Spectroscopic characteristics of langasite (La3Ga5SiO14) and langatate (La3Ga5.5Ta0.5O14) crystals doped with Eu3+

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Optical Materials 30 (2008) 1007–1012 www.elsevier.com/locate/optmat

Spectroscopic characteristics of langasite (La3Ga5SiO14) and langatate (La3Ga5.5Ta0.5O14) crystals doped with Eu3+ S. Georgescu *, O. Toma, A.M. Chinie, L. Gheorghe, A. Achim, A.S. Stefan National Institute for Lasers, Plasma, and Radiation Physics, P.O. Box MG-36, 76900 Magurele, Romania Received 13 February 2007; received in revised form 11 May 2007; accepted 19 May 2007 Available online 10 July 2007

Abstract The spectroscopic properties of two partially disordered crystals langasite (LGS) and langatate (LGT) doped with Eu3+ were investigated. The fluorescence lifetimes of 5D0 and 5D1 excited levels are measured. The spectroscopic figures of merit R0, R2 (the areas of the electric-dipole transitions 5D0 ! 7F0 and, respectively, 5D0 ! 7F2 divided by the area of the magnetic-dipole one – 5D0 ! 7F1) and the maximum splitting of the 7F1 level (DE) are calculated from the luminescence spectra. The spectral overlapping between the luminescence from 5D0 and 5D1 levels was eliminated using pulsed excitation at 532 nm and suitable delays. We obtained R0(LGS) > R0(LGT), R2(LGS) < R2(LGT) and DE (LGS) > DE (LGS). Possible reasons (covalency, J-mixing effects) for these inequalities are discussed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.43.j; 78.55 Keywords: Partially disordered crystals; Langasite; Langatate; Fluorescence; Eu3+; Fluorescence probe

1. Introduction The crystals from the langasite family were initially intended as hosts for laser active media [1], but in present their main application is based on their very good piezoelectric characteristics [2,3]. Nowadays, langasite (La3Ga5SiO14) tends to replace quartz in high temperature applications [4]. Also, langasite is used for electro-optic Q-switch [5], for gas sensor [6], and so on. Recently, selftuning of Nd3+ in langasite crystals was obtained [7]. Two other members of the langasite family, langatate (La3Ga5.5Ta0.5O14) and langanite (La3Ga5.5Nb0.5O14) proved to have superior characteristics in some applications [8]. Langasite, langatate and langanite are partially disordered crystals, i.e., one of the crystallographic positions is shared by two different ions. This results in inhomogeneously broadened absorption and fluorescence lines. This was first considered an advantage for laser emis*

Corresponding author. E-mail address: [email protected]fim.ro (S. Georgescu).

0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.05.035

sion: wider absorption lines result in an efficient pumping while wider emission lines means tunability. Besides the inhomogeneous line broadening, the partial disorder could lead, in some instances, to the splitting of the optical lines. Such an effect was observed in other partially disordered crystals [9], namely calcium niobium gallium garnet and calcium lithium niobium gallium garnet doped with Nd3+. A multisite structure was observed in the low temperature spectra, and was assigned to various configurations in the first cationic coordination sphere. Langasite (LGS), langatate (LGT) and langanite (LGN) crystallize in the P321 space group, symmetry class 32 and are isostructural with the calciumgallogermanate (Ca3Ga2Ge4O14) [10]. The general formula is A3BC3D2O14 where A represents the dodecahedral positions (distorted Thompson cubes), B – octahedral positions and C, D – tetrahedral positions. La3+ occupies the position A. The local symmetry at this site is C2 [11]. In contrast with LGS where Ga3+ and Si4+ share with equal probability the tetrahedral positions D, in LGT the octahedral positions B are occupied by two different ions, Ga3+ and Ta5+ (also, with equal

