Subsolidus phase relationships in the system ZnO–Li2O–WO3

Journal of Alloys and Compounds 460 (2008) 142–146

Subsolidus phase relationships in the system ZnO–Li2O–WO3 Peiwen Lv a , Dagui Chen a , Wei Li a , Liping Xue a , Feng Huang a,∗ , Jingkui Liang a,b,c,∗ a

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China b Beijing National Laboratory for Condensed Matter Physics, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China c International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China Received 8 February 2007; received in revised form 28 May 2007; accepted 1 June 2007 Available online 7 June 2007

Abstract The subsolidus phase relationships of the system ZnO–Li2 O–WO3 have been investigated by X-ray diffraction (XRD) analyses. There are one ternary compound, five binary compounds and eight 3-phase regions in this system. The new ternary compound Li2 Zn2 W2 O9 was found by the powder diffraction pattern. The corresponding crystal structure of this compound was refined by Rietveld profile fitting method. It belongs to a ¯ and lattice constants are a = 5.1438(2) A, ˚ c = 14.1052(3) A, ˚ and its thermal property was studied. trigonal system with space group P 3c1 © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; X-ray diffraction; Phase diagram; Rietveld profile fitting; Li2 Zn2 W2 O9

1. Introduction ZnO has attracted numerous attentions for its promising applications on UV light-emitters, transparent high power electronics, surface acoustic wave devices, piezoelectric transducers, gas sensing and solar cells [1,2]. For achieving such applications, one of the most important goals is to obtain high quality ZnO films homoepitaxially grown on the ZnO single crystal substrates. Currently, there are three major methods for bulk ZnO single crystal growth: hydrothermal solution growth [3–5], the vapor phase transmission growth [6,7] and melting growth [8–10]. The growth speed used in hydrothermal solution method is very slow and the size of the crystal is restricted by the container [3–5]. The growth condition of the vapor phase transmission growth is very difficult to manipulate [6] and it should procure more progress in growth speed and cost cutting. The Czochralski method is frequently used for growing large single

∗

Corresponding authors. E-mail addresses: [email protected] (F. Huang), [email protected] (J. Liang). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.005

crystal [8,9] and the use of a modified Bridgman configuration in the melting growth process promise very good quality crystals with low defects [9–12]. However, it seems not suitable for growing ZnO single crystal because of the high melting point (1975 ◦ C) and high volatility of ZnO at high temperature. In order to suppress the ZnO evaporation and reduce the crystal defects during crystal growth, the crystal must be grown from a solvent at a relative low growth temperature [8]. Accordingly, appropriate fluxes must be found for the crystal growth. Based on the phase relationship of ZnO–Li2 O–MoO3 [13] and K2 O–ZnO–AO3 (A = Mo, W) [14] studied early, we focus on the element Li and W which are widely used in fluxes. According to previous study on the Li2 O–WO3 system, five binary compounds (Li6 WO6 , Li4 WO5 , Li2 WO4 , Li2 W2 O7 and Li2 W4 O13 ) were reported [15,16]. Moreover, Parmentier reported another binary compound Li2 W5 O16 [17]. Accordingly, the phase transition point for Li4 WO5 is 690.1 ◦ C, the congruous melting point for Li2 W2 O7 is 745.1 ◦ C and the incongruous melting point for Li2 WO4 and Li2 W5 O16 is 740.1 ◦ C and 820 ◦ C, respectively [15–17]. Although the phase relationships of ZnO–Li2 O binary system have not well studied, experimenter has synthesized three binary compounds Li2 ZnO2 , Li4 ZnO3 and Li6 ZnO4 [18–20].

P. Lv et al. / Journal of Alloys and Compounds 460 (2008) 142–146

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ation (45 kV × 40 mA) using continuous mode at a rate of 2θ = 4◦ /min. XRD data used for ternary compound Li2 Zn2 W2 O9 structural analyses and the patterns for Rietveld profile fitting were collected by step scan mode with a step width of 2θ = 0.017◦ and a sampling time of 0.0071◦ /s. The DTA investigation was conducted by NETZSCH-STA449C (Germany) in platinum crucible. The measurements were performed in the atmosphere of air in the temperature range 30–1200 ◦ C. The heating rate was 10 K/min and the reference substance was ␣-Al2 O3.

