Luminescence properties and charge compensation of Sr3Al2O6 doped with Ce3+ and alkali metal ions

Luminescence properties and charge compensation of Sr3Al2O6 doped with Ce3+ and alkali metal ions

Materials Chemistry and Physics 124 (2010) 1094–1099 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www...

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Materials Chemistry and Physics 124 (2010) 1094–1099

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Luminescence properties and charge compensation of Sr3 Al2 O6 doped with Ce3+ and alkali metal ions Guanghuan Li a , Yawen Lai b , Tianjie Cui a , Hong Yu a , Darui Liu c , Shucai Gan a,∗ a b c

College of Chemistry, Jilin University, Changchun 130026, PR China College of Geoexploration Science and Technology, Jilin University, Changchun 130026, PR China College of Materials Science and Engineering, Jilin University, Changchun 130026, PR China

a r t i c l e

i n f o

Article history: Received 11 April 2010 Received in revised form 5 August 2010 Accepted 10 August 2010 Keywords: Inorganic compounds Ceramics Luminescence Optical properties

a b s t r a c t Sr3 Al2 O6 :Ce3+ phosphors were synthesized by a solid-state reaction method under mild reducing atmosphere of activated carbon. The effects of H3 BO3 flux on the luminescence intensity and the optimum concentration of Ce3+ for luminescence property have been investigated. The effect of a small amount of charge compensators like Li+ , Na+ , K+ and Rb+ on Sr3 Al2 O6 :Ce3+ phosphor has also been studied. Sr3 Al2 O6 :Ce3+ , R+ (R = Li, Na, K and Rb) exhibit superior blue emission around 460 nm to Sr3 Al2 O6 :Ce3+ and can be effectively excited by 395 nm light, which implies that efficient charge compensation can promote the luminescence of Ce3+ in Sr3 Al2 O6 . The Sr3 Al2 O6 :Ce3+ , R+ (R = Li, Na, K and Rb) have potential application as a blue phosphor for n-UV chip excited white LEDs. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The white light emitting diodes (LEDs) have attracted increasing attention in recent years due to their excellent properties such as low power consumption, long operating time, and environmental benefit [1,2]. Due to these excellent properties, white LEDs are expected to be new light sources in the illumination field. There are several ways to gain white light [3,4]. Among these, The most common and simple method to realize white-light LEDs is to combine an InGaN-based blue LED with a yellow phosphor material, such as YAG:Ce (YAG denotes yttrium aluminum garnet). In 1997, white LEDs, based on blue LED chips coated with a yellow emitting phosphor YAG:Ce were first reported [5]. The blue chip/YAG:Ce system has many advantages. However, because YAG:Ce emits a greenishyellow light, the emission from an InGaN-based blue LED coated with YAG:Ce is deficient in the red spectral region and its color rendering property is poor. White LEDs can also be made by coating a near ultraviolet (n-UV) emitting LED with a mixture of high efficient red, green and blue emitting phosphors [6], analogous to the way a fluorescent lamp work. This method yields light with better spectral characteristics, which render color better. Recently, many efforts have been made to improve the emission efficiency of LED chips in the near UV-to-deep blue range [7]. Therefore, it is necessary to provide phosphor compositions that are excitable in the near UV range and emit in the visible range.

∗ Corresponding author. Tel.: +86 431 88502259. E-mail address: [email protected] (S. Gan). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.08.039

Strontium based aluminate phosphors are novel and efficient luminescence materials. They are an important class of phosphors for their high quantum efficiency in the visible region [8], and can provide durable properties with ultraviolet light [9]. Sr3 Al2 O6 is a member family of cubic crystalline structure. The noteworthy feature is a puckered six-membered AlO4 tetrahedral ring, with the average bridging Al–O bond length slightly greater than the average non-bridging Al–O bond length. In the unit cell, eight such separate Al6 O18 rings surround each of eight vacant sites, and account for all 48 Al and 144 O atoms [10]. Recently, many studies on phosphors with strontium aluminate as a host based on their persistent luminescence and photoconductivity spectrum have been found. Zhang prepared Sr3 Al2 O6 :Eu2+ phosphor, and found that it exhibited a red broad emission band with emission peak at 612 nm under 472 nm excitation [11]. Sharma et al. synthesized Sr3 Al2 O6 :Pr3+ phosphor, and found that it is a good phosphor for UV application [12]. However, the optical properties of Ce3+ ions doped Sr3 Al2 O6 host have not been reported before. Ce3+ has only one outer electron and only two spin-orbital splitting 4f states (2 F5/2 , 2 F 7/2 ). Thus, its excited state energy structure is simpler than that of the other trivalent rare earth ions. Ce3+ -doped materials can generate luminescence from red to UV due to the strong crystal field dependence of its 5d–4f transition energy [13]. Actually, the Ce3+ ion has a wide application in luminescent materials, for example, Ce3+ has been researched extensively in YAG systems [14,15]. Generally, when a metal ion is substituted for an element with a different valency in the matrix, charge compensation is needed using ions such as Na+ or Li+ [16,17]. Liu et al. [18] studied the effect of different charge compensation models on the luminescent property of CaMoO4 : Eu3+ . Our pre-