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probability), while in LGN the positions B are occupied by Ga3+ and Nb5+. Ga3+ occupies the remaining positions (C and D). The structural difference between LGS and LGT (or LGN) consists in the placement of the shortest distance positions randomly occupied around the A site: four positions in the plane perpendicular on the C2 axis for LGS and two positions along the C2 axis in LGT and LGN. Besides, there is a larger charge difference between Ta5+ (or Nb5+) and Ga3+ than between Si4+ and Ga3+. Since the only difference between LGT and LGN consists in the nature of the pentavalent cation, we shall examine here only LGS and LGT. Eu3+ (4f6 configuration) is largely used as a probe of the surrounding symmetry in crystals (see Ref. [12] and references therein). In LGS and LGT crystals, Eu3+ substitutes the La3+ ion in the dodecahedral position. Eu3+ ion has a very convenient energy level scheme. The fluorescence emitted by the 5D0 level (non-degenerate) is commonly used to evaluate the site symmetry of the luminescent centres involving Eu3+. Thus, the number of resolved lines in the transition 5D0 ! 7F0 (if the site symmetry is low enough to allow this transition [13]) indicates the number of inequivalent luminescent centers. The ratio of the areas of the lines corresponding to the 5D0 ! 7F2 (hypersensitive electric-dipole [14]) and 5D0 ! 7F1 (magnetic-dipole) transitions is affected by the asymmetry and covalency – higher ratio for increasing deviation from inversion symmetry and/or higher covalency [15,16]. The aim of this paper is a comparative study of the spectroscopic properties of Eu3+ in LGS and LGT. We try to find out how the structural differences are reflected in the emission peculiarities. The interest of this study is motivated by the strong red luminescence of Eu3+ doped LGS and LGT when excited in UV or in green suggesting potential applications as phosphors of this type of materials.

The lifetime experiments were performed on europiumdoped LGS and LGT single crystals; in order to avoid the problems related to the anisotropy of the LGS and LGT crystals, in the fluorescence measurements powder samples were used. All the measurements were performed at room temperature. 3. Results and discussion The transition 5D0 ! 7F1 is a pure magnetic-dipole transition, allowed by the parity selection rule. Its probability can be calculated provided that the free ion wavefunctions are known. This transition is practically insensitive to the changes in the vicinity of the Eu3+ ion and is used as an internal standard [12,17,18]. It is a common practice to use the ratios 5

R2 ¼

areað D0 !7 F2 Þ 5

areað D0 !7 F1 Þ

ð1Þ

and 5

R0 ¼

areað D0 !7 F0 Þ 5

areað D0 !7 F1 Þ

ð2Þ

2. Experimental

to obtain structural information about the luminescence centres. For pumping in the 5L6 level of Eu3+ (Xenon lamp) the 5 D0, 5D1, 5D2, and 5D3 levels are populated. A simpler experimental situation is obtained for pumping with the second harmonic of the Nd:YAG laser. In this case, only 5 D0 and 5D1 levels are populated. At room temperature the 7F1 level is populated and the pump transition is 7 F1 ! 5D1 (Fig. 1) followed by a rather rapid transition to 5D0. In crystals with large phonons (as, for example, YAG) the luminescence emitted from the 5D1 level is very weak and, practically, all the recorded luminescence origi-

Eu-doped langasite and langatate were synthesized in our laboratory from high-purity La2O3, Ga2O3, SiO2, Ta2O5, Eu2O3, according to (La0.95Eu0.05)3Ga5SiO14 and (La0.97Eu0.03)3Ga5.5Ta0.5O14 formulas. The oxides were mixed in an agate balls mill and calcinated at 1500 °C for 24 h. Then the powder was pressed in pallets and the crystals were grown along the C-axis in platinum crucibles in nitrogen atmosphere, using the Czochralski method. The Eu3+ luminescence was excited with the second harmonic of the Solar II Nd:YAG laser or with Xenon lamp equipped with suitable filters and gathered into a GDM 1000 double monochromator equipped with a S20 photomultiplier in ‘photon counting’ configuration. The luminescence spectra and the decays were recorded with a TURBO MCS scaler. The preamplifier (EG&G Parc Model 115) included in the experimental set-up was modified to allow variable measure gates. Various delays between the trigger signal and the opening of the measure gate are also allowed.

Fig. 1. Part of the energy level scheme of Eu3+. For 532 nm excitation, the pump transition is 7F1 ! 5D1. The two pairs of downward arrows represent possible overlapping of fluorescence spectra.

