The structure and luminescence properties of long afterglow phosphor Y3−xMnxAl5−xSixO12

Journal of Luminescence 131 (2011) 676–681

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

The structure and luminescence properties of long afterglow phosphor Y3  xMnxAl5  xSixO12 Zhongfei Mu a,b, Yihua Hu a,n, Yinhai Wang a, Haoyi Wu a, Chujun Fu a, Fengwen Kang a a b

School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, PR China Experimental Teaching Department, Guangdong University of Technology, Guangzhou 510006, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 June 2010 Received in revised form 3 November 2010 Accepted 14 November 2010 Available online 20 November 2010

A series of phosphors with the composition Y3  xMnxAl5  xSixO12 (x ¼ 0, 0.025, 0.050, 0.075, 0.150, 0.225, 0.300) were prepared with solid state reactions. The X-ray powder diffraction analysis of samples shows that the substitution of Mn2 + and Si4 + does not change the garnet structure of phosphors, but makes the interplanar distance decrease to a certain extent. The emission spectra show that Mn2 + in Y3Al5O12 emits yellow–orange light in a broad band. With the increment of substitution content, the emission intensity of the phosphors increases firstly then decreases subsequently, and the emission peak moves to longer wavelength. Afterglow spectra and decay curves show that all the Mn2 + and Si4 + co-doped samples emit yellow–orange light with long afterglow after the irradiation of ultraviolet light. The longest afterglow time is 18 min. Thermoluminescence measurement shows that there exist two kinds of traps with different depth of energy level and their depth decreases with the increment of substitution content. & 2010 Elsevier B.V. All rights reserved.

Keywords: Y3Al5O12 Mn2 + Afterglow Substitution

1. Introduction In recent years, rare earth ions doped yttrium aluminum garnet (Y3Al5O12, YAG) has been widely applied in the field of luminescent materials because of its excellent mechanical robustness, transparency, thermal conductance and chemical stability [1–9]. YAG is a kind of host with very good structural compatibility. Y3 + and Al3 + in YAG can be replaced by many kinds of other cations with different valency and size within a suitable range. Many previous works investigated the effect of host substitution on the luminescence properties of usual luminescent centers in YAG. It is common that only one kind of cations in YAG is substituted. Trivalent rare earth ions such as La3 + , Gd3 + , Lu3 + , Tb3 + and others, can be used to replace Y3 + [10–13]. Ga3 + in boron family can be used to replace homogeneous Al3 + [14]. It is also possible that two or more kinds of cations are doped into YAG at the same time. Katelnikovas et al. [15] prepared and investigated the structure and luminescence properties of Ce3 + doped Y3Mg2AlSi2O12. The results of X-ray diffraction (XRD) analysis show that samples present single garnet structure. Phosphors emit red light in a broad band peaking at 600 nm. The luminescence shows red shift in comparison with YAG: Ce3 + . Kuru et al. [16] investigated the solubility of Ca2 + and Si4 + in YAG and found that the solubility of sole Ca2 + or Si4 + is very small (less than 1%), but if Ca2 + and Si4 + with the same mole number are put into YAG, their co-solubility can be lifted up to 8%.

When only one kind of cations dissolves in YAG, trivalent cations are optimal because substituted cations and replaced ones have the same valence. It is unsuitable that only one kind of divalent or tetravalent cations is chosen to replace Y3 + or Al3 + because the substitution content must be small for the sake of charge unbalance, otherwise new phases will appear which destroy the single garnet structure of samples. Zhou et al. [17] prepared and studied phosphor Ca1.5Y1.5Al3.5Si1.5O12:Tb3 + . Their results of XRD analysis show that samples possess garnet structure with a little impurity of Y2O3. Obviously, in the literature [17], Si4 + is employed as a charge compensator to increase the solubility of Ca2 + in YAG. This work provides us an approach of how to increase the substitution content of divalent cations in YAG. Mn2 + is a kind of transition metal ions with 3d5 configuration. Its luminescence transition is parity-forbidden d-d transition. Thus its emission intensity is too small to be detected by present instruments if the doping concentration is low. Generally speaking, parity selection rule can be relaxed to a certain extent if only there exists charge transfer transition in the host. It is easy to bring new phases if one increases the doping concentration of Mn2 + in YAG. In the present work, Si4 + was employed as a charge compensator. The problem of charge unbalance was solved. A large amount of Mn2 + was doped into YAG successfully. On this basis we investigated the luminescence properties of Mn2 + in YAG.

