Ce3+ to Mn2+ energy transfer in Sr3Y2Ge3O12:Ce3+, Mn2+ garnet phosphor

Ce3+ to Mn2+ energy transfer in Sr3Y2Ge3O12:Ce3+, Mn2+ garnet phosphor

Journal of Alloys and Compounds 653 (2015) 636e642 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

2MB Sizes 3 Downloads 30 Views

Journal of Alloys and Compounds 653 (2015) 636e642

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Ce3þ to Mn2þ energy transfer in Sr3Y2Ge3O12:Ce3þ, Mn2þ garnet phosphor  ski, Eugeniusz Zych, Jerzy Sokolnicki* Damian Pasin Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie Street, 50-383 Wrocław, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 July 2015 Received in revised form 16 August 2015 Accepted 31 August 2015 Available online 10 September 2015

Ce3þ and Mn2þ singly doped and co-doped Sr3Y2Ge3O12 phosphors were synthesized by the solidestate reaction. In these phosphors Ce3þ ions occupy exclusively the Sr2þ site while Mn2þ ions mostly the Y3þ site with some traces in the Sr2þ site. Under excitation at 430 nm into the Ce3þ absorption band the Sr3Y2Ge3O12: Ce3þ, Mn2þ phosphor emits green light from Ce3þ (530 nm) and red light from Mn2þ (630 nm) due to the Ce3þ / Mn2þ energy transfer. By appropriate ratio of the active ion concentrations the Ce3þ and Mn2þ co-doped Sr3Y2Ge3O12 phosphor can generate lights from green to orange region. We showed that the mechanism of energy transfer from Ce3þ to Mn2þ is of the resonance type and it occurs via an electric dipoleedipole interaction. Furthermore, we calculated the critical distance for Ce3þ / Mn2þ energy transfer to be 16.90 Å by concentration quenching methods. © 2015 Elsevier B.V. All rights reserved.

Keywords: Garnet structure Ce3þ Mn2þ Energy transfer White LEDs

1. Introduction It is generally accepted that LEDs will replace conventional incandescent and fluorescent lamps for general lighting in the near future. High efficient phosphor converted LEDs (pcLED) lamps when introduced into general lighting should significantly reduce power consumption due to their superior lifetime, efficiency and reliability compared to conventional light sources. But before this happens the progress is needed in the brightness and colorrendering properties of LEDs. Currently, the phosphor most commonly utilized in bichromatic white LEDs is the yellowemitting garnet structure Y3Al5O12:Ce3þ (YAG:Ce) [1]. This phosphor has a deficient red emission and high color temperature (CCT > 4500 K), thus low color rendering index (CRI z 70e80), which cannot meet the requirements of phosphors for indoor lighting [2]. To improve the spectral distribution of Ce3þ emission in order to meet the requirements of white LEDs, an effective solution is to co-doping with Ce3þ ions, Mn2þ ions. Mn2þ emits in green (weak crystal field) and red (stronger crystal field) spectral region but is difficult to pump because its ded absorption transition is both parity and spin forbidden. Ce3þ has been already demonstrated to be very effective sensitizing ions for Mn2þ emission because its absorption/emission transitions are allowed by the

* Corresponding author. E-mail address: [email protected] (J. Sokolnicki). http://dx.doi.org/10.1016/j.jallcom.2015.08.277 0925-8388/© 2015 Elsevier B.V. All rights reserved.

selection rules. Ce3þ / Mn2þ energy transfer has been investigated in many host lattices including Ca3Sc2Si3O12 [3], KBaY(BO3)2 [4], Ca2Gd8(SiO4)6O2 [5] and Ca9Al(PO4)7 [6], CaSr2Al2O6 [7]. The intensity ratio of Ce3þ and Mn2þ emissions depends on concentration of both ions in the host lattice, thus varying their concentrations the color tuning can be realized [8e11]. Garnet structure compounds are of considerable practical importance and are also of great interest in crystal chemistry. The reason for the latter is that they have a relatively simple structure (with four parameters completely specifying the structure) which can be composed of almost half the elements. Moreover, garnet structure is the only one oxide structure in which Ce3þ absorbs blue light and emits green/yellow light. A broad absorption band in the UV/blue spectral region coincides well with the emission of excitation sources used in pc-LEDs application. The excitation sources for phosphors in LEDs are UV (360e410 nm) or blue light (420e480 nm). Although the garnet structure silicates of the general formula A3B2X3O12 are relatively well known, knowledge about the germania based garnets is still insufficient. Garnets crystallize in a body-centered system with eight formula units per unit cell. The space group is Ia3d. In this structure A atoms (Ca2þ, Sr2þ) are located in the dodecahedral site where they are each surrounded by 8 oxygen atoms, forming a polyhedron with D2 symmetry point. The atoms in the site B are each surrounded by 6 oxygen atoms in an octahedral site with the C3i symmetry point.

