The crystal structure and properties of Ga2Cr1.33Se5, a new layered semiconductor

Journal of Alloys and Compounds 366 (2004) 21–27

The crystal structure and properties of Ga2 Cr1.33Se5, a new layered semiconductor I. Oko´nska-Kozłowska a , K. Szamocka c,∗ , E. Malicka a , A. Wa´skowska b , J. Heimann c , T. Mydlarz d , A. Gilewski d , T. Gro´n c a

b

Institute of Chemistry, University of Silesia, 40007 Katowice, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50422 Wrocław, Poland c Institute of Physics, University of Silesia, 40007 Katowice, Poland d International Laboratory of High Magnetic Fields and Low Temperatures, 53529 Wrocław, Poland Received 10 June 2003; received in revised form 8 July 2003; accepted 8 July 2003

Abstract A new selenide of the ternary system Ga–Cr–Se has been obtained by chemical transport reactions. Single crystals of the pseudobinary compound (Ga2 Se3 )3x –(Cr2 Se3 )1−x with x = 0.333, are trigonal and have the unit cell of dimensions a = 3.781(1), c = 46.807(9) Å, V = 579.5(1) Å3 with Z = 3 in the hexagonal setting of space group R3. The compound has been characterised as to the crystal structure and composition related to the magnetic properties and electric transport behaviour. The structure can be described as a close packed hexagonal arrangement of selenium ions, forming layers perpendicular to the c-axis. The cations occupy tetrahedral and octahedral voids between the layers. Along the c-axis there is a repetition c/3 with five symmetrically different layers. The sequence of the layers results from the metal vacancies having also impact on the properties of the crystal. The temperature dependence of the magnetic susceptibility obeys the Curie–Weiss law with positive θC−W = 137.7 K, pointing to ferromagnetic interactions. Nevertheless, no evident magnetic transition was found down to 4.2 K, and also a thermomagnetic effect was not observed. There was no saturation of magnetisation at 4.2 K in a stationary field of 150 kOe; the moment has scarcely reached the value µ = 2.0µB /Cr3+ . The electrical transport measurements showed the compound to be the p-type semiconductor. © 2003 Elsevier B.V. All rights reserved. Keywords: Magnetically ordered materials; Semiconductors; Crystal structure and symmetry; X-ray diffraction; Magnetic measurements

1. Introduction Ternary selenide spinels, exhibiting interesting structural, magnetic and electrical transport properties have been the subject of numerous studies [1–9]. It was shown that replacement of the di- or trivalent cation by a third metal resulted in new chemical compounds with essentially changed properties interesting for both basic research and technological aspects [10–15]. Quaternary selenides, crystallising with the spinel-type structure, can usually be obtained in a whole concentration range of the dopant [16–21], while those with non-spinel structure show a limited solubility and often the end-member of the system remains unknown [21–23]. In the course of our studies on the Cd, Zn–Ga–Cr–Se solid solutions it was interesting to prepare the end-member of the sys∗

Corresponding author. E-mail address: [email protected] (K. Szamocka).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00689-3

tem, i.e. Ga2x/3 Cr2 Se4 . To our knowledge, there are reports on the ternary single crystal of GaCrSe3 with orthorhombic structure [24] and on polycrystalline forms of hexagonal Ga1.2 Cr0.8 Se3 and monoclinic Ga1.3 Cr0.7 Se3 [25]. Using chemical transport reactions we aimed at preparing single crystals of the spinel-type. However, our attempts to grow the compound resulted in the alloy resembling the pseudobinary system (ZnSe)3x –(In2 Se3 )1−x [26,27]. The present paper describes a single crystal of Ga2 Cr1.33 Se5 characterised by X-ray diffraction and the results of magnetic susceptibility and electrical resistivity measurements. 2. Experimental 2.1. Sample preparation Single crystals were grown by chemical vapour transport in closed quartz ampoules with anhydrous chromium

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I. Oko´nska-Kozłowska et al. / Journal of Alloys and Compounds 366 (2004) 21–27 Table 1 Crystal data, experimental conditions and structure refinement parameters