2. Experimental n

Corresponding author. Tel.: + 86 20 39322262; fax: + 86 20 39322265. E-mail address: [email protected] (Y. Hu).

0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.11.016

All the phosphors were synthesized with solid state reactions. At first, raw materials Y2O3(99.99%), Al2O3(99.99%), MnCO3(99.0%),

Z. Mu et al. / Journal of Luminescence 131 (2011) 676–681

(4,4,4)

(6,4,0)

(6,4,2)

consistent with JCPDS card no.88-2048. Thus all the samples can be indexed to YAG with single garnet structure. Fig. 2(a) shows the magnified part of the XRD patterns (to be clear, the XRD patterns of Sam1 and Sam2 were omitted). It is observed that the main diffraction peaks gradually shift to higher angles with the increment of substitution content. According to Bragg’s equation, 2d sin y ¼ l (here l is the wavelength of employed X-ray, y is the diffraction angle of the corresponding diffraction peak, d is the interplanar distance of corresponding crystal plane), the movement of 2y to higher angles shows that the interplanar distance decreases gradually. The radii of the ions are as follows: R(Y38 + )¼0.1019 nm, R(Al36 + ) ¼0.0535 nm, R(Al34 + ) ¼ 0.039 nm, R(Mn28 + ) ¼0.096 nm, R(Mn26 + ) ¼0.067 nm, R(Mn24 + )¼ 0.066 nm, R(Si44 + )¼0.026 nm [18], the subscript here denotes the coordination number(CN) of the cation. According to the difference among the radii of these ions and the ratio of raw materials, it is assumed that Mn2 + and Si4 + replace the Y38 + and Al34 + , respectively. There is no any new phase appearing in the XRD patterns of samples. This indicates that our assumption is right. Both Mn2 + and Si4 + come into the expected sites. According to the interplanar distance equation of cubic system (d2 ¼ a2 =ðh2 þ k2 þl2 Þ, here h, k, l are the crystal indices of relevant crystal plane, a is cell parameter), cell parameter decreases with the decrement of interplanar distance

Sam6 intensity (a.u.)

SiO2 (99.0%) were accurately weighed according to the composition of Y3  xMnxAl5  xSixO12(x ¼0, 0.025, 0.050, 0.075, 0.150, 0.225, 0.300). A small amount of H3BO3 was added into the raw materials as a flux. The weighed materials were put into an agate mortar and ground with an agate muller for 2 h in order to mix them thoroughly. Then they were heated up to 1550 1C with a constant heating rate of 5 1C per minute in a tubular furnace. A weak reducing atmosphere with H2 (5%) and N2 (95%) was employed in order to prevent Mn2 + from being oxidized. The samples were preserved at 1550 1C for 4 h, then cooled naturally with the furnace. To be simple, the samples are named as Sam0 (x¼0), Sam1 (x¼0.025), Sam2 (x¼0.050), Sam3 (x ¼0.075), Sam4 (x ¼0.150), Sam5 (x ¼0.225) and Sam6 (x¼0.300), according to the value of x, respectively. The prepared phosphors were analyzed by XRD to perform the phase identification. The diffractometer worked with Cu Ka irradiation (l ¼0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The excitation spectra, emission spectra and afterglow spectra of the samples were measured with a Hitachi F-7000 Fluorescence Spectrophotometer. Decay curves and thermoluminescence (TL) curves of the samples were detected by a Beijing FJ-427SA1 thermoluminescent dosimeter. Before the measurement of afterglow time, afterglow spectra, decay curves and TL curves, the samples were excited by ultraviolet (UV) light for 300 s. The counting of afterglow time began immediately after the removal of the excitation resource and ended while one can hardly see the luminescence of samples with naked eyes in the dark. For afterglow spectra and decay curves, the measurement began as quickly as possible after the irradiation. For TL curves, the measurement started with 300 s delaying time after the removal of the excitation source.