S. Georgescu et al. / Optical Materials 30 (2008) 1007–1012

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nates in 5D0. On the other hand, for crystals with lower phonons, if the excitation takes place in 5D1 (or in a higher level), the 5D1 luminescence cannot be neglected any longer and the luminescence lines from 5D0 and 5D1 could overlap. An analysis of the energy level scheme of Eu3+ in various crystals and glasses indicates some possible spectral overlapping: 5D0 ! 7F1 with 5D1 ! 7F3 and 5D0 ! 7F2 with 5D1 ! 7F4 (see Fig. 1). In this case, in order to calculate the correct R2 and R0 ratios, the luminescence spectra from 5D0 and 5D1 levels should be separated. This can be easily performed if we take into account that the lifetime T1 of the 5D1 is shorter than the lifetime T0 of 5D0. 3.1. Lifetimes measurements For pumping with the second harmonic of the Nd:YAG laser, both 5D1 and 5D0 levels are populated. The decay of 5 D0 can be influenced by the decay of 5D1 in two ways: (i) by de-excitation of the population of the pump level 5 D1–5D0 and (ii) by the simultaneous measurement of both decays at a given wavenumber E. As an example, in Fig. 2 we present two normalized decay curves of 5D0 measured in LGT:Eu (3%): one measured at 17290 cm1 and other measured at 17257 cm1. The first illustrates the case (i) while the second corresponds to case (ii). The luminescence decay of 5D0 in LGS:Eu (5%) (recorded at 17290 cm1) is shown in Fig. 3. The level 5D1 lies at higher energy (19000 cm1) than the pump quanta (532 nm, 18797 cm1), but it can be reached by the pump transition (7F1 ! 5D1) since 7F1 level has a significant population at room temperature. Thus, the direct measurement of the lifetime of this level is possible. The experimental decays are shown in insets in Figs. 2 and 3. The values of the lifetimes T0 (5D0) and T1 (5D1) for langasite and langatate are given in Table 1. The decays of 5D0 level are close to exponential in both crystals. By

Fig. 2. The decay curves of 5D0 measured in LGT:Eu(3%) recorded at two different wavenumbers (17290 cm1 and 17257 cm1). Inset: the decay curve of 5D1 measured on the transition 5D1 ! 7F0 (19035 cm1).

Fig. 3. Decay curve of 5D0 measured in LGS:Eu (5%) at 17290 cm1. Inset: the decay curve of 5D1 measured on the transition 5D1 ! 7F0 (19030 cm1).

Table 1 Luminescence lifetimes T0 (5D0) and T1 (5D1) in LGS and LGT crystals Crystal

T0 (ls)

T1 (ls)

LGS:Eu (5%) LGT:Eu (3%)

1010 1035

26a 62a

a

Measured as the normalized area of the 5D1 decay.

contrast, the kinetics of 5D1 is not exponential due to cross-relaxation processes [19], so that the values given in Table 1 are averaged values (the lifetime of the 5D1 level was taken as the area under the normalized decay curve – the so-called ‘‘efficiency lifetime’’). 3.2. Fluorescence spectra For Xenon lamp continuous pumping in the level 5L6, the 5D0, 5D1, 5D2, and 5D3 levels are populated and their emission could overlap. This is the case of the transitions 5 D0 ! 7F1 and 5D1 ! 7F3. Though the population of 5D1 is much lower than the population of 5D0 (according to their lifetimes), the emission probability on the transition 5 D1 ! 7F3 can be greater and its emission intensity can be significant. As an example regarding the emission probability of the transition 5D1 ! 7F3, in the work [18], the probability of this transition was found to be about two times greater than the probability of 5D0 ! 7F1 for an europium-doped lithium borate glass. A comparison of the fluorescence spectra recorded for LGT:Eu (powder sample) in the 16500–17500 cm1 spectral domain for CW pumping (Xenon lamp, in 5L6 level) and pulsed excitation (in 5D1, delay 330 ls, gate duration 2500 ls) is given in Fig. 4. Since the maximum of the transition 5D0 ! 7F0 seems to be less affected by the overlapping with the transition 5D1 ! 7F3 (we checked the shape of 5D0 ! 7F0 transition for various delays), the spectra were normalized to have the same maximum intensity of

1010

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Fig. 4. Black line: evidence of the overlapping of 5D0 ! 7F1 and 5 D1 ! 7F3 transitions in the fluorescence spectra of a LGT:Eu sample excited with a CW Xenon lamp in 5L6 level. Grey line: the influence of 5D1 luminescence was removed using pulsed laser pumping and a 330 ls delay for the measure gate of the preamplifier. 5