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Fig. 2. Diffuse reflectance spectra of the Sr3 Al2 O6 host and Sr2.96 Al2 O6 :0.04Ce3+ . 3+

Fig. 1. XRD patterns of undoped host lattice (a) and Sr2.96 Al2 O6 :0.04Ce

(b).

vious work discussed the effects of charge compensation on the CaAl2 O4 :Eu3+ phosphor, and found that efficient charge compensation could enhance the luminescence intensity [19]. However, it should be noted that there are few reports about alkali metal ions and Ce3+ co-doped into strontium aluminates host [20]. In this paper, observed properties of Sr3 Al2 O6 :Ce3+ phosphor synthesized by solid state reaction are reported. The effects of concentration of Ce3+ , H3 BO3 and charge compensation on photoluminescence (PL) have been presented and analyzed. These new phosphors show great potential to act as blue phosphors for n-UV chip excited white LEDs. 2. Experimental The samples were synthesized through the solid-state reaction technique. Sr3 Al2 O6 :Ce3+ , R+ (R = Li, Na, K and Rb) were initially prepared by mixing stoichiometric amounts of SrCO3 (A.R.), Al2 O3 (A.R.), Ce2 (C2 O4 )3 (4N), and R2 CO3 (R = Li, Na, K, Rb) (A.R.), then adding a certain quantity of flux H3 BO3 . These powders were blended and milled thoroughly in an agate mortar for 3 h. Then the mixtures were transferred to alumina crucibles and annealed successively at 1250 ◦ C for 4 h under a reducing atmosphere created by burning activated carbon. In order to study the effects of adding H3 BO3 and alkali metal ions, three recipes are used during the preparation. (1) Sr3−x Al2 O6 :xCe3+ phosphors were prepared. The Ce3+ concentration is controlled as 1.0, 2.0, 3.0, 4.0 and 5.0 mol% and the H3 BO3 mass concentration remains constant at 6 wt%; (2) Sr2.96 Al2 O6 :0.04Ce3+ phosphors were prepared with 1.0, 2.0, 6.0 and 10.0 wt% H3 BO3 and the Ce3+ concentration remains constant at 4 mol%; (3) Sr2.92 Al2 O6 :0.04Ce3+ , 0.04R+ (Li, Na, K and Rb) were prepared with 6 wt% H3 BO3 . All the crystalline structures of samples were measured by a Rigaku D/maxIIB X-ray diffractometer with Cu K␣ radiation. The excitation and emission spectra were measured by a Hitachi F-7000 fluorescence spectrofluorometer equipped with a 150 W Xe lamp. The relative intensity was measured by PR-302 relative brightness meter. The diffuse reflectance spectra were obtained at room temperature by a Hitachi U-4100 spectrophotometer with the reflection of black felt (reflection 3%) and white Al2 O3 (reflection 100%) in the wavelength region of 200–600 nm. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) under 395 nm pulsed laser excitation. All the experiments were performed at room temperature.

Sr3 Al2 O6 host shows a high reflection in the visible range. The spectrum of Sr2.96 Al2 O6 :0.04Ce3+ displays four absorption bands between 200 and 420 nm attributed to the absorption of Ce3+ ion with the 4f–5d transition. The absorption band covers the spectral region of 200–420 nm, implying that the phosphor is suitable to UV or near-UV LED excitation. The excitation spectrum (PLE) and emission spectrum of Sr2.96 Al2 O6 :0.04Ce3+ phosphor are shown in Fig. 3. The PLE of Ce3+ in Sr3 Al2 O6 shows a broad absorption band in the range of 275–420 nm and peaking at 395 nm, which is due to The Ce3+ 4f1 → 5d1 transition of the Ce3+ ions. The lowest excitation band is situated at 395 nm, so Strokes shift is about 3577 cm−1 . The red shift of the excitation peak, which is expressed by the energy difference of the lowest 5d-excited level of Ce3+ (25,316 cm−1 ) in the present host compared with the free ion of Ce3+ (49,278 cm−1 ) [13], is 23,962 cm−1 . When excited under 395 nm, the emission spectrum of Sr2.96 Al2 O6 :0.04Ce3+ phosphor exhibits one dissymmetrical band at 460 nm. In general, the energy distribution of the fluorescence emission band accords with Gaussian distribution [21], namely,