677

3. Results and discussion 3.1. Crystal structure of phosphors

Sam5 Sam4 Sam3

In order to determine the crystalline phase structure, the XRD phase analysis was carried out. Fig. 1 shows the XRD patterns of all prepared samples. As can be seen in Fig. 1, there is no obvious difference among seven XRD patterns. Comparing these patterns with standard JCPDS cards, the main diffraction peaks are all

Sam0 51

52

53

54

55 56 2θ (°)

57

58

59

60

1.200

cell parameter /nm

1.199

intensity (a.u.)

Sam6 Sam5 Sam4 Sam3 Sam2

40 2θ (°)

50

Fig. 1. XRD patterns of samples.

60

1.197 fitting curve 1.196

1.194

JCPDS 88-2048 30

Sam4

Sam6

1.195

Sam0

20

1.198

Sam5

Sam1

10

Sam3

Sam0

70

0.00

0.05

0.10 0.15 0.20 the value of x

0.25

0.30

Fig. 2. (a) Magnified XRD patterns of samples from 511 to 601 and (b) the relationship curve of cell parameter with the value of x (calculated according to the data of crystal plane (6,4,0)).

678

Z. Mu et al. / Journal of Luminescence 131 (2011) 676–681

Table 1 Change of the cell parameter a with the value of x.

charge transfer absorpsion 4T (4D) 2

Samples

x

2y(6,4,0)/deg.

d(6,4,0)/nm

a/nm

Sam0 Sam3 Sam4 Sam5 Sam6

0 0.075 0.150 0.225 0.300

55.20 55.22 55.28 55.36 55.40

0.166265 0.166210 0.166043 0.165822 0.165712

1.198955 1.198555 1.197356 1.195762 1.194967

4A

4E (4D)

4G)

4T (4G) 2

intensity (a.u.)

(d) (b) X10

(a)

200

250

300

(c)

X10

4T (4G) 1

350 400 450 500 wavelength (nm)

550

600

Fig. 3. Excitation spectra and emission spectra of Sam3. (a) charge transfer band monitoring 580 nm emission; (b) sharp line spectrum in UV and blue–violet region monitoring 580 nm emission (  10); (c) emission spectrum excited by 266 nm and (d) emission spectrum excited by 409 nm(  10).

Sam3: 580nm

Sam4: 586nm

Sam2: 579nm Sam1: 579nm

Sam5: 590nm Sam6: 592nm

intensity (a.u.)

which results in the shrink of crystal lattice. Table 1 listed the relevant data figured out according to Bragg’s equation and the interplanar distance equation of cubic system. On the basis of these data, the relationship curve of cell parameter with the value of x can be shown in Fig. 2(b). From Fig. 2(b), it is observed that the cell parameter of samples is approximately linear with the value of x and cell parameter decreases monotonically when the value of x increases. Obviously, the decrement of cell parameter from Sam0 to Sam6 can be attributed to that the radii of two kinds of doped ions are smaller than the ones substituted. Hodges et al. [19] pointed out that when Mn2 + is doped into four kinds of garnet structures (Lu3Al5O12, Lu3Ga5O12, Y3Al5O12,Y3Ga5O12), Mn2 + can come into any of three kinds of sites (which are in the center of dodecahedron, octahedron and tetrahedron, respectively). In our experiments, the doping concentration is higher and Si4 + was employed as charge compensator, thus a large amount of Mn2 + came into dodecahedron composed of eight O2 with a coordination number of 8 (CN¼8). The quantity of electric charge of Mn2 + is less than Y3 + , thus its attractive force to O2 is smaller than Y3 + , while the attractive force of Si4 + to O2 is bigger than Al3 + , which are substituted by Si4 + . On this occasion, O2 get closer to Si4 + , but away from Mn2 + , thus the dodecahedrons which accommodate Mn2 + are distorted, the effective CN of Mn2 + is less than 8.

