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Origin of the generally defined absorption edge of non-stoichiometric lithium niobate crystals
Solid State Communications 141 (2007) 113–116 www.elsevier.com/locate/ssc
Origin of the generally defined absorption edge of non-stoichiometric lithium niobate crystals Xiaochun Li a , Yongfa Kong a,∗ , Hongde Liu a , Lei Sun a , Jingjun Xu a , Shaolin Chen b , Ling Zhang b , Ziheng Huang b , Shiguo Liu b , Guangyin Zhang b a The Key Lab of Advanced Technique and Fabrication For Weak-Light Nonlinear Photonics Materials, Ministry of Education, Nankai University,
Tianjin 300457, China b Tianjin Key Lab of Photonics Material and Technology for Information Science, Nankai University, Tianjin 300457, China
Received 6 June 2006; received in revised form 10 September 2006; accepted 14 October 2006 by D.J. Lockwood Available online 2 November 2006
Abstract The ultraviolet absorption edges of LiNbO3 crystals with different Li2 O contents and MgO doping concentrations were investigated. The generally defined absorption edges at absorption coefficient α = 15 or 20 cm−1 of all these crystals fit the Urbach rule perfectly. The origin of this absorption edges in non-stoichiometric LiNbO3 crystals is attributed to the presence of Li vacancies. c 2006 Elsevier Ltd. All rights reserved.
PACS: 78.70.Dm; 61.72.Ji Keywords: C. Intrinsic defects; D. Absorption edge
1. Introduction Lithium niobate (LiNbO3 , LN) is still the object of intense studies in basic and applied research. Though extensive papers have been published related with the ultraviolet (UV) absorption edge of LiNbO3 , a basic question is still not clear, that is what is the meaning of the generally defined absorption edge at absorption coefficient α = 15 or 20 cm−1 ? Interpretations of the optical properties of ABO3 compounds in the visible and ultraviolet range have generally been based on the electronic band-structure calculation of Kahn and Leyendeker for SrTiO3 [1]. In that work the fundamental optical transition is shown to occur in the BO6 octahedral, and the corresponding absorption coefficient α is 105 ∼ 106 cm−1 . Therefore, the absorption edge at α = 15 or 20 cm−1 is far from the intrinsic absorption. The shape of the generally defined absorption edge of congruent LiNbO3 (CLN) is spectrally consistent with the widely observed Urbach rule shape [2] α = α0 exp[σ (h¯ ω − h¯ ω0 )/kB T ] ∗ Corresponding author.
E-mail address: [email protected] (Y. Kong). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.10.021
(1)
where, kB is Boltzmann’s constant; h¯ is Planck’s constant divided by 2π ; α0 is the maximum value of α, h¯ ω0 is the photon energy corresponding to α0 , T is the room temperature with the value of 300 K, and σ is the fitting parameter corresponding to impurity. It has been known that the Urbach absorption is caused by the band-tail. While it is still unclear what causes the band-tail absorption in LiNbO3 , and what is the absorption centre at α = 15 or 20 cm−1 . Redfield et al. reported that a blue shift occurs to the position of UV absorption edge with increased Li2 O composition in non-stoichiometric LiNbO3 [2]. S. Kar et al. recently reported that the same blue shift occurred to the VTE processed LiNbO3 crystal [3]. For the MgO doped crystal, with the increase of MgO doping concentration, the blue shift was also observed [4]. So the absorption edge at α = 15 or 20 cm−1 has a direct relationship with the intrinsic defects. In this paper, the UV absorption edges of LiNbO3 crystals with different Li2 O contents and MgO dopants were investigated. From these experimental results, we deduced that the generally defined absorption edge at absorption coefficient α = 15 or 20 cm−1 is corresponding to Li ion vacancy.
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X. Li et al. / Solid State Communications 141 (2007) 113–116
Table 1 Description of MgO-doped LiNbO3 crystals in our experiments Sample no.
Doping level (mol%)
σ
Mg0.5 Mg1 Mg2 Mg3 Mg4 Mg6.5 Mg7
0.5 1.0 2.0 3.0 4.0 6.5 7.0
0.4027 0.3890 0.3683 0.3344 0.3416 0.4551 0.4303
σ is the fitting parameter of Urbach rule.