 ski et al. / Journal of Alloys and Compounds 653 (2015) 636e642 D. Pasin

Recently, we investigated the relation between structure and emission properties in Ce, Mn doped garnets of general formula A3B2X3O12 (where A ¼ Ca,Sr; B]Sc,Y; X ¼ Si,Ge) and correlated the emission wavelength with the distortion around the rare-earth ion in the garnet structure [11]. In the present paper we chose the Sr3Y2Ge3O12 garnet as the host lattice for Ce and Mn doping to study the energy transfer between both ions in the view of better understanding the processes that occur in the garnet structure phosphors considering for application in white LEDs. In this garnet, having the largest unit cell among germanate garnets, Mn2 þ ions occupy almost exclusively the position B. Variable concentration of Mn2þ allowed determining the critical concentration for efficient energy transfer and critical distance between the Ce3þ and Mn2þ as well as to established the mechanism of Ce3þ to Mn2þ energy transfer. 2. Material and methods Sr3Y2Ge3O12:Ce3þ, Mn2þ powders were obtained using high temperature solid state synthesis with 5% wt. of LiF flux. The samples obtained without the fluxing agent crystallized at temperature higher of 200e300  C and showed weaker luminescence properties. For this reason, we decided to further investigate the samples obtained with the use of the flux. LiF also served as a charge compensator to avoid the charge unbalance caused by Mn2þ substituting for Y3þ and formation of vacancies caused by Ce3þ substituting Sr2þ. The Ce dopant concentration was 1% with respect to strontium and Mn dopant concentrations were in the range of 0e10% with respect to yttrium. Stoichiometric amounts of GeO2 (99.9999%, Alfa Aesar), SrCO3 (Alfa Aesar, 99,994%), Y2O3 (Stanford Materials, 99,999%), CeO2 (Stanford Materials, 99.99%), MnCO3 (Aldrich, 99.9%) and LiF (Strem Chemicals, 99.9%) as a flux, were mixed and ground thoroughly in alumina mortar. After grinding powders were heated in corundum crucible in forming gas containing 5% of H2 and 95% of N2. The synthesis temperature was 1000  C and the heating time was 5 h. The X-ray diffraction (XRD) patterns were measured with D8 Advance (BRUKER) diffractometer using CuKa1 radiation (l ¼ 1.54056 Å) filtered with Ni. The diffractograms were recorded over the range of 2q 10e90 . Photoluminescence (PL) and PL excitation (PLE) spectra as well as decay times were recorded using FSL980 spectrofluorometer from Edinburgh Instruments at 298 K. As an excitation source 450 W Xenon





Fig. 1. XRD patterns for Sr3Y2Ge3O12: 1% Ce , x% Mn (x ¼ 0e10) samples and the standard data for Sr3Y2Ge3O12 (ICSD No. 80582) and Y2O3 (ICSD No. 647563).

637

lamp or Nanosecond Flashlamp nF900 were used. The resolution of the measurements was about 0.25 nm. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-synthesized Sr3Y2Ge3O12: 1% Ce3þ, x%Mn2þ (x ¼ 0e10) phosphors, which match well with ICSD file No. 80582. However, for some samples, traces of unreacted Y2O3 were detected. It follows from the fact that Mn ions can substitute for both Sr and Y in amounts hard to predict, which makes it difficult to weigh substrates in stoichiometric quantities. No impurity phases associated with the Ce3þ and Mn2þ doping were detected indicating that the dopants dissolves in the host materials giving solid solutions. Coordination polyhedra and interatomic distances for the two types of site in Sr3Y2Ge3O12: D2 site and C3i site are given in Fig. 2. It was already proved that in the A3B2X3O12 garnet structure the Ce3þ ions substitute for atoms in the A site, while the Mn2þ ions can exchange atoms in both A and B sites [8,9]. However, as we showed in our previous paper, in case of Ca3Y2Ge3O12 and Sr3Y2Ge3O12 Mn2þ occupies B site and its emission from the A site is undetectable, possibly due to the covering by the Ce3þ emission band. To determine whether Mn2þ can substitute to some extent in Sr position, we investigated single Mn2þ doped Sr3Y2Ge3O12. Fig. 3 shows the emission and excitation spectra of Sr3Y2Ge3O12 doped with 4 mol % of Mn2þ. The phosphor excited at 270 nm exhibits weak emission band centered at 630 nm with an inflection in the high energy side centered at 560. According to the TanabeeSuganodiagram of d5 configuration [12] the band corresponds to the spin forbidden 4T1(4G) / 6A1(6S) transition in Mn2þ. The band at 630 nm was previously attributed to the octahedral (B) site [11] due to the stronger crystal field experienced by Mn2þ in this site compared to dodecahedral (A) site. Consequently, the inflection can be attributed to the Mn2þ emission from the A site. The excitation spectrum monitoring the emission at the maximum of the emission band (630 nm) consists of two broad bands in the UV region