1(

3.2. Photoluminescence analysis The excitation spectra and emission spectra of the samples except for Sam0 were measured in order to study the luminescence properties of Mn2 + in YAG and the effect of the substitution of Mn2 + and Si4 + on the luminescence properties of the samples. The excitation spectra and emission spectra of Sam3 are shown in Fig. 3. The excitation spectra of Sam3 monitoring the emission at 580 nm are composed of the broad band in UV region and sharp line spectrum in blue–violet region. The broad band from 200 to 325 nm peaking at 266 nm can be attributed to Mn2 + –O2- charge transfer transition [20–22]. There are several sharp lines from 330 to 525 nm in the excitation spectrum. The emission spectra excited with 266 and 409 nm (one of the main excitation peaks in sharp line spectrum) are shown in the right of Fig. 3. Their emission spectra are all broad bands peaking at 580 nm. This indicates that the sample can be excited by UV light and visible light in blue–violet region. According to the analysis about XRD patterns, Mn2 + in the samples are mainly in the center of the distorted dodecahedron with CNo8, there is also a small amount of Mn2 + which locates in the center of octahedron (CN¼6) and tetrahedron (CN¼ 4). According to the Tanabe–Sugano diagram of 3d5 configuration, observed broad band emission can be attributed to the transition of Mn2 + from 4T1(4G) to 6A1(6S). Because the two energy levels have different slopes, the emission of Mn2 + is a broad band. The wavelength of emission peaks is affected by the crystal field strongly. It is well known that the emitting color can vary from green to deep red. Tetrahedrally coordinated Mn2 + (weak crystal

540

560

580 600 wavelength (nm)

620

640

Fig. 4. Emission spectra of samples.

field) usually gives a green emission, whereas octahedrally coordinated Mn2 + (strong crystal field) gives an orange to deep red emission [23–25]. All the Mn2 + and Si4 + co-doped samples show yellow–orange emission. This indicates that effective luminescent centers in the phosphors are mainly the Mn2 + that locates in the center of octahedron or distorted dodecahedron. According to the Tanabe–Sugano diagram of 3d5 configuration and some related literatures [24–26], several sharp lines at 492, 467, 450, 419 and 409 nm can be assigned to the transition of Mn2 + from the ground state 6A1(6S) to the excited states 4T1(4G), 4T2(4G), 4A1(4G), 4T2(4D) and 4E(4D), respectively. Fig. 4 shows the emission spectra of six samples under the excitation of 409 nm. From Fig. 4, the emission intensity of samples increases firstly, but decreases subsequently, with the increment of substitution content. The maximum of emission intensity appears when x ¼0.075 for Sam3. The wavelength of the emission peak moves to longer wavelength gradually (579, 579, 580, 586, 590 and

Z. Mu et al. / Journal of Luminescence 131 (2011) 676–681

592 nm). The electronic configuration of Mn2 + is (1s22s22p63s23p6) 3d5 3d5 electrons are in the outside of the other sublayers without any shield. Thus the energy level of 3d5 electrons is affected by crystalline field strongly. The analysis of XRD patterns shows that with the increment of substitution content, the interplanar distance decreases gradually, which results in the decrease in the bond length. Consequently, the crystal field effects around Mn2 + get stronger which enhances the splitting of 3d5 energy level. Thus the lowest energy of 3d5 energy level gets lower. This shows red shift in emission spectra macroscopically. Singh et al. [27] synthesized Mn2 + doped YAG with combustion process. In their work, Mn2 + doped YAG shows broad band emission peaking at 519 nm that is attributed to the transition of Mn2 + . The emission color in their work is obviously different from ours. The difference in emission color can be ascribed to the difference of chemical environment around Mn2 + . The Mn2 + in their work mainly locates in the center of tetrahedron with CN¼ 4. But in our experiments, Mn2 + participating in the luminescence locates in the center of octahedron with CN¼6 or distorted dodecahedron with CNo8. According to the literature [19], it is possible that some Mn2 + locate in the center of tetrahedron in our samples but their emission was not observed in our experiments. After repeating comparison and analysis, three possible reasons which result in the difference of the Mn2 + environment between the samples synthesized by Singh et al. and ours are as follows. Firstly, there exists difference in preparing methods. Combustion process was employed in the former but solid state reactions for the latter. Secondly, synthesized samples are not pure with impurity of YAlO3 but in our experiments all the samples present single garnet structure. Thirdly, in the former, only Mn2 + was doped into YAG with a small concentration(x¼0.01). Whereas in our experiments, Mn2 + and Si4 + replaced Y3 + and Al3 + with a high substitution content (xmax ¼0.30).