3. Result and discussion 3.1. Phase relationship of the system ZnO–Li2 O–WO3

Fig. 1. Subsolidus phase relationships of the system ZnO–Li2 O–WO3 .

In the ZnO–WO3 binary system, there is only one intermediate phase ZnWO4 exists [21,22]. The congruous melting point for ZnWO4 is as high as 1210.5 ◦ C [21]. Therefore, in this work, we study on the phase relationship of the ZnO–Li2 O–WO3 ternary system, as part of work in finding suitable fluxes for growing large ZnO single crystal at relative low temperature. 2. Experimental All samples were prepared by standard solid-state chemistry reaction in air with the purity of the starting materials (ZnO, Li2 CO3 , WO3 ) higher than 99.9%. The compositions of prepared samples (more than 40 samples, and their chemical composition are shown in Fig. 1) in appropriate chemical proportions were weighed precisely, and then homogenized by carefully ground, and pressed into pellets under the pressure around 108 Pa. The pellets were sintered in air for several times. Each time would last for about 72 h at around 550–700 ◦ C and then the pellets were cooled in the furnace to room temperature. The sintering temperature and times of individual samples with different composition depend on chemical properties of their initial mixtures. After each time the pellets were triturated and identified by X-ray powder diffraction (XRD) method, until the powder diffraction patterns of the samples showed no changes between two consecutive heating stages. The temperature of the furnace was measured with a Pt–PtRh thermocouple and was precisely controlled to within ±2 ◦ C up to 1200 ◦ C with an intelligent controller. Phase identification of the samples was carried out on an automated Philips PW3040/60 high resolution diffractometer with Cu K␣ radi-

3.1.1. Binary System 3.1.1.1. Li2 O–WO3 System. Under our experimental condition, we only found four binary compounds: Li4 WO5 , Li2 WO4 , Li2 W2 O7 and Li2 W5 O16 and the sintering temperature was 700, 640, 690, 690 and 700 ◦ C and metastable Li6 WO6 could not be obtained. The known structures of binary compounds Li2 WO4 and Li2 W2 O7 were refined via Rietveld structure refinement [23,24]. The indexing result of Li4 WO5 is also in good agreement with refs. [26] and [27]. Structure parameters are listed in Table 1, which shows that our results are consistent with references. 3.1.1.2. Li2 O–ZnO binary System. While under our experimental conditions (solid-state reactions below 700 ◦ C in the air), no binary compound was found which may due to the short sinter time [16] and the different conditions for sample treatment [19,20]. 3.1.1.3. ZnO–WO3 binary System. In the ZnO–WO3 binary system, our result is in good agreement with the report that there is only one intermediate phase ZnWO4 exists [21,22]. The compound was easily obtained via solid-state reaction under 700 ◦ C. The known structures of ZnWO4 [25] were also refined via Rietveld structure refinement (listed in Table 1). 3.1.2. Subsolidus phase relationship of the system ZnO–Li2 O–WO3 There was no study of ZnO–Li2 O–WO3 ternary system and no ternary compound has been reported before. In this work, we have found a new compound: Li2 Zn2 W2 O9 through solid-state

Table 1 Comparison of crystal parameters of ternary compounds Binary compound

Li2 WO4 Li2 W2 O7 ZnWO4 ␣Li4 WO5 ␤Li4 WO5

Space group

R-3 R-3 P-1 P-1 P2/c P2/c Cubic Cubic P-1 P-1

Cell parameter

Reference (◦ )

˚ a (A)

˚ b (A)

˚ c (A)

α

14.3497 14.3610 8.2860 8.2830 4.6895 4.6920 8.3157 8.2900 5.1090 5.109

14.3497 14.3610 7.0460 7.0500 5.7171 5.7210 8.3157 8.2900 7.7407 7.715

9.5948 9.6030 5.0545 5.0370 4.9242 4.9280 8.3157 8.2900 5.0691 5.060

90 90 85.3621 85.4000 90 90 90 90 101.502 101.80

β

(◦ )