 Ev = Ev0 exp

−2.772(v − v0 )2 ()

2

 (1)

3. Results and discussion The XRD patterns of undoped host lattice and Sr2.96 Al2 O6 :0.04Ce3+ are presented in Fig. 1. All peaks are well indexed to the Sr3 Al2 O6 phase (JCPDS card no. 24-1187). The doped Ce3+ ions do not induce any significant phase change. The host lattice Sr3 Al2 O6 has a cubic crystalline structure with space group Pa3, and its lattice parameters are a = 1.58440 nm [10]. The diffuse reflectance spectra of the Sr3 Al2 O6 host and Sr2.96 Al2 O6 :0.04Ce3+ are shown in Fig. 2. The spectrum of the

Fig. 3. The excitation and emission spectra of Sr2.96 Al2 O6 :0.004Ce3+ phosphor under 395 nm excitation and monitored at 460 nm.

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Table 1 Parameters describing the Gaussian fit of the 5d–4f emissions of Ce3+ in the Sr2.96 Al2 O6 :0.004Ce3+ phosphor. 5d–2 F5/2

Transition −1

Energy (cm E R2

)

5d–2 F7/2

21 978

20 534 1444 cm−1 0.999 53

where v is frequency, v0 is the frequency of peak value, Ev0 is the peak value,  is the full width at half maximum, and Ev is the luminescence energy at v. According to the above formula and using the Origin 7.5 software, the emission spectrum of the phosphor can be obtained through fitting the two Gaussian peaks, corresponding to the 5d–2 F5/2 and 5d–2 F7/2 transitions. The results of the Gaussian fits applied to the emission spectra are presented in Table 1. One band is centered at 455 nm (21,978 cm−1 , curve (c)), the other band is centered at 487 nm (20,534 cm−1 , curve (d)). The energy difference of the two bands is approximately 1444 cm−1 , and the broad extending from 400 to 600 nm can be obtained by combination of the two bands. Although this value is smaller than that between the two ground states 2 F5/2 and 2 F7/2 of Ce3+ (about 2000 cm−1 ), it is consistent with that of the Ce3+ doped YAG phosphors under high pressures [22]. We suggest that the crystal field in the Sr3 Al2 O6 lattice can probably influence the Ce3+ ions and reduce the energy difference between the levels of the ground state of Ce3+ . More details should be studied in future. According to Sánchez-Benítez work [23], adding H3 BO3 to reduce the synthesis temperature can not only provide a liquid medium and increase the Ce3+ diffusion rate but also yield crystal defects, which can trap the holes generated by the excitation of ions. Sr2.96 Al2 O6 :0.04Ce3+ phosphor was therefore prepared with the H3 BO3 flux fritting in order to improve the luminescence properties. The effect of H3 BO3 concentration on the emission spectrum of Sr2.96 Al2 O6 :0.04Ce3+ phosphor is illustrated in Fig. 4. The peak intensity at 460 nm of Sr2.96 Al2 O6 :0.04Ce3+ prepared with H3 BO3 flux fritting is much stronger that those prepared without H3 BO3 . The strongest PL intensity is obtained from the sample prepared with 6.0 wt% H3 BO3 . Further increase of H3 BO3 over more than 6 wt% decreases the PL intensity which is probably due to the excess crystal deformation of lattice [24]. Additionally, the structures of Sr2.96 Al2 O6 :0.04Ce3+ are found by XRD analysis shown in Fig. 5. The crystal structure of Sr2.96 Al2 O6 :0.04Ce3+ phosphors remains as pure phase at H3 BO3 concentration below 6.0 wt%. However, the boron ion can replace the Al ions in the structure and result in

Fig. 4. The effect of H3 BO3 concentration on the emission spectrum of Sr2.96 Al2 O6 :0.04Ce3+ phosphor.