2. Experiments Congruent pure and MgO doped LiNbO3 crystals were grown using Czochralski method along c-axis. The congruent composition was selected as [Li]/[Nb] = 48.4/51.6. The notations and doping levels of MgO were listed in Table 1. All doping levels refer to the values introduced into the melt. The thickness of CLN and LiNbO3 :Mg samples used in this study is 63 µm and 1 mm respectively. The near-stoichiometric LiNbO3 (NSLN) was prepared by the vapour transport equilibration (VTE) method [5]. The as-grown pure crystal was firstly cut to 20 × 20 × 1.2 mm3 (x × y × z) plates, then put in a sealed platinum crucible with LiNbO3 powder with a [Li]/[Nb] ratio of 55/45. The VTE treatment was proceeded under 1050 ◦ C for 120 h. Finally these VTE-treated plates were polished to optical grade with a thickness of 1.0 mm (z). The Li2 O content of these VTE-treated plates is 49.9 mol% calibrated by the Raman scattering method [6]. The visible–ultraviolet transmission spectra of all crystals were measured with the BECKMAN DU-8B spectrophotometer at room temperature. The spectrum of NSLN after reduction was also measured. During reduction, the NSLN crystal was kept in an oven with argon atmosphere at 700 ◦ C for 6 h. Absorption coefficients were calculated based on the Mclean formula: ! r 1 1 (1 − R)2 2 α = − ln −b + b + 2 , b= , d R 2T R 2 where d is the thickness of the sample, T = I /I0 the transmittance, R = (n − 1)2 /(1 + n)2 with n denoting the refractive index. The wavelength and composition dependences of R were taken into account via Sellmeier equation [7]. 3. Results and discussion Fig. 1 shows the fitting results of the absorption edges of CLN and NSLN according to the Urbach rule. From this figure, we can see that the absorption edge perfectly fits Urbach rule, which indicates that the generally defined absorption edge is band-tail absorption caused by defects in the crystal lattice. Fig. 2 shows the transmittance spectra of CLN and NSLN, where the thickness of CLN and NSLN is 63 µm and 1.0 mm, respectively. From this figure we can see the transmission spectrum of CLN overlaps with that of NSLN at about 310 nm.
Fig. 1. Spectral dependences of Lnα for LiNbO3 crystals. The squares and circles indicate measured values for CLN and NSLN, respectively, and the real lines are fitting results according to Urbach rule.
Fig. 2. The transmittance spectra of LiNbO3 crystals, the thinkness of CLN and NSLN is 63 µm and 1 mm, respectively.
At this wavelength, the light intensity absorbed by these two samples can be thought as the same. It is known that the light intensity absorbed by crystal is proportional to the product of Lambert Beer absorption coefficient α and the sample thickness d along the light propagation direction. So, we can infer Eq. (2): ds αc = = 15.87 αs dc
(2)
where αs and αc are the Lambert Beer absorption coefficient of CLN and NSLN, respectively. Lambert Beer absorption coefficient α of material is defined as [8–10] α = shν N
(3)
where h is the Planck constant, s is the absorption crosssection for the special wave number ν, and N is the density of the absorption centre. For the same wavelength and the same absorption centre, the value of s for CLN and NSLN crystals can be regarded as equal. According to Eq. (3), we can get Eq. (4) : Nc = 15.87. Ns
(4)
X. Li et al. / Solid State Communications 141 (2007) 113–116
Fig. 3. Spectral dependences of Lnα for MgO-doped LiNbO3 crystals. The squares, circles and triangles indicate measured values for Mg0.5 , Mg2 and Mg7 , respectively, and the real lines are fitting results according to Urbach rule.
115
Fig. 4. The relationship between the fitting parameter σ of Urbach rule and MgO concentration in melt.