90 90 102.1864 102.1300 90.6479 90.6320 90 90 101.397 101.78

γ

(◦ )

120 120 110.3743 110.2900 90 90 90 90 108.671 108.77

This work Ref. [23] This work Ref. [24] This work Ref. [25] This work Ref. [26] This work Ref. [27]

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P. Lv et al. / Journal of Alloys and Compounds 460 (2008) 142–146

Table 2 List of calculated and observed intensities and d-spacing for the powder pattern of Li2 Zn2 W2 O9 hkl

dcalc

dobs

Icalc

Iobs

hkl

dcalc

dobs

Icalc

Iobs

002 010 012 004 104 110 112 113 020 016 114 024 008 116 120 018

7.053 4.455 3.766 3.526 2.765 2.572 2.416 2.256 2.227 2.079 2.078 1.883 1.763 1.735 1.684 1.639

7.053 4.454 3.766 3.526 2.765 2.572 2.416 2.256 2.227 2.079 2.078 1.883 1.763 1.735 1.684 1.640

9.7 74.5 10.3 14.6 100.0 42.4 4.7 6.4 10.2 8.7 18.3 40.5 2.2 17.1 11.3 18.0

4.4 66.1 9.0 12.9 95.0 42.9 5.5 6.5 9.8 9.3 19.2 36.6 2.4 19.6 14.2 20.2

026 124 030 118 028 216 034 0 1 10 220 036 1 1 10 130 128 224 0 2 10 134

1.617 1.519 1.485 1.454 1.382 1.369 1.369 1.345 1.286 1.255 1.237 1.236 1.218 1.208 1.192 1.166

1.617 1.519 1.485 1.454 1.383 1.369 1.369 1.345 1.286 1.255 1.237 1.236 1.218 1.208 1.192 1.166

6.4 41.9 16.8 5.0 7.5 6.9 7.7 10.5 5.7 1.6 3.6 5.0 10.5 4.1 7.6 15.6

6.7 42.1 18.5 6.9 8.4 6.8 7.5 13.1 6.1 1.1 4.0 5.8 10.3 3.6 8.6 15.7

Note: The diffraction peaks with relative intensity lower than 1 are not list in the table.

reactions which were sintered at 660 ◦ C. The XRD pattern and indexes of Li2 Zn2 W2 O9 are given in Fig. 2 and Table 2. According to the results of XRD phase identification, the subsolidus phase relationships of the system ZnO–Li2 O–WO3 is shown in Fig. 1, which consists of one ternary compound, five binary compounds. There are eight 3-phase regions as follow: (I) WO3 –Li2 W5 O16 –ZnWO4 ; (II) Li2 W5 O16 –ZnWO4 –Li2 W2 O7 ; (III) Li2 W2 O7 –ZnWO4 –Li2 WO4 ; (IV) Li2 Zn2 W2 O9 –ZnWO4 – Li2 WO4 ; (V) Li2 Zn2 W2 O9 –ZnWO4 –ZnO; (VI) Li2 Zn2 W2 O9 – Li2 WO4 –ZnO; (VII) Li2 WO4 –Li4 WO5 –ZnO; (VIII) Li4 WO5 – ZnO–Li2 O.

powder pattern [28]. The result of the indexing procedure was a ˚ and c = 14.105 A, ˚ hexagonal/trigonal unit cell with a = 5.144 A the reliability indices is M31 /F31 = 33/27 (0.016081, 73). The observed reflections and their indexes are listed in Table 2 (the extinction condition for (h 0 l) reflections is l = 2n), which ¯ allowed these space groups: P 3c1, P3c1, P63 /mcm, P63 cm and ¯ P 62c.

3.2. Structure determination of Li2 Zn2 W2 O9 3.2.1. Data collection and indexing We collected powder diffraction data of Li2 Zn2 W2 O9 via step scan mode with a step width of 2θ = 0.017◦ and a sampling time of 0.0071◦ /s. The precise peak positions were evaluated using the program PEAKFIT, and the program Treor was used to index the

Fig. 2. Final Rietveld refinement pattern at 300 K of Li2 Zn2 W2 O9 . Small crosses represent the experimental values and solid lines the calculated pattern. The vertical bars at the top indicate Bragg reflection positions, and the bottom curve is the difference between the observed and calculated values.