679

From Sam1 to Sam6, afterglow time increases firstly and decreases subsequently with the increment of substitution content, which is consistent with the changing trend of emission intensity of phosphors. Sam0 does not show afterglow while the others show yellow–orange long afterglow. Obviously, afterglow originates from doped Mn2 + in YAG. Fig. 5 shows the afterglow spectra of samples measured with Hitachi F-7000 fluorescence spectrophotometer. All samples can emit yellow–orange afterglow in only one broad band peaking near 585 nm. The position of afterglow emission peaks is consistent with the one of fluorescent emission peaks. The intensity of afterglow emission has the same changing trend with afterglow time when substitution content increases. Both of them can prove each other.

3.4. Decay curves Decay curves of all six samples were measured with thermoluminescent dosimeter. As shown in Fig. 6, the decay of samples is composed of an initial fast decay process and a subsequent slow decay process. In order to compare the afterglow properties of samples, the decay curves were fitted with the equation as follow [28]:     t t I ¼ I1 exp þ I2 exp ð1Þ

t1

t2

Here I denotes the afterglow emission intensity. I1 and I2 are two constants which are related with the initial intensity. t1 and t2 denote fast decay constant and slow decay constant, respectively. The fitting results are listed in Table 2. The data in Table 2 show that both two decay constants have the same changing trend. That is to say, they increase firstly, decreases subsequently with the increment of

Sam6

Sam1 Sam2 Sam3 Sam4 Sam5 Sam6

intensity (a.u.)

We investigated the long persistent luminescence properties of prepared samples under the excitation of UV light. Before measurement, samples were irradiated under the UV light with a peaking wavelength of 254 nm for 300 s. Then we observed the phenomenon of long afterglow in the dark. All the samples can long persistent emit yellow–orange light with different afterglow time except for Sam0. Sam3 has the longest afterglow time (18 min).

intensity (a.u.)

3.3. Afterglow spectra

50

60

70 80 time (s)

90

100

intensity (a.u.)

Sam5 Sam4 300

400

Fig. 6. Afterglow curves of samples. Subgraph shows the magnified part surrounded by the rectangle.

Sam3 Sam2

Table 2 Decay curve fitting results of samples.

Sam1 400

200 time (s)

100

0

500

600 wavelength (nm)

Fig. 5. Afterglow spectra of samples.

700

Samples

Sam1

Sam2

Sam3

Sam4

Sam5

Sam6

x

0.025 13.6 101.2

0.050 16.0 111.7

0.075 19.6 122.0

0.150 15.4 106.3

0.225 13.9 99.5

0.300 12.5 95.2

t1/s t2/s

680

Z. Mu et al. / Journal of Luminescence 131 (2011) 676–681

substitution content. This is in accordance with the changing trend of fluorescence intensity.