D0 ! 7F0. We attributed the difference between the spectral features in wavenumber domain 16500–17250 cm1 to the contribution of 5D1 level. In the fluorescence spectra shown in Figs. 5 and 6 the contribution of 5D1 was eliminated and the correct values of the ratios R2 and R0 could be calculated. In a recent paper [20], we analyzed the low temperature fluorescence spectra of Eu3+ in LGS and LGT in relation with the structural differences. Thus, the 5D0 ! 7F0 fluorescence line of LGS:Eu is well fitted with a single Gaussian suggesting the presence of only one fluorescent center (or quasi-center [1]). In contrast, at least two fluorescence lines were observed in the fluorescence spectrum of 5D0 ! 7F0 transition in LGT:Eu. We explained these differences by different configurations in the anionic positions randomly occupied at shortest distances: many (16) possibilities of occupation of four D positions by Ga3+ and Si4+ (with

Fig. 6. Fluorescence spectrum of LGT:Eu powder excited at 532 nm (delay: 330 ls, gate: 2500 ls).

lower charge difference) in LGS and few (only 4) possibilities of occupation of two B positions by Ga3+ and Ta5+ (with higher charge difference) in LGT. In order to compare materials with different refraction indexes, in [21] was introduced a ‘‘corrected’’ R2 ratio: " # 9n2 areað5 D0 !7 F2 Þ 0 R2 ¼ ð3Þ 5 2 ðn2 þ 2Þ areað D0 !7 F1 Þ h i 2 where ðn29nþ2Þ2 is the correction factor (which takes into account electric- and magnetic-dipole crystal field corrections) and n is the refraction index of the material. A similar correction will be applied to the ratio R0. The refractive indexes of both LGS and LGT crystals as well as the Sellmeier coefficients are given in [22]. Since the LGS and LGT are anisotropic and the fluorescence spectra were recorded on powders, we considered values averaged over the orientations. We obtained the following values for the correction factors: 1.03 for LGS and 1.02 for LGT (we h i 2 note that ðn29nþ2Þ2 ¼ 1 for n = 2 and the refraction indexes for both LGS and LGT are close to 2). The values R2 ðR02 Þ and R0 ðR00 Þ are collected in Table 2. The first result is R02 ðLGTÞ > R02 ðLGSÞ. The value of R2ðR02 Þ (which, for Eu3+, is proportional with the Judd– Ofelt parameter X2) depends on the deviation from the inversion symmetry and the covalency [12,13,15,23,24]. The larger asymmetry and/or covalency, the larger R2ðR02 Þ is obtained. The site symmetry at the La3+ (Eu3+) position is C2 for both crystals. Although the neighborhood of the Eu3+ ion in LGS and LGT is not identical, Table 2 R2 ðR02 Þ; R0 ðR00 Þ and R ¼ R00 =R02 ratios, crystal field parameter B20, and the maximum splitting of 7F1 level DE

Fig. 5. Fluorescence spectrum of LGS:Eu powder excited at 532 nm (delay: 330 ls, gate: 2500 ls).

Crystal

R2 ðR02 Þ

R0 ðR00 Þ

R ¼ R00 =R02

B20 (cm1)

DE (cm1)

LGS LGT

4.1 (4.2) 5.9 (6.0)

0.25 (0.26) 0.11 (0.11)