Fig. 3. Perspective projection of the structure of Li2 Zn2 W2 O9 .

P. Lv et al. / Journal of Alloys and Compounds 460 (2008) 142–146 Table 3 Crystallographic data for Li2 Zn2 W2 O9 Identification code Empirical formula Formula weight Wavelength Crystal system/space group Unit cell dimensions

Li2 Zn2 W2 O9 Li2 O(ZnWO4 )2 656.3366 ˚ 1.540562 A ¯ Trigonal/P 3c1 ˚ a = b = 5.1435(2) A, ˚ α = β=90◦ , c = 14.1052(3) A, γ = 120◦ 323.175 (1) 859 ◦ C 2 6.7466 g/cm3

Volume Melting point Formula per unit cell Z Calculated density

Note: Cell parameters listed here are the final refined results, which are slightly different from the results of indexing procedure.

3.2.2. Crystal structure determination of Li2 Zn2 W2 O9 We found the structure of Li2 Zn2 W2 O9 is isostructural to that of known Zn1.44 Ni2.56 Ta2 O9 by comparing the X-ray pattern of these two compounds [29]. Both compounds crystallize ¯ and the lattice conin a trigonal system with space group P 3c1 stants are similar. According to the initial structure model of Zn1.44 Ni2.56 Ta2 O9 , Rietveld structure refinement is introduced to refine the structure using computer program DBWS-9807 [30,31]. The refined result is shown in Fig. 2 (the temperature factor was fixed at zero because it always changed to negative when it was refined). The crystal parameters and atomic positions of Li2 Zn2 W2 O9 are listed in Table 3 and Table 4, respectively, and the residual factors are Rp = 8.83%, Rwp = 11.43% and Re = 4.88%. 3.2.3. Structure description and evaluation The crystal structure of Li2 Zn2 W2 O9 with coordination polyhedra is shown in Fig. 3. Zn (or Li) atoms could partially be substituted by Li (or Zn) atoms. Each cation is bonded to six oxygen atoms to form an octahedron and the octahedron is distorted due to the different cation–O bond lengths and angles. The selected bond lengths and bond angles are listed in Table 5. According to bond lengths between the cation and oxygen, we used Brown’s bond valence empirical formula [32] to calculate the valence sum of the i-ions. The formula

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Table 4 Final Refined structure parameters of Li2 Zn2 W2 O9 with Rp = 8.83%, Rwp = 11.43%, Re = 4.88% Atom

Wyckoff position

x

y

z

Sof

W Zn1 Li1 Zn2 Li2 O1 O2

4c 4d 4d 4d 4d 12g 6f

0 1/3 1/3 1/3 1/3 0.3400 (1) 0.2598 (0)