3.5. TL curves It is known that the long persistent luminescence is related to the concentration and depth of trap energy levels formed in the material for the sake of the existence of defects [29]. Generally speaking, materials can long persistent emit visible light in room temperature if traps with suitable concentration and depth are introduced. Usually, the effective method employed to study trap energy level is to detect the TL curves of materials. Fig. 7 shows the TL curves of six samples. From Fig. 7, Sam3 possesses the highest TL intensity with two obvious TL peaks at 137 and 269 1C, respectively. From Sam1 to Sam6, the TL intensity increases firstly, but decreases subsequently, which is consistent with the changing trend of fluorescence intensity. All the consistency mentioned above confirm that the long persistent luminescence comes from the transitional emission of Mn2 + in YAG. TL curves of samples were fitted to two Gaussion peaks with Origin software. The data obtained were listed in Table 3. From Table 3, one can see with the increment of substitution content, both TL peaks move to lower temperature. The temperatures corresponding to TL peaks of all samples are higher than 100 1C, which shows that samples present good TL efficiency under UV irradiation at higher temperature than room temperature. The depth of trap energy level corresponding to TL peaks can be

calculated according to the equation as follows [30]: E ¼ Tm =500

ð2Þ

Here Tm is the temperature corresponding to TL peaks with the unit of K (1 eV is corresponding to 500 K). From Eq. (2), the depth of trap energy level is in direct proportion to the absolute temperature corresponding to TL peaks. There exist two trap energy levels with different depth in all six samples. The data in Table 3 show that the depth of trap energy level corresponding to both TL peaks decreases gradually with the increment of substitution content. For the peak with lower temperature, the depth of trap energy level decreases from 0.834 eV of Sam1 to 0.782 eV of Sam6. For the peak with higher temperature, it decreases from 1.096 to 0.950 eV. Generally speaking, many factors affect TL intensity of materials, such as the concentration and transition probability from excited state to ground state of luminescent centers, the concentration of traps and depth of trap energy level, and so on. From luminescent centers, the product of concentration and transition probability, namely fluorescence intensity, has a very good consistence with TL intensity. The changing trend of trap concentration is too complicated to be analyzed with the present experiments. The analysis of TL spectra shows that the depth of trap energy levels decreases with the increment of substitution content, which is advantageous to the release of trapped electrons. Obviously, the change of fluorescence intensity plays a dominant role in the change of TL intensity, and the other factors are negligible.

4. Conclusions Yellow–orange long persistent emitting phosphors Mn2 + and Si co-doped YAG were prepared with solid state reactions. The structure and luminescence properties of phosphors were investigated in detail. The analysis of XRD patterns of samples shows that the substitution of Mn2 + and Si4 + to Y3 + and Al3 + make the interplanar distance decrease but does not change the single garnet crystal phase of the samples. The emission spectra show that samples can emit yellow–orange light in a broad band peaking from 579 to 592 nm with long afterglow under the excitation of UV and blue–violet light. With the increment of substitution content, the emission intensity of samples increases firstly, decreases subsequently. The highest emission intensity occurs when x ¼0.075 for Sam3. The emission peaks move to longer wavelength. Afterglow spectra and decay curves show that all the Mn2 + and Si4 + co-doped samples emit yellow–orange light with long afterglow after the irradiation of ultraviolet light. The longest afterglow time is 18 min for Sam3. Two kinds of traps with different energy level depth in co-doped samples were observed by means of TL detection, and their depth decreases with the increment of substitution content. Higher TL efficiency shows that the phosphors present a good potential for UV irradiation dosimeter applications. 4+

intensity (a.u.)

Sam3 Sam2

Sam4

Sam1 Sam5 Sam6

50

100

150

200 250 temperature (°C)

300

350

Fig. 7. TL curves of samples.