Fig. 2. Coordination polyhedra and interatomic distances for the two types of site in Sr3Y2Ge3O12: red, blue, orange and green circles represent O, Ge, Y and Sr atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

638

ski et al. / Journal of Alloys and Compounds 653 (2015) 636e642 D. Pasin

Fig. 3. PL and PLE spectra of Sr3Y2Ge3O12: 4% Mn2þ.

centered at 260 and 300 nm. When the emission is monitored at 560 nm the excitation spectrum consist of two broad bands in the same spectral region but their intensities ratio is different. These bands in both spectra can be attributed to Mn2þ / O2 charge transfer transitions. Two different excitation spectra indicate the presence of Mn2þ in two different sites in the host structure. It is worth noting that absorption transitions within 3d configuration of Mn2þ, usually occur in the range 340e480 nm, are in this case not observed due to high intensity of the charge transfer transition. The ionic radius of Mn2þ (r6 ¼ 0.83 Å) is close to the Y3þ (r6 ¼ 0.9 Å) and remarkable smaller than that of Sr2þ (r8 ¼ 1.26 Å) vs Mn2þr8 ¼ 0.96 Å (Table 1). For this reason, Mn2þ is more likely to take the B site than A site. Similarly, Eu3þ takes the B site [13,14] while the emission of Ce3þ from the B site has not been so far registered. It seems that a charge mismatch plays less important role in sites occupation, especially that the fluxing agent (LiF) compensates the charge. However, some role can play a degree of distortion of the A site because it affects its size. The increase of the d88/d81 ratio leads to compression of dodecahedron site. For Ca3Y2Ge3O12 and Sr3Y2Ge3O12 host lattices bigger Y3þ cation in the B site relaxes the deviation from the cubic structure (smaller d88/ d81 ratio) causing the A site enlarges and probably becomes too big to compete for Mn2þ, even though in this site the ion charge need

Fig. 4. PL and PLE spectra of Sr3Y2Ge3O12: 1% Ce3þ.

not be compensated. When cation B is Sc3þ, the emission of Mn2þ from the A site is observed [11]. Fig. 4 shows the PLE and PL spectra of Sr3Y2Ge3O12: 1% Ce. The excitation spectrum of the sample consists of three broad excitation bands peaking at 430, 300 and 260 nm, which correspond to electron transitions of the Ce3þ ions from the 4f ground state to the different components of excited 5d states split by the crystal field. The lowest energy band, located at about 430 nm, is the most intense, suggesting that higher excited levels are in the vicinity of the conduction band and therefore may be quenched by photoionization. Under excitation at 430 nm, the sample exhibits a green emission with band peaking at 530 nm, which originates from the 5d4f transition of Ce3þ ions. Fig. 5 shows the PLE and PL spectra of Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10). The emission spectra (Fig. 5a) consist of two broad bands peaking at 530 and 630 nm, which can be ascribed to the Ce3þ and Mn2þ emissions, respectively. It can be seen that the emission intensity of the Ce3þ ions decreases with the increment of the Mn2þ concentration, whereas the emission intensities of the Mn2þ first increase to a maximum at 7% and then reach saturation due to concentration quenching. Changes in intensity are not fully adequate to changes in concentration, suggesting a non-uniform distribution of dopant ions. The emission color of Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ phosphors is then tuned by adjusting the Mn2þ concentration, as expected. It should be noted that the emission band of Mn2þ residually present in the A position (560 nm) is covered by Ce3þ emission band or Mn2þ does not occupy the A position when co-doped with Ce3þ. The excitation spectra of Mn2þ monitoring the emission at 630 nm are shown in Fig. 5b. The spectra consist of three bands assigned above to the absorption transitions in Ce3þ. This indicates the Ce3þ to Mn2þ energy transfer. The band centred at 260 nm is a superposition of Ce3þ absorption band and Mn2þ charge transfer band. This is proved by the increasing intensity ratio between this band and the band peaking at 430 nm with increasing concentration of Mn2þ. The critical distance between the Ce3þ and Mn2þ ions can be estimated using eq. (1) given by Blasse [15]:

 Rc ¼ 2

3V 4pxc Z

1=3 (1)

where V is the volume of the unit cell, Z represents the formula units per unit cell, and xc is the critical concentration (the total concentration of sensitizer ions of Ce3þ and activator ions of Mn2þ), where the emission of Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ phosphors reaches the maximum. By taking the values of V ¼ 2241.46 Å3, Z ¼ 8, and xc ¼ 0.11, the critical transfer distance RC was found to be 16.90 Å. It should be noted that this estimates do not account both for Ce3þeCe3þ energy migration, which would tend to increase our estimates for the critical radius for energy transfer and Mn2þeMn2þ energy migration which would decrease this estimates. There are two types of energy transfer: one is exchange interaction, and the other one is multipolar interaction [16,17]. In the case of the exchange interaction, the critical distance between the sensitizer and activator is always shorter than 4 Å, thus we can assumed the electric multipolar interaction between Ce3þ and Mn2þ. In order to investigate the dynamics of luminescence process between Ce3þ and Mn2þ, the decay curves of Ce3þ and Mn2þ in the Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10) phosphors excited at 425 nm and monitored at 530 or 630 nm were measured and shown in Fig. 6. The decay time values together with Ce3þ / Mn2þ energy transfer efficiencies are collected in Table 2. The decay

 ski et al. / Journal of Alloys and Compounds 653 (2015) 636e642 D. Pasin

639

Fig. 5. a. Emission spectra of Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10), lexc ¼ 420 nm, b. Excitation spectra of Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10), lmon ¼ 630 nm.

Table 1 Ionic Radii (Å) of different cations for the given coordination number (CN). Ionic radii (Å) Ion

CN ¼ 6

CN ¼ 8

Ce3þ Mn2þ Sr2þ

1.01 0.83 1.18

1.143 0.96 1.26

process of the Ce3þ emission is characterized by an average lifetime t, which can be calculated using eq. (2) as follows [17]:

Z0

hT ¼ 1  IðtÞt dt

t¼∞ Z0

(2) IðtÞ dt



where I(t) is the luminescence intensity at time t and t is the decay lifetime. Curve fitting using the above equation gave the decay constant for Ce3þ emission decreased monotonically from 34.02 to 25.45 ns as x increased from 0 to 0.1 demonstrates an energy transfer from Ce3þ to Mn2þ. At the same time the decay constant of Mn2þ emission decreased from 9.49 to 7.29 ms due to the energy migration between Mn2þ ions with increasing concentration. The energy transfer efficiency (hT) from a sensitizer to an activator can be calculated by the following formula [17e19]:

tS I ¼1 S tS0 IS0

(3)

where tS0 and tS are the decay lifetimes of the sensitizer (Ce3þ) in the absence and presence of the activator (Mn2þ), respectively. IS0 and IS are the luminescence intensities of the sensitizer Ce3þ in the absence and presence of the activator Mn2þ, respectively. hT, the energy transfer efficiency from Ce3þ to Mn2þ in Sr3Y2Ge3O12: 1%

ski et al. / Journal of Alloys and Compounds 653 (2015) 636e642 D. Pasin

640

Fig. 6. a. Decay time curves of Ce3þ emission at 530 nm, b. Decay time curves of Mn2þ emission at 630 nm.