Eq. (4) indicates that absorption centres of CLN are 15.87 times than those of NSLN. There are many intrinsic defects in non-stiochiometric LiNbO3 crystals. According to Li vacancy model [11], the − main intrinsic defects are Li vacancies (VLi ) and antisite Nb 4+ ions (NbLi ). Because the energy level of Nb4+ Li is in about 1.6 eV [12], it can’t contribute to the UV absorption. Therefore, the band-tail absorption in LiNbO3 should be caused by Li vacancy. − The density of VLi can be calculated as: N=
y × 6.02 × 1023 M/ρ
(5)
− where y is the mole per cent of VLi , M is mole mass, and ρ is the density of crystal. According to Li-vacancy model [11] [Li1−x (NbLi )x/5 (VLi )4x/5 ]NbO3 , y is 4.18% and 0.264% for CLN and NSLN samples, respectively. So we get that the VLi density ratio of CLN and NSLN is 15.84, which is in good agreement with the experimental results of absorption spectra. Because the amount of VLi varies with the doping concentration of MgO, the UV absorption edges of series of Mg-doped LiNbO3 crystals were measured. The MgO concentration in the melt varied from 0.5 to 7 mol%. As shown in Fig. 3, the absorption edges of LiNbO3 :Mg crystals also vary exponentially with the photon energy following the Urbach rule, only Mg0.5 , Mg2 and Mg7 were drawn for clarity. The absorption slope is given as S = kBσT according to Eq. (1). So, σ can be calculated graphically from slope and the calculated results were also collected in Table 1. The relationship between σ and MgO concentration is shown in Fig. 4. The change trend of this relationship is almost identical with that between the amount of VLi and MgO concentration, which is reported by Iyi et al. to illuminate the defect model of MgO-doped LiNbO3 crystals [13]. The coincidence of the change trends of the two relationships also indicates that the UV absorption edge is strongly dependent on Li vacancy. Though oxygen vacancy (VO ) has been proved not to be the main intrinsic defect in LiNbO3 crystal, the amount of oxygen vacancy can be changed by oxidation–reduction. Fig. 5 shows the absorption spectra of NSLN before and after reduction.
Fig. 5. The absorption spectra of NSLN before and after reduction.
From this figure we can see that the oxygen vacancy density does not clearly influence the UV absorption. 4. Summary The ultraviolet absorption edges of LiNbO3 crystals with different Li2 O contents and MgO doping concentrations were investigated. The generally defined absorption edges at absorption coefficient α = 15 or 20 cm−1 of all these crystals fit the Urbach rule perfectly. From the absorption edge of CLN and NSLN, we deduced that the absorption centres are Li vacancies. The relationship between σ and Mg concentration indicates that the absorption edge is also decided by VLi content. Thus we can conclude that the generally defined absorption edge in nonstoichiometric LiNbO3 crystals is attributed to the presence of Li vacancies. In general, the energy band will widen with the decrease in the over all defects, which leads to the blue shift of the UV edge. Acknowledgements This work is partly supported by the National Natural Science Foundation of China (Grant No. 60578019) and Programme for Changjiang Scholars and Innovative Research Team in University.
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X. Li et al. / Solid State Communications 141 (2007) 113–116
References [1] A.H. Kahn, A.J. Leyendeker, Phys. Rev. A 135 (1964) 1321. [2] D. Redfield, W.J. Burke, J. Appl. Phys. 45 (1974) 4566. [3] S. Kar, R. Bhatt, V. Shukla, R.K. Choubey, P. Sen, K.S. Bartwal, Solid State Commun. 137 (2006) 283. [4] R.K. Choubey, P. Sen, P.K. Sen, R. Bhatt, S. Kar, V. Shukla, K.S. Bartwal, Opt. Mater. 28 (2006) 467. [5] P.F. Bordui, R.G. Norwood, D.H. Jundt, M.M. Fejer, J. Appl. Phys. 71 (1992) 875.
[6] U. Schlarb, S. Klauer, M. Wesselmann, K. Betzler, M. W¨ohlecke, Appl. Phys. A 56 (1993) 311. [7] U. Schlarb, K. Betzler, Phys. Rev. B 48 (1993) 15613. [8] F. Jermann, J. Otten, J. Opt. Soc. Amer. B 10 (1993) 2085. [9] Y. Liu, L. Liu, J. Opt. Soc. Amer. B 19 (2002) 2413. [10] Q. Dong, L. Liu, D. Liu, C. Dai, L. Ren, Appl. Opt. 43 (2004) 5016. [11] P. Lerner, C. Legras, J.P. Dumas, J. Cryst. Growth 3–4 (1968) 231. [12] H.J. Reyher, R. Schulz, O. Thiemann, Phys. Rev. B 50 (1994) 3609. [13] N. Iyi, K. Kitamura, Y. Yajima, S. Kimura, J. Solid State Chem. 118 (1995) 148.
