinate (0.3664, 0.3260). Therefore, MSLSO:Eu²⁺/Eu³⁺ (0.075) is predicted to be a promising candidate for WLEDs.

Acknowledgements

The work is financially supported by the National Natural Science Foundation of China (Grant no. 51202166).

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