0 2/3 2/3 2/3 2/3 0.3099 (8) 0

0.3597 (6) 0.0246 (1) 0.0246 (1) 0.2945 (7) 0.2945 (7) 0.0910(0) 1/4

1.0000 0.3720 0.6280 0.6280 0.3720 1.0000 1.0000

 is Vi = j exp (r0 − rij /B), where rij are band length of i–j ions, r0 and B are valence parameters. The valence parameters (r0 , B) are exacted from ref. [33]: r0 = 1.91, B = 0.36 for W ion, r0 = 1.292, B = 0.48 for Li ion and r0 = 1.675, B = 0.39 for Zn ion [33]. For Zn (or Li) atoms could be disorderedly substituted by Li (or Zn) atoms partially, we give the r0 = 1.4345, B = 0.4465 for Li1(Zn1) ion and r0 = 1.5325, B = 0.4235 for Zn2(Li2) ion. The results of the calculations are given in Table 5. Furthermore, each oxygen atom is bonded to four cations so that octahedron would share vertex, edge or surface. And we find another isostructural compound Mg2 Ni2 Ta2 O9 /Mg4 Ta2 O9 [34]. As their XRD ¯ patterns are similar and they both have the space group P 3c1 and the similar cell parameter with Li2 Zn2 W2 O9 . Calculated and observed intensities and d-spacing of selected reflections of XRD pattern of Li2 Zn2 W2 O9 have listed in Table 2, so we think that the result of structure analysis for Li2 Zn2 W2 O9 is reliable, which shows a good match. 3.3. Thermal property of Li2 Zn2 W2 O9 According to previous report and our analysis, we concentrate on the phase-region in order to find clue for ZnO crystal growth. The XRD results indicate that there is no intermediate compound in ZnO–Li2 Zn2 W2 O9 system and no solid solubility region. DTA results and XRD analysis reveal Li2 Zn2 W2 O9 melted incongruently at 859 ◦ C and decomposed to ZnO, Li2 WO4 and ZnWO4 . We find that the second endothermic peak too slight to observe when the corresponding exothermic peak can

Table 5 ˚ and valences of ions in Li2 Zn2 W2 O9 Selected bond lengths (A) Bonds

˚ Length (A)

Angle (◦ )

Valence sum

Nominal valence

Li1(Zn1)–O1 × 3 Li1(Zn1)–O1 × 3 Zn2(Li2)–O1 × 3 Zn2(Li2)–O2 × 3 W–O1 × 3 W–O2 × 3 O1–Zn1 O1–Zn1 O1–Zn2 O1–W O2–Zn2 × 2 O2–W × 2

2.301(0) 2.075(2) 2.289(3) 2.030(7) 1.815 (1) 2.045 (8) 2.075 (2) 2.301 (0) 2.289 (3) 1.815 (1) 2.030(7) 2.045 (8)

O1–Li1(Zn1)–O1 = 75.32(7) O1–Li1(Zn1)–O1 = 101.21(7) O1–Zn2(Li2)–O1 = 75.78(2) O2–Zn2(Li2)–O2 = 110.86(9) O1–W–O1 = 106.28(3) O2–W–O2 = 68.95(9) O1–Li1(Zn1)–O1 = 88.80(6)/161.56(5)/91.80(6)

1.147

1.372

1.429

1.628

5.968

6.0

1.852

2.0

1.99

2.0

O1–Zn2(Li2)–O2 = 75.80(9)/91.47(5)/150.95(2) O1–W–O2 = 91.73(3)/86.95(5)/153.07(4)

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Fig. 4. DTA analysis of (x)ZnO–(1 − x)Li2 Zn2 W2 O9 pseudo–binary system (a) heating curves and (b) cooling curves. The annotations describe the ZnO content (mol%) in the sample.

be seen at 975 ◦ C. According to relative shift of first peak at heating–cooling curves of DTA analysis, the second endothermic peak should be around 1120 ◦ C. According to refs. [21] and [22], the liquidus line should be observed near 1200 ◦ C (Fig. 4) in ZnO–ZnWO4 pseudo–binary system. As Li2 Zn2 W2 O9 decomposed to ZnO, Li2 WO4 and ZnWO4 , the temperature of the liquidus line of ZnO–Li2 Zn2 W2 O9 system may lower than ZnO–ZnWO4 system. Further analysis of Li2 Zn2 W2 O9 is on the way. 4. Conclusion The subsoildus phase relation of the system ZnO–Li2 O–WO3 comprises eight 3-phase regions. There are five binary compounds in the binary system Li2 O–WO3, ZnO–WO3 and ZnO–Li2 O under our experimental condition. A new ternary compound Li2 Zn2 W2 O9 has been found in the ternary system. The crystal structure of Li2 Zn2 W2 O9 has been refined by Rietveld profile fitting method. It has trigonal system with space ¯ ˚ group P 3c1, and the lattice parameters are a = 5.1438(2) A, ˚ The unit of formula per unit cell is 2. It inconc = 14.1052(3) A. gruently melts at 859 ◦ C and decomposes to ZnO, Li2 WO4 and ZnWO4 . Acknowledgements This research is supported by One Hundred Talent Program of Chinese Academy of Sciences (CAS), National Natural Science Foundation of China and Funding of State Key Laboratory of Structural Chemistry at FJIRSM (No. 50672123). References [1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293. [2] D.C. Look, Mat. Mater. Sci. Eng. B—Solid State Mater. Adv. Technol. 80 (2001) 383. [3] R.A. Laudise, E.D. Kolb, A.J. Caporaso, J. Am. Ceram. Soc. 7 (1964) 9.