Origin of the generally defined absorption edge of non-stoichiometric lithium niobate crystals Xiaochun Li a , Yongfa Kong a,∗ , Hongde Liu a , Lei Sun a , Jingjun Xu a , Shaolin Chen b , Ling Zhang b , Ziheng Huang b , Shiguo Liu b , Guangyin Zhang b a The Key Lab of Advanced Technique and Fabrication For Weak-Light Nonlinear Photonics Materials, Ministry of Education, Nankai University,
Tianjin 300457, China b Tianjin Key Lab of Photonics Material and Technology for Information Science, Nankai University, Tianjin 300457, China
Received 6 June 2006; received in revised form 10 September 2006; accepted 14 October 2006 by D.J. Lockwood Available online 2 November 2006
Abstract The ultraviolet absorption edges of LiNbO3 crystals with different Li2 O contents and MgO doping concentrations were investigated. The generally defined absorption edges at absorption coefficient α = 15 or 20 cm−1 of all these crystals fit the Urbach rule perfectly. The origin of this absorption edges in non-stoichiometric LiNbO3 crystals is attributed to the presence of Li vacancies. c 2006 Elsevier Ltd. All rights reserved.
PACS: 78.70.Dm; 61.72.Ji Keywords: C. Intrinsic defects; D. Absorption edge
1. Introduction Lithium niobate (LiNbO3 , LN) is still the object of intense studies in basic and applied research. Though extensive papers have been published related with the ultraviolet (UV) absorption edge of LiNbO3 , a basic question is still not clear, that is what is the meaning of the generally defined absorption edge at absorption coefficient α = 15 or 20 cm−1 ? Interpretations of the optical properties of ABO3 compounds in the visible and ultraviolet range have generally been based on the electronic band-structure calculation of Kahn and Leyendeker for SrTiO3 [1]. In that work the fundamental optical transition is shown to occur in the BO6 octahedral, and the corresponding absorption coefficient α is 105 ∼ 106 cm−1 . Therefore, the absorption edge at α = 15 or 20 cm−1 is far from the intrinsic absorption. The shape of the generally defined absorption edge of congruent LiNbO3 (CLN) is spectrally consistent with the widely observed Urbach rule shape [2] α = α0 exp[σ (h¯ ω − h¯ ω0 )/kB T ] ∗ Corresponding author.
E-mail address: [email protected] (Y. Kong). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.10.021
(1)
where, kB is Boltzmann’s constant; h¯ is Planck’s constant divided by 2π ; α0 is the maximum value of α, h¯ ω0 is the photon energy corresponding to α0 , T is the room temperature with the value of 300 K, and σ is the fitting parameter corresponding to impurity. It has been known that the Urbach absorption is caused by the band-tail. While it is still unclear what causes the band-tail absorption in LiNbO3 , and what is the absorption centre at α = 15 or 20 cm−1 . Redfield et al. reported that a blue shift occurs to the position of UV absorption edge with increased Li2 O composition in non-stoichiometric LiNbO3 [2]. S. Kar et al. recently reported that the same blue shift occurred to the VTE processed LiNbO3 crystal [3]. For the MgO doped crystal, with the increase of MgO doping concentration, the blue shift was also observed [4]. So the absorption edge at α = 15 or 20 cm−1 has a direct relationship with the intrinsic defects. In this paper, the UV absorption edges of LiNbO3 crystals with different Li2 O contents and MgO dopants were investigated. From these experimental results, we deduced that the generally defined absorption edge at absorption coefficient α = 15 or 20 cm−1 is corresponding to Li ion vacancy.
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X. Li et al. / Solid State Communications 141 (2007) 113–116
Table 1 Description of MgO-doped LiNbO3 crystals in our experiments Sample no.
Doping level (mol%)
σ
Mg0.5 Mg1 Mg2 Mg3 Mg4 Mg6.5 Mg7
0.5 1.0 2.0 3.0 4.0 6.5 7.0
0.4027 0.3890 0.3683 0.3344 0.3416 0.4551 0.4303
σ is the fitting parameter of Urbach rule.