Fig. 5. XRD spectrum of Sr2.96 Al2 O6 :0.04Ce3+ phosphors with various H3 BO3 concentrations (Ce = 4.0 mol%).

the Sr3 Al2 O6 lattice distortions due to its low ionic radius (B3+ : 0.023 nm) compared to the Al ion (Al3+ : 0.057 nm) [24]. As the concentration of H3 BO3 reaches 10 wt%, a second Sr3 B2 O6 phase is found. From Fig. 5, the peaks of SrAl2 O4 also appear when the H3 BO3 concentration is up to 10 wt%. A possible reason is that Al2 O3 reacted with Sr3 Al2 O6 to form SrAl2 O4 at this concentration. The addition of H3 BO3 during the preparation of Sr2.96 Al2 O6 :0.04Ce3+ phosphors is proved as an effective method to enhance the PL intensity and the optimized H3 BO3 content is 6.0 wt%. The addition of H3 BO3 during the preparation of Sr3 Al2 O6 :Ce3+ phosphors is proved as an effective method to enhance the PL intensity and the optimized H3 BO3 content as 6.0 wt%. In the following, we fixed the H3 BO3 content as 6.0 wt% and studied the optimized Ce dopant concentration for maximum PL intensity. As shown in Fig. 6, the emission intensities increase with increasing Ce3+ concentrations, and then decreases due to concentration quenching. When the value of x is about 0.04, the maximum emission intensity occurs. Fig. 7(a) and (b) shows the excitation and emission spectra of Sr2.92 Al2 O6 :0.04Ce3+ , 0.04R+ (R = Li, Na, K and Rb), respectively. The luminescence intensities are remarkably enhanced when the

Fig. 6. Emission spectra of Sr3−x Al2 O6 :xCe3+ phosphors with various Ce concentrations (H3 BO3 = 6.0 wt%; ex = 395 nm).

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Fig. 7. Excitation and emission spectra of Sr2.92 Al2 O6 :0.04Ce3+ ,0.04R+ (R = Li, Na, K and Rb).

lithium, sodium, potassium and rubidium ions are co-doped in Sr3 Al2 O6 :Ce3+ samples. The ionic radii size of Al3+ , Sr2+ , and Ce3+ are 0.057, 0.113 and 0.103 nm, respectively. When Ce3+ ions doped into Sr3 Al2 O6 host lattice, they may prefer to occupy the Sr2+ site rather than Al3+ site, because the radius of the Ce3+ is closer to that of the Sr2+ ion. Due to the nonequivalent substitution, the excess of positive charge in the lattice should be compensated for R+ (R = Li, Na, K and Rb), which may take place according to: 2Sr2+ = Ce3+ + R+ . Ce3+ and R+ co-doped in Sr3 Al2 O6 lattice leads to a charge balance and moreover a reduction in the Sr2+ vacancy concentration. Therefore, the charge compensation pattern of the present Sr3 Al2 O6 :Ce3+ /R+ samples is most likely of the form, since Ce3+ and R+ co-doped in Sr3 Al2 O6 matrix may induce a lattice distortion. In addition, by careful analyzing the spectra of all samples, it is easy to notice that the shapes and positions are very similar in the PLE spectra and PL spectra for all the phosphors by 395 nm excitation and monitoring at 460 nm, respectively. Namely, varying the charge compensators does not affect the shape and positions of PLE and PL in the same host lattices. The evident changes in the relative intensity of the excitation and emission spectra of the phosphors with different charge compensation approaches can be clearly observed. Lithium, sodium and potassium ions as charge compensation has been reported [25]. In the lattice of Sr3 Al2 O6 , the strontium ions Sr(1), Sr(2) and Sr(3) have six oxygen coordinations and Sr(4), Sr(5) and Sr(6) have nine, eight and seven oxygen coordinations, respectively. The distances of Sr–O bond length are ranging from 0.245 to 0.294 nm [10]. This distance appears to be appropriate for a significant influence of R+ on the surroundings of the Ce3+ ion [26]. The alkaline ions probably effectively incorporated into suitable lattice of Sr ions under high temperature. In Sr3 Al2 O6 :Ce3+ phosphors, the doped Ce3+ and alkaline ions are randomly distributed in the cation (Sr2+ ) sites of the host. The ionic radii of Li+ , Na+ , K+ and Rb+ are 0.060, 0.095, 0.133 and 0.148 nm, respectively. When alkali metal ions are co-doped into Sr3 Al2 O6 :Ce3+ , the coordination conditions for Ce3+ will be influenced, because the alkali metal cations with different radii in the host compound can result in some distortions of the sub-lattice structure around the luminescent centre ions and change the distances of O–Al and O–Ce. Thus, the relative intensities of excitation and emission for Sr3 Al2 O6 :Ce3+ would vary with different charge compensation approaches. K and