[4] T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, N. Akagami, J. Cryst. Growth 214/215 (2000) 72. [5] E. Ohshima, H. Ogino, I. Niikura, K. Maeda, M. Ito, T. Fukuda, Semicond. Sci. Technol. 20 (2005) 49. [6] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jnones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 105 (1998) 399. [7] A. Mycielski, L. Kowalczyk, A. Szadkowski, B. Chwalisz, A. Wysmolek, J. Alloys Comp. 371 (2004) 150. [8] K. Oka, H. Shibata, S. Kashiwaya, J. Cryst. Growth 237–239 (2002) 509. [9] J. Nause, B. Nemeth, Semicond. Sci. Technol. 20 (2005) 45. [10] J.W. Nielsen, E.F. Dearborn, J. Phys. Chem. 64 (1960) 1762. [11] B.M. Wanklyn, J. Cryst. Growth 7 (1970) 107. [12] D. Schulz, S. Ganschow, D. Klimm, M. Neubert, M. Ro␤berg, M. Schmidbauer, R. Fornari, J. Cryst. Growth 296 (2006) 27. [13] L. Xue, et al., J. Alloys Compd. 452 (2008) 263. [14] L. Xue, et al., J. Alloys Compd. 430 (2007) 67. [15] J. Hauck, J. Inorg. Nucl. Chem. 36 (1974) 2291. [16] L.L.Y. Chang, S. Sachdev, J. Am. Ceram. Soc. 58 (1975) 267. [17] M. Parmentier, J.M. Reau, C. Fouassier, C. Gleitzer, Bull. Soc. Chim. Fr. 5 (1972) 1743. [18] H. Untenecker, R. Hoppe, Z. Anorg. Allg. Chem. 551 (1987) 147. [19] K. Tsukamoto, et al., J. Mater. Sci. 19 (1984) 2493. [20] R. Hoppe, P. Kastner, Z. Anorg. Allg. Chem. 393 (1972) 105. [21] A.V. Shchenev, Yu. F. Kargin, V.M. Skorikov, Zh. Neorg. Khim. 33 (1988) 2165. [22] T.M. Yanushkevich, V.M. Zhukovskii, E.V. Tkachenko, Zh. Neorg. Khim. 23 (1978) 2485. [23] W.H. Zachariasen, H.A. Plettinger, Acta Cryst. 14 (1961) 229. [24] H. Morikawa, K. Okada, F. Marumo, S.I. Iwai, Acta Cryst. B 31 (1975) 1451. [25] P.F. Schofield, K.S. Knight, G. Cressey, J. Mater. Sci. 31 (1996) 2873. [26] J.M. Reau, C. Fouassier, P. Hagenmuller, Bull. Soc. Chim. Fr. (1967) 3873. [27] R. Hoffmann, R. Hoppe, Z. Anorg. Allg. Chem. 573 (1989) 143. [28] P.E. Werner, L. Eriksson, M. Westdahl, J. Appl. Crystallogr. 18 (1985) 367. [29] G. Halle, H. Mueller-Buschbaum, Z. Anorg. Allg. Chem. 563 (1988) 65. [30] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [31] R.A. Young, A. Sakthirel, T.S. Moss, C.O. Paiva-Santos, J. Appl. Crystallogr. 28 (1995) 336. [32] I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (1985) 244. [33] I.D. Brown, The bond-valence method: an empirical approach to chemical structure and bonding, in: M. O’Keeffe, A. Navrotsky (Eds.), Structure and Bonding in Crystals, Acad. Press Inc., New York, 1981, pp. 1–30. [34] G. Halle, H. Mueller-Buschbaum, Z. Anorg. Allg. Chem. 562 (1988) 87.