Table 3 Data about the TL of samples. Samples

Sam1 Sam2 Sam3 Sam4 Sam5 Sam6

Substitution content

0.025 0.050 0.075 0.150 0.225 0.300

Composition

+ + , Si40.025 Y2.975Al4.975O12: Mn20.025 + + Y2.950Al4.950O12: Mn20.050 , Si40.050 + + Y2.925Al4.925O12: Mn20.075 , Si40.075 + + Y2.850Al4.850O12: Mn20.150 , Si40.150 + + Y2.775Al4.775O12: Mn20.225 , Si40.225 + + Y2.700Al4.700O12: Mn20.300 :Si40.300

Temperature of TL peaks/1C

Depth of traps/eV

Lower

Higher

Lower

Higher

144 141 137 134 124 118

275 272 269 236 217 202

0.834 0.828 0.820 0.814 0.794 0.782

1.096 1.090 1.084 1.018 0.980 0.950

Z. Mu et al. / Journal of Luminescence 131 (2011) 676–681

Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos.21071034 and 20871033). References [1] P. Schlotter, J. Baur, C. Hielscher, et al., Mater. Sci. Eng., B 59 (1999) 390. [2] P. Schlotter, R. Schmidt, J. Schneider., Appl. Phys. A: Mater. Sci. Process 64 (1997) 417. [3] M. Veith, S. Mathur, A. Kareiva, et al., J. Mater. Chem. 9 (1999) 3069. [4] J.R. Lu, K. Ueda, H. Yagi, et al., J. Alloys Compd. 341 (2002) 220. [5] O.A. Lopez, J. McKittrick, L.E. Shea., J. Lumin. 71 (1997) 1. [6] D. Ravichandran, R. Roy, A.G. Chakhovskoi, et al., J. Lumin. 71 (1997) 291. [7] A. Yoshikawa, G. Boulon, L. Laversenne, et al., J. Appl. Phys. 94 (2003) 5479. [8] J.B. Gruber, B. Zandi, M.F. Reid, Phys. Rev. B. 60 (1999) 15643. [9] Y.M. Cheung, S.K. Gayen., Phys. Rev. B. 49 (1994) 14827. [10] M. Kottaisamy, P. Thiyagarajan, J. Mishra, et al., Mater. Res. Bull. 43 (2008) 1657. [11] K. Zhang, H. Liu, Y. Wu, et al., J. Mater. Sci. 42 (2007) 9200.

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

681

J.D. Kmetec, T.S. Kubo, T.J. Kane., Opt. Lett. 19 (1994) 186. H.S. Jang, W. Bin lm, D.C. Lee, et al., J. Lumin. 126 (2007) 371. M. Gervais, S. LeFloch, N. Gautier, et al., Mater. Sci. Eng. B. 45 (1997) 108. A. Katelnikovas, H. Bettentrup, D. Uhlich, et al., J. Lumin. 129 (2009) 1356. Y. Kuru, E.O. Savasir, S.Z. Nergiz, et al., Phys. Status Solidi C 5 (2008) 3383. L.Y. Zhou, W.C.H. Choy, J.X. Shi, et al., Mater. Chem. Phys. 100 (2006) 372. R.D. Shannon, Acta Crystallogr., A. Sect., Found. Crystallogr. 32 (1976) 751. J.A. Hodges., J. Phys. Chem. Solids. 35 (1974) 1385. J. Wang, S.B. Wang, Q. Su., J. Math. Chem. 14 (2004) 2569. J. Lin, D.U. Sanger, M. Mennig, et al., Thin Solid Films 360 (2000) 39. L. Lin, M. Yin, C.S. Shi, et al., J. Alloys Compd 455 (2008) 327. C.Y. Li, Q. Su, S.B. Wang., Mater. Res. Bull. 37 (2002) 1443. V. Singh, R.P.S. Chakradhar, J.L. Rao, et al., J. Lumin. 128 (2008) 1474. C.J. Duan, A.C.A. Delsing, H.T. Hintzen., J. Lumin. 129 (2009) 645. Y. Zorenko, V. Gorbenko, T. Voznyak, et al., J. Lumin. 130 (2010) 380. V. Singh, R.P.S. Chakradhar, J.L. Rao, et al., Appl. Phys. B: Lasers Opt 98 (2010) 407. A. Nag, T.R.N. Kutty., J. Alloys Compd 354 (2003) 221. A.J.J. Bos., Radiat. Meas. 41 (2006) S45. C.F. Guo, Q. Tang, C.X. Zhang, et al., J. Lumin. 126 (2007) 333.