Fig. 1. Scanning microscope picture of the single crystal of the title compound.

chloride, CrCl3 , as a transporting agent and with the selenide Ga2 Se3 as a solid phase. These ampoules were heated in a horizontal zone furnace for 7 days (temperature gradient was between 25 and 38 K). The temperature of the solution and crystallisation zone was 1100 and 1067 K, respectively. After 7 days the furnace was cooled to room temperature with a rate of 20 K/h. Single crystals of hexagonal plate-like shape with edges of about 0.5–5 mm have been obtained (Fig. 1). The Ga2 Se3 was synthesised from elemental gallium and selenium (purity 99.999%) in evacuated quartz ampoules by heating at 1073 K for 7 days. An X-ray powder analysis showed that the products contained only the synthesised phase. 2.2. Single crystal X-ray diffraction A crystal of dimensions 0.33 mm × 0.31 mm × 0.11 mm selected for standardless energy dispersive absorption analysis (EDAX), showed the presence of Cr, Ga and Se. The intensity data were measured using a KM-4 diffractometer (Oxford Diffraction) equipped with a two-dimensional area CCD detector (graphite monochromated Mo K␣ radiation). A hemisphere of data was collected using the ␻-scan technique (ω = 1.0◦ per image, 30 s exposure time). Nine hundred images from eight runs with different orientations in reciprocal space have been obtained, covering almost 96% of the Ewald sphere. One image was monitored as a standard after every 40 images. Integration of the intensities and, corrections for Lorentz and polarization effects were done. Numerical, face-indexed analytical absorption correction lowered R(int) from 0.19 to 0.086 (Kuma KM-4/CCD software [28]). The unit cell calculations based on 1403 reflections led to a larger cell with the dimensions a = 3.781(1), c = 46.807(6) Å. Systematic absences fulfilled the conditions hkil: −h + k + l = 3n + 1 and 000l: l = 3n + 1, indicating the rhombohedral system. The possible ¯ R3m, R3m ¯ and R32. The attempts space groups are: R3, R3, to solve the structure in the centrosymmetric space groups ¯ failed completely. Statistical tests strongly R3¯ and R3m

Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group

Ga2 Cr1.33 Se5 603.4 293(2) 0.71073 Rhombohedral R3, hexagonal setting

Unit cell dimensions (Å)

a = 3.7810(10) c = 46.807(9) γ = 120◦

Volume (Å3 ) Z Density (calculated) (Mg/m3 ) Absorption coefficient (mm−1 ) Crystal size (mm3 ) θ range for data collection Index ranges

579.5(2) 3 5.189 31.786 0.33 × 0.31 × 0.11 6.25–30.45◦ −5 ≤ h ≤ 4, −4 ≤ k ≤ 5, −60 ≤ l ≤ 65 2158 899 0.1622/0.085

Reflections collected Independent reflections R(int) before/after absorption correction Completeness to θ = 30.45◦ Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2σ(I)] Extinction coefficient Largest diffraction peak and hole (e (Å−3 ))

96.6% Full-matrix least-squares on F2 727/1/36 0.985 R1 = 0.0612, wR2 = 0.135 0.00263(5) 2.914 and −2.66

suggested the absence of an inversion centre. Therefore the non-centrosymmetric space group R3 and R3m have been chosen. The best convergence was obtained in R3 (no. 146). The structure was solved with direct methods. The calculations were performed using the SHELXL’97 program system [29], from which the atomic scattering factors were also taken. As the structure is polar, the refinement program generates the floating origin constraints. A summary of data acquisition and the results of the least-squares structure refinement are given in Table 1. Good convergence and lack of correlation between the refined parameters confirmed that the choice of the acentric space group was appropriate. The atoms the occupied positions (a): (1/3 2/3 2/3; 2/3 1/3 1/3) + 00z and their final coordinates with equivalent isotropic temperature factors are given in Table 2. The atomic positional parameters in Table 2 refer to the absolute structure; the Flack parameter was 0.045 (0.05). The selected bond distances and angles are collected in Table 3. 2.3. Magnetic measurements The temperature dependence of the magnetic susceptibility was measured in the range 4.2–620 K using the Faraday method in stationary magnetic fields (335 Oe at low temperatures and 2000 Oe above 77 K). The susceptibility values were corrected for diamagnetism by applying Slater–Angus constants and are shown in Fig. 2.