Table 2 The calculated lattice parameters of the phosphors. Phosphors

a (nm)

v (nm3 )

Sr2.96 Al2 O6 :0.04Ce3+ Sr2.92 Al2 O6 :0.04Ce3+ ,0.04Li+ Sr2.92 Al2 O6 :0.04Ce3+ 0.04Na+ Sr2.92 Al2 O6 :0.04Ce3+ 0.04K+ Sr2.92 Al2 O6 :0.04Ce3+ 0.04Rb+

1.58440 1.58447 1.58448 1.58596 1.58653

3.97736 3.97789 3.97796 3.98912 3.99342

Rb ions may induce a lattice distortion when they compensate the excess of positive charge in the lattice due to the ionic radius larger than Sr2+ (0.113 nm) [27]. In contrast, Li+ and Na+ , with an ionic radius smaller than Sr2+ , would not induce a lattice distortion. The incorporation of lithium and sodium ions can neutralize the charge generated resulting from Ce3+ substitution for Sr2+ , and thus stabilize the structure and enhance the luminescence. This deduction is in good agreement with the cell constants and unit cell volume of the sample shown in Table 2. Fig. 8 shows the fluorescence decay curve of d-f transition of Ce3+ ion in Sr3 Al2 O6 :Ce3+ , R+ (R = Li, Na, K and Rb) excited by pulsed laser (ex = 395 nm). The decay curve can be well fitted by an exponential equation:

 t

I = A exp −

(2)



where I is the phosphorescence intensity; A is a constant; t is the time, and  is decay time for the exponential components. The results, as shown in Table 3, indicate that the phosphors have different decay times. The obtained results in Table 3 show the values for the amplitude A and decay time . The order of decay time is Sr3 Al2 O6 :Ce3+ ,Li+ > Sr3 Al2 O6 :Ce3+ ,Rb+ > Sr3 Al2 O6 :Ce3+ ,K+ > Sr3 Al2 Table 3 Decay time for the exponential components of phosphors. Phosphors 3+

+

Sr2.92 Al2 O6 :0.04Ce ,0.04Li Sr2.92 Al2 O6 :0.04Ce3+ ,0.04Na+ Sr2.92 Al2 O6 :0.04Ce3+ ,0.04K+ Sr2.92 Al2 O6 :0.04Ce3+ ,0.04Rb+

A

 (ns)

0.00057 0.00007 0.00013 0.00050

25.373 17.779 18.972 23.881

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Fig. 8. fluorescence decay curve of Sr2.92 Al2 O6 :0.04Ce3+ ,0.04R+ (R = Li, Na, K and Rb).

O6 :Ce3+ ,Na+ . The fluorescence life times are short enough for potential applications in white LEDs.

4. Conclusions (1) Sr2.96 Al2 O6 :0.04Ce3+ phosphor has been synthesized by solidstate reaction. The excitation spectrum is a broadband extending from 300 to 400 nm. The emission spectrum shows a broadband, which can be resolved into two emission bands peaking at 455 and 487 nm corresponding to the transitions of 5d states to 4f 2 F5/2 and 2 F7/2 of the Ce3+ ion. The energy difference of the two bands is approximately 1444 cm−1 . We suggest that the crystal field in the Sr3 Al2 O6 lattice can probably influence the Ce3+ ions and reduce the energy difference between the levels of the ground state of Ce3+ . (2) Samples of phosphor powders prepared with 2.0, 3.0, 4.0 and 5.0 mol% Ce show that the fluorescence intensity increases with the concentration of Ce up to 4.0 mol% and then begin to decrease. (3) The addition of H3 BO3 during the preparation of Sr3 Al2 O6 :Ce3+ phosphors is proved as an effective method to enhance the PL intensity and the optimized H3 BO3 content is 6.0 wt%. (4) The luminescence intensities are remarkably enhanced when the lithium, sodium, potassium and rubidium ions are co-doped in Sr3 Al2 O6 :Ce3+ samples. Sr3 Al2 O6 :Ce3+ , R+ (R = Li, Na, K and Rb) phosphors may be promising blue phosphors for near UVexcited white LEDs.

Acknowledgments This present work was financially supported by the High Technology Research and Development Program Foundation of China, no.: 2007AA06Z202 (863), Natural Science Foundation of Jilin Province of China, no.: 20070405 and National Science and Technology Major Projects, no.: 2008ZX05018.

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