0.62 0.18

950 700

350 260

S. Georgescu et al. / Optical Materials 30 (2008) 1007–1012

we considered that this difference is not sufficiently large to explain the important variation of R2. Therefore, we examined the influence of the covalency on the variation of this parameter. A measure of covalency is the nephelauxetic effect [25]. The nephelauxetic effect lowers the barycenters of the energy levels in rapport with the free ion values. A measure of the nephelauxetic effect is the red shift of the 5D0 ! 7F0 transition [26,27]. If Eu3+ manifests a different degree of covalency in LGS and LGT this should be reflected in a different red shift of 5D0 ! 7F0. In Fig. 7 are shown the fluorescence lines corresponding to the 5D0 ! 7F0 transition in LGS and LGT. In rapport with the position of the gaseous Eu3+ (17374 cm1, [28]) we observe a different red shift: 87 cm1 for LGS and 98 cm1 for LGT. It results that the Eu–O bond is more covalent in LGT:Eu. This fact could explain the higher R2 ðR02 Þ value in LGT:Eu. Another interesting result is R00 ðLGTÞ < R00 ðLGSÞ. The non-vanishing probability of the 5D0 ! 7F0 electric-dipole transition was explained by the contribution of the linear crystal field terms and J-mixing effects [23,29,30]. In the frame of the J-mixing model the 5D0 ! 7F0 transition ‘‘borrows’’ intensity from 5D0 ! 7F2. According to [30], the ratio R of the intensities of 5D0 ! 7F0 and 5D0 ! 7F2 transitions is proportional with the square of the 0 crystal R field parameter B20. With our notations, R ¼ R00 / B220 . 2 The crystal field parameter B20 can be estimated from the 5 7 positions of the three lines of the D0 ! F1 transition [30]. Though the energy level scheme of Eu3+ in LGT and LGS is not yet clarified and the multisite structure in LGT is proven by the splitting of the line corresponding to the 5D0 ! 7F0, a rough estimation of B20 from the fluorescence spectra (Figs. 5, 6) could be done. We obtained B20  950 cm1 for LGS and B20  700 cm1 for LGT (Table 2). We can now calculate the ratio R(LGT)/ R(LGS) = 0.18/0.62 = 0.29 < 1 from the experimental data (Table 2). On the other hand, B220 ðLGTÞ=B220 ðLGSÞ  0:54 < 1. Therefore, we can say that, if not dominant,

Fig. 7. 5D0 ! 7F0 transition of Eu3+ in LGS (thick line) and LGT (thin line). The position of this transition for gaseous Eu3+ [28] is marked with an arrow.

1011

J-mixing could play a significant role in the intensity of D0 ! 7F0 transition in LGS and LGT. Another parameter which can be extracted from the fluorescence spectra is the maximum splitting of 7F1 level, DE. We obtained 350 cm1 in LGS (see Fig. 6) and 260 cm1 in LGT (Fig. 5). Since the maximum splitting of 7F1 is proportional with crystal field strength parameter [31,32], we conclude that the strength of the crystal field is higher in LGS than in LGT. This result is consistent with the increase of the distance La–O from LGS to LGT [33,34]. 5

4. Conclusions The fluorescence properties of LGS:Eu and LGT:Eu were investigated and compared. We measured the R0 and R2 ratios with accuracy eliminating the spectral overlapping of transitions 5D0 ! 7F1 with 5D1 ! 7F3 and 5 D0 ! 7F2 with 5D1 ! 7F4 using a convenient delay in the recording of the fluorescence spectra. We found that R2(LGT) > R2(LGS); this result is in agreement with the increase of the red shift of 5D0 ! 7F0 line from LGS to LGT, denoting an increase of the covalency from LGS to LGT. On the other hand, R0(LGT) < R0(LGS). We found a qualitative explanation of this relation in the frame of J-mixing model. The maximum splitting of 7F1 level was greater in LGS than in LGT; since this splitting is proportional with crystal field strength parameter, we concluded that the strength of the crystal field is higher in LGS than in LGT. References [1] A.A. Kaminskii, I.M. Silvestrova, S.E. Sarkisov, G.A. Denisenko, Phys. Stat. Solid A 80 (1983) 607. [2] J. De´taint, J. Schwartzel, A. Zarka, B. Capelle, J.P. Denis, E. Phillipot, in: Proceedings of the IEEE International Frequency Control Symposium, 1994, pp. 58–71. [3] R.C. Smythe, R.C. Helmbold, G.E. Hague, K.A. Snow, in: IEEE Trans. UFFC 47, 2000, pp. 355–360. [4] H. Fritze, H. Seh, H.L. Tuller, G. Borchardt, J. Eur. Ceram. Soc. 21 (2001) 1473. [5] H. Kong, J. Wang, H. Zhang, X. Yin, S. Zhang, Y. Liu, X. Cheng, L. Gao, X. Hu, M. Jiang, J. Cryst. Growth 254 (2003) 360. [6] H. Seh, H.L. Tuller, H. Fritze, Sensor Actuat. B 93 (2003) 169. [7] I. Aramburu, I. Iparraguirre, M.A. Illarramendi, J. Azkargorta, J. Ferna´ndez, R. Balda, Opt. Mat. 27 (2005) 1692. [8] B. Chai, H. Qiu, Y.Y. Ji, J.L. Lefaucheur, in: Proceedings of the IEEE International Frequency Control Symposium, 1999, pp. 821–828. [9] A. Lupei, V. Lupei, L. Gheorghe, L. Rogobete, E. Osiac, A. Petraru, Opt. Mat. 16 (2001) 403. [10] B.V. Mill, A.V. Butashin, G.G. Kodzhabagian, E.L. Belokoneva, N.V. Belov, Doklady Akademii Nauk SSSR 264 (1982) 1385. [11] V.N. Molchanov, B.A. Maksimov, A.F. Kondakov, T.S. Chernaya, Yu. V. Pisarevsky, V.I. Simonov, JETP Lett. 74 (2001) 222. [12] R. Reisfeld, E. Zigansky, M. Gaft, Mol. Phys. 102 (2004) 1319. [13] G. Blasse, A. Bril, Philips Res. Repts. 21 (1966) 368. [14] C.K. Jørgensen, B.R. Judd, Mol. Phys. 8 (1964) 281. [15] G. Blasse, A. Bril, W.C. Nieuwpoort, J. Phys. Chem. Sol. 27 (1966) 1587. [16] H. Ebendorff-Heidepriem, D. Ehrt, J. Non-Cryst. Solid 248 (1999) 247.