Ce3þ, x% Mn2þ, calculated as a function of x. hT was found to increase with an increasing Mn2þ dopant content. This means that saturation of emission intensity for the sample wit 10% of Mn2þ results from energy migration between the Mn2þ ions, which is consistent with the monotonic shortening of the Mn2þ decay times with increasing concentration. On the basis of the Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained [19e21]:

where hS0 and hS are the luminescence quantum efficiencies of Ce3þ in the absence and presence of Mn2þ, respectively; the value of hS0 /hS can be approximately estimated from the luminescence intensity ratio (IS0 ); C is the concentration of Mn2þ and n ¼ 6, 8, and 10 for dipoleedipole, dipole-quadrupole, and quadrupoleequadrupole interactions, respectively. Finally, Equations (2) and (3) can thus be represented by the following equation:

hSo a=3 I aC and So a C n=3 hS IS

jPSA j ¼

(4)

1 1  aC n=3 tS0 tS

(5)

Table 2 Decay times of Ce3þ and Mn2þ emissions from Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10) and Ce3þ / Mn2þ energy transfer efficiencies calculated as a function of x (hT). Mn2þ concentration

0%

0.5%

1%

2%

4%

7%

10%

Decay time [ns] lem ¼ 530 nm Mn2þ Concentration Decay time [ms] lem ¼ 630 nm

34.02

32.45 0.5% 9.49 0.05

30.90 1% 9.45 0.09

30.00 2% 8.77 0.12

29.04 4% 8.37 0.15

27.11 7% 7.44 0.20

25.45 10% 7.29 0.25

hT

 ski et al. / Journal of Alloys and Compounds 653 (2015) 636e642 D. Pasin

where PSA is the probability of energy transfer. Fig. 7 depicts dependence of ln[PSA] on C1/3 based on Equation (5) for the Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10) phosphors with a slope of 5.9. The observed value is close to 6 implies that energy transfer from Ce3þ to Mn2þ may occurred via the dipoleedipole mechanism. In Fig. 8 the emission spectra of Sr3Y2Ge3O12: 1% Ce3þ, 7% Mn2þ in wide range of temperatures are shown. Intensity of emission is decreasing monotonously with increasing temperature and at 500 K is completely quenched. The intensity ratio of Ce3þ and Mn2þ emissions is constant over the entire range of temperatures which means that thermally activated photoionization of excited electron on Ce3þ is the main mechanism of emission quenching. The emission intensity drops to half the beginning intensity, at T50% ¼ 375 which is too low for high-power LED application. At this point it should be emphasized that manganese ions may incorporate the host lattice at different oxidation states. In the sample obtained in the reducing atmosphere one can consider the presence of Mn3þ or Mn2þ. Because both ions exhibit similar spectroscopic properties Mn3þ can be misinterpreted as Mn2þ. Considering all the results obtained, we must conclude that they confirm attribution of the emission from Mn to Mn2þ and not to Mn3þ. The emission spectra of Mn3þ are influenced by the Jahn-Teller effect so that two emission bands should be observed (especially at low temperature). The intensity ratio of these bands is strongly temperature dependent because at low temperatures, the 1T2 emitting level is mostly populated and population of the 5T2 emitting level increases with temperature. Thus, the change of the intensity and position of respective bands is clearly seen [22,23]. Such behavior is inconsistent with what we observe for our sample. Please note the only one emission band related to Mn, which does not change its position and width with temperature. The intensity ratio of Ce3þ and Mn emissions is roughly temperature independent. Also decay times measured by us indicate the presence of Mn2þ. The decay times measured by S. Kuck for Mn3þ in Gd3Sc2Ga3O12 garnet is < 0.5 ms [22]. In our case, the decays are 7.29e9.49 ms at RT. For the higher energy Mn emission, which occurs in garnets of different composition (e.g. Ca3Sc2Si3O12) the lifetimes are also of the millisecond order [11]. The decays of 1T2 and 5T2 levels should be of different order because the transition from 5T2 to 5E is spin allowed. Another factor that should be taken into account is absorption spectrum of Mn3þ. It usually consists of two intense bands between 400 and 550 nm due to the spinallowed transitions from split 5E ground state, which is not a case

Fig. 7. Dependence of ln[PSA] on C1/3 of Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10).

641

Fig. 8. Temperature dependence on the luminescence intensity in 41e500 K range for Sr3Y2Ge3O12: 1% Ce3þ, 7% Mn2þ. The inset shows the integrated intensity of emission.

in our spectra, where only a plateau is observed. The Commission International de I'Eclairage (CIE) chromaticity coordinates for Sr3Y2Ge3O12: 1% Ce, x% Mn (x ¼ 0e10) are presented in Fig. 9. With increasing Mn2þ content, the chromaticity coordinates (x, y) vary systematically from (0.295, 0.536) to (0.480, 0.443); corresponding color tone of the sample changes gradually from green to orange. Various white lights are expected to obtain when the tunable emission of Sr3Y2Ge3O12: Ce3þ, Mn2þ coupled with blue LEDs. The obtained samples have more emission intensity in the red region than the commercial phosphor Y3Al5O12:Ce3þ, thus, the white light with higher CRI is expected to be generated. These results indicate that this single-phased tunable-emitting phosphor may be useful for the development of white LEDs, due to the stronger emission in the red region.