2. Experiments Congruent pure and MgO doped LiNbO3 crystals were grown using Czochralski method along c-axis. The congruent composition was selected as [Li]/[Nb] = 48.4/51.6. The notations and doping levels of MgO were listed in Table 1. All doping levels refer to the values introduced into the melt. The thickness of CLN and LiNbO3 :Mg samples used in this study is 63 µm and 1 mm respectively. The near-stoichiometric LiNbO3 (NSLN) was prepared by the vapour transport equilibration (VTE) method [5]. The as-grown pure crystal was firstly cut to 20 × 20 × 1.2 mm3 (x × y × z) plates, then put in a sealed platinum crucible with LiNbO3 powder with a [Li]/[Nb] ratio of 55/45. The VTE treatment was proceeded under 1050 ◦ C for 120 h. Finally these VTE-treated plates were polished to optical grade with a thickness of 1.0 mm (z). The Li2 O content of these VTE-treated plates is 49.9 mol% calibrated by the Raman scattering method [6]. The visible–ultraviolet transmission spectra of all crystals were measured with the BECKMAN DU-8B spectrophotometer at room temperature. The spectrum of NSLN after reduction was also measured. During reduction, the NSLN crystal was kept in an oven with argon atmosphere at 700 ◦ C for 6 h. Absorption coefficients were calculated based on the Mclean formula: ! r 1 1 (1 − R)2 2 α = − ln −b + b + 2 , b= , d R 2T R 2 where d is the thickness of the sample, T = I /I0 the transmittance, R = (n − 1)2 /(1 + n)2 with n denoting the refractive index. The wavelength and composition dependences of R were taken into account via Sellmeier equation [7]. 3. Results and discussion Fig. 1 shows the fitting results of the absorption edges of CLN and NSLN according to the Urbach rule. From this figure, we can see that the absorption edge perfectly fits Urbach rule, which indicates that the generally defined absorption edge is band-tail absorption caused by defects in the crystal lattice. Fig. 2 shows the transmittance spectra of CLN and NSLN, where the thickness of CLN and NSLN is 63 µm and 1.0 mm, respectively. From this figure we can see the transmission spectrum of CLN overlaps with that of NSLN at about 310 nm.
Fig. 1. Spectral dependences of Lnα for LiNbO3 crystals. The squares and circles indicate measured values for CLN and NSLN, respectively, and the real lines are fitting results according to Urbach rule.
Fig. 2. The transmittance spectra of LiNbO3 crystals, the thinkness of CLN and NSLN is 63 µm and 1 mm, respectively.
At this wavelength, the light intensity absorbed by these two samples can be thought as the same. It is known that the light intensity absorbed by crystal is proportional to the product of Lambert Beer absorption coefficient α and the sample thickness d along the light propagation direction. So, we can infer Eq. (2): ds αc = = 15.87 αs dc
(2)
where αs and αc are the Lambert Beer absorption coefficient of CLN and NSLN, respectively. Lambert Beer absorption coefficient α of material is defined as [8–10] α = shν N
(3)
where h is the Planck constant, s is the absorption crosssection for the special wave number ν, and N is the density of the absorption centre. For the same wavelength and the same absorption centre, the value of s for CLN and NSLN crystals can be regarded as equal. According to Eq. (3), we can get Eq. (4) : Nc = 15.87. Ns
(4)
X. Li et al. / Solid State Communications 141 (2007) 113–116
Fig. 3. Spectral dependences of Lnα for MgO-doped LiNbO3 crystals. The squares, circles and triangles indicate measured values for Mg0.5 , Mg2 and Mg7 , respectively, and the real lines are fitting results according to Urbach rule.
115
Fig. 4. The relationship between the fitting parameter σ of Urbach rule and MgO concentration in melt.