1012

S. Georgescu et al. / Optical Materials 30 (2008) 1007–1012

[17] C. Go¨rller-Walrand, L. Fluyt, A. Ceulemans, J. Chem. Phys. 95 (1991) 3099. [18] P. Babu, C.K. Jayasankar, Physica B 279 (2000) 262. [19] M. Bettinelli, C.D. Flint, J. Phys.: Condens. Matter 3 (1991) 4433. [20] S. Georgescu, O. Toma, A. Achim, A.M. Chinie, L. Gheorghe, A. Stefan, in: International Conference Micro- to Nano-Photonics ROMOPTO 06, August 28-31, 2006, Sibiu, Romania, to be published in Proceedings SPIE. [21] M. Wachtler, A. Speghini, K. Gatterer, H.P. Fritzer, David Ajo, M. Bettinelli, J. Am. Ceram. Soc. 81 (1998) 2045. [22] J. Stade, L. Bohaty´, M. Hengst, R.B. Heimann, Cryst. Res. Technol. 37 (2002) 1113. [23] W.C. Nieuwpoort, G. Blasse, Solid State Commun. 4 (1966) 227. [24] J.A. Capobianco, P.P. Proulx, M. Bettinelli, F. Negrisolo, Phys. Rev. B 42 (1990) 5936.

[25] C.K. Jørgensen, Modern Aspects of Ligand Field Theory, NorthHolland, Amsterdam, 1971. [26] E. Antic-Fidancev, J. Alloys Comp. 300–301 (2000) 2. [27] L.D. Carlos, O.L. Malta, R.Q. Albuquerque, Chem. Phys. Lett. 415 (2005) 238. [28] G.S. Ofelt, J. Chem. Phys. 38 (1963) 2171. [29] Z.J. Kiss, H.A. Weakliem, Phys. Rev. Lett. 15 (1965) 457. [30] M. Tanaka, G. Nishimura, T. Kushida, Phys. Rev. B 49 (1994) 16917. [31] F. Auzel, O.L. Malta, J. Phys. 44 (1983) 201. [32] O.L. Malta, E. Antic-Fidancev, M. Lemaitre-Blaise, A. Milicic-Tang, M. Taibi, J. Alloys Comp. 228 (1995) 41. [33] A.A. Kaminskii, B.V. Mill, G.G. Khodzhabagyan, A.F. Konstantinova, A.I. Okorochkov, I.M. Silvestrova, Phys. Stat. Solid (a) 80 (1983) 387. [34] H. Takeda, K. Sugiyama, K. Inaba, K. Shimamura, T. Fukuda, Jpn. J. Appl. Phys. 36 (1997) L919.