Fig. 9. CIE chromaticity coordinates of Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10).

642

ski et al. / Journal of Alloys and Compounds 653 (2015) 636e642 D. Pasin

4. Conclusions In conclusion, Sr3Y2Ge3O12: 1% Ce3þ, x% Mn2þ (x ¼ 0e10) phosphors were synthesized by a high-temperature solidestate reaction. In these phosphors Ce3þ substitutes for Sr2þ while Mn2þ almost exclusively substitutes for Y3þ. These phosphors are effectively excited in the wavelength range from 380 to 470 nm and due to due to the Ce3þ / Mn2þ energy transfer emit green to orange light depending on the Ce3þ/Mn2þ concentration ratio. The Ce3þ / Mn2þ energy transfer was shown to be mainly resonant type via dipoleedipole interaction and the critical distance of energy transfer from Ce3þ to Mn2þ was calculated to be about 16.90 Å by concentration quenching methods. Acknowledgment This work was supported by POIG.01.01.02-02-006/09 project cofounded by European Regional Development Fund within the Innovative Economy Program. Priority I, Activity 1.1. Subactivity 1.1.2. References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [2] A.A. Setlur, W.J. Heward, M.E. Hannah, U. Happek, Chem. Mater 20 (2008) 6277. [3] Y. Liu, X. Zhang, Z. Hao, Y. Luo, X. Wang, L. Ma, J. Zhang, J. Lumin. 133 (2013)

21. [4] Z. Lian, J. Sun, L. Zhang, D. Shen, G. Shen, X. Wang, Q. Yan, RSC Adv. 3 (2013) 16534. [5] G. Li, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, J. Mater. Chem. 21 (2011) 13334. [6] J. Hou, W. Jiang, Y. Fang, Y. Wang, X. Yin, F. Huang, ECS J. Solid State Sci. Technol. 1 (2012) R57. [7] Y. Li, Y. Shi, G. Zhu, Q. Wu, H. Li, X. Wang, Q. Wang, Y. Wang, Inorg. Chem. 53 (2014) 7668. ndez-Pozos, C. Flores, A. Speghini, M. Bettinelli, J. Phys. [8] U. Caldino, J.L. Herna Condens. Matter 17 (2005) 7297. [9] C. Guo, L. Luan, Y. Xu, F. Gao, L. Liang, J. Electrochem. Soc. 155 (2008) J310. [10] S. Ye, J. Zhang, X. Zhang, S. Lu, X. Ren, X. Wang, J. Appl. Phys. 101 (2007) 033513. [11] D. Pasinski, E. Zych, J. Sokolnicki, J. Lumin (2015), http://dx.doi.org/10.1016/ j.jlumin.2015.02.044. [12] M. Weber, J. Phosphor Handbook, second ed., CRC Press, Boca Raton, 2007. [13] M. Bettinelli, A. Speghini, F. Piccinelli, C.A.N. Neto, O.L. Malta, J. Lumin 131 (2011) 1026. [14] E. Antic-Fidancev, M. Lemaitre-Blaise, P. Porcher, M. Taibi, J. Aride, J. Alloys Compds 188 (1992) 75. [15] G. Blasse, Philips. Res. Rep. 131 (1969) 131. [16] G. Blasse, Phys. Lett. 28A (1968) 444. [17] R. Reisfeld, E. Greenberg, R. Velapoldi, B. Barnett, J. Chem. Phys. 56 (1972) 1698. [18] P.I. Paulose, G. Jose, V. Thomas, N.V. Unnikrishnan, M.K.R. Warrier, J. Phys. Chem. Solids 64 (2003) 841. [19] Y. Tan, C. Shi, J. Phys. Chem. Solids 60 (1999) 1805. [20] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [21] R. Reisfeld, L. Boehm, J. Solid State Chem. 4 (3) (1972) 417. [22] S. Kuck, S. Hartung, S. Hurling, K. Petermann, G. Huber, Spectrochim. Acta A 54 (1998) 1741. [23] K. Petermann, G. Huber, J. Lumin (1984) 71, 31/32.