Eq. (4) indicates that absorption centres of CLN are 15.87 times than those of NSLN. There are many intrinsic defects in non-stiochiometric LiNbO3 crystals. According to Li vacancy model [11], the − main intrinsic defects are Li vacancies (VLi ) and antisite Nb 4+ ions (NbLi ). Because the energy level of Nb4+ Li is in about 1.6 eV [12], it can’t contribute to the UV absorption. Therefore, the band-tail absorption in LiNbO3 should be caused by Li vacancy. − The density of VLi can be calculated as: N=
y × 6.02 × 1023 M/ρ
(5)
− where y is the mole per cent of VLi , M is mole mass, and ρ is the density of crystal. According to Li-vacancy model [11] [Li1−x (NbLi )x/5 (VLi )4x/5 ]NbO3 , y is 4.18% and 0.264% for CLN and NSLN samples, respectively. So we get that the VLi density ratio of CLN and NSLN is 15.84, which is in good agreement with the experimental results of absorption spectra. Because the amount of VLi varies with the doping concentration of MgO, the UV absorption edges of series of Mg-doped LiNbO3 crystals were measured. The MgO concentration in the melt varied from 0.5 to 7 mol%. As shown in Fig. 3, the absorption edges of LiNbO3 :Mg crystals also vary exponentially with the photon energy following the Urbach rule, only Mg0.5 , Mg2 and Mg7 were drawn for clarity. The absorption slope is given as S = kBσT according to Eq. (1). So, σ can be calculated graphically from slope and the calculated results were also collected in Table 1. The relationship between σ and MgO concentration is shown in Fig. 4. The change trend of this relationship is almost identical with that between the amount of VLi and MgO concentration, which is reported by Iyi et al. to illuminate the defect model of MgO-doped LiNbO3 crystals [13]. The coincidence of the change trends of the two relationships also indicates that the UV absorption edge is strongly dependent on Li vacancy. Though oxygen vacancy (VO ) has been proved not to be the main intrinsic defect in LiNbO3 crystal, the amount of oxygen vacancy can be changed by oxidation–reduction. Fig. 5 shows the absorption spectra of NSLN before and after reduction.
Fig. 5. The absorption spectra of NSLN before and after reduction.
From this figure we can see that the oxygen vacancy density does not clearly influence the UV absorption. 4. Summary The ultraviolet absorption edges of LiNbO3 crystals with different Li2 O contents and MgO doping concentrations were investigated. The generally defined absorption edges at absorption coefficient α = 15 or 20 cm−1 of all these crystals fit the Urbach rule perfectly. From the absorption edge of CLN and NSLN, we deduced that the absorption centres are Li vacancies. The relationship between σ and Mg concentration indicates that the absorption edge is also decided by VLi content. Thus we can conclude that the generally defined absorption edge in nonstoichiometric LiNbO3 crystals is attributed to the presence of Li vacancies. In general, the energy band will widen with the decrease in the over all defects, which leads to the blue shift of the UV edge. Acknowledgements This work is partly supported by the National Natural Science Foundation of China (Grant No. 60578019) and Programme for Changjiang Scholars and Innovative Research Team in University.
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X. Li et al. / Solid State Communications 141 (2007) 113–116
References [1] A.H. Kahn, A.J. Leyendeker, Phys. Rev. A 135 (1964) 1321. [2] D. Redfield, W.J. Burke, J. Appl. Phys. 45 (1974) 4566. [3] S. Kar, R. Bhatt, V. Shukla, R.K. Choubey, P. Sen, K.S. Bartwal, Solid State Commun. 137 (2006) 283. [4] R.K. Choubey, P. Sen, P.K. Sen, R. Bhatt, S. Kar, V. Shukla, K.S. Bartwal, Opt. Mater. 28 (2006) 467. [5] P.F. Bordui, R.G. Norwood, D.H. Jundt, M.M. Fejer, J. Appl. Phys. 71 (1992) 875.
[6] U. Schlarb, S. Klauer, M. Wesselmann, K. Betzler, M. W¨ohlecke, Appl. Phys. A 56 (1993) 311. [7] U. Schlarb, K. Betzler, Phys. Rev. B 48 (1993) 15613. [8] F. Jermann, J. Otten, J. Opt. Soc. Amer. B 10 (1993) 2085. [9] Y. Liu, L. Liu, J. Opt. Soc. Amer. B 19 (2002) 2413. [10] Q. Dong, L. Liu, D. Liu, C. Dai, L. Ren, Appl. Opt. 43 (2004) 5016. [11] P. Lerner, C. Legras, J.P. Dumas, J. Cryst. Growth 3–4 (1968) 231. [12] H.J. Reyher, R. Schulz, O. Thiemann, Phys. Rev. B 50 (1994) 3609. [13] N. Iyi, K. Kitamura, Y. Yajima, S. Kimura, J. Solid State Chem. 118 (1995) 148.