I. Oko´nska-Kozłowska et al. / Journal of Alloys and Compounds 366 (2004) 21–27 Table 2 Atomic coordinates (×104 ) and equivalent isotropic displacement parameters (×103 Å2 ) y

z

U (eq.)

0 6667 0 6667 3333 3333 0 6667 0 6667

0 3333 0 3333 6667 6667 0 3333 0 3333

29(1) 762(1) 1327(1) 1986(1) 2718(1) 1043(1) 3018(3) 0255(1) 1844(1) 2480(1)

11(1) 7(1) 6(1) 27(1) 11(1) 11(1) 87(3) 17(1) 45(2) 23(1)

U (eq.) is defined as one-third of the trace of the orthogonalized Uij tensor.

Temperature runs of the magnetisation for a sample cooled in zero field (ZFC) and cooled in a field of 500 Oe (FC) were performed in the range 4.2–50 K at the measuring field 500 Oe (Fig. 3). The magnetisation measurements were carried out at 4.2 and 20 K in stationary magnetic fields up to 140 kOe using an induction magnetometer and pulsed fields up to 350 kOe at 4.2 K with a pulse duration of 10 ms (Fig. 4). 2.4. Electrical measurements The electrical conductivity in the present has been measured in the [0 0 1] direction and in the temperature range from 80 to 310 K by the standard four-point method [30] using a HP 34401A digital multimeter. The activation energy, EA , and the temperature coefficient of resistivity, αTCR , were determined from the formulae σ = σ0 exp(−EA /kT) and ρ = ρ0 (1 + αTCR T), respectively. The thermoelectric power was measured at 300 K using a differential method

Table 3 Bond lengths (Å) and angles (◦ ) Cr(1)–Se(2) Cr(1)–Se(3) Cr(2)–Se(1) Cr(2)–Se(5) Ga(1)–Se(2) Ga(1)–Se(1) Ga(2)–Se(3) Ga(2)–Se(4) Ga(3)–Se(4) Ga(3)–Se(5)

2.551(3) 2.556(3) 2.693(6) 2.616(7) 2.365(3) 2.427(1) 2.404(4) 2.289(1) 2.311(5) 2.446(2)

Se(2)–Cr(1)–Se(3) Se(2)–Cr(1)–Se(2)#1 Se(3)–Cr(1)–Se(2)#2 Se(3)#2–Cr(1)–Se(3)#3 Se(5)–Cr(2)–Se(5)#4 Se(5)–Cr(2)–Se(1)#5 Se(5)–Cr(2)–Se(1)#6 Se(1)#7–Cr(2)–Se(1)#6 Se(1)–Ga(1)–Se(2) Se(1)#8–Ga(1)–Se(1) Se(3)–Ga(2)–Se(4) Se(4)–Ga(2)–Se(4)#1 Se(4)–Ga(3)–Se(5) Se(5)–Ga(3)–Se(5)#8

The crystal structure can be described as a hexagonal arrangement of Se2− ions forming layers perpendicular to the

Magnetic susceptibility [emu/mol]

χ=2.35/(T-137.7)

200

1 /χ χ

150

µ /f.u.= 4.33 [µ /f.u.] B

+3

µ /Cr = 3.86 [µ /atom] B

100

1 50

0

0 0

100

200

300

3×

3. Results and discussion

250

eff

3×

[31]. The electrical and thermal contacts were made by silver paste.

Ga2Cr1.33Se5

eff

3×

Symmetry transformations used to generate equivalent atoms: #1: x − 1, y, z; #2: x, y + 1, z; #3: x + 1, y + 1, z; #4: x − 1, y − 1, z; #5: x − 1/3, y + 1/3, z + 1/3; #6: x − 1/3, y − 2/3, z + 1/3; #7: x + 2/3, y + 1/3, z + 1/3; #8: x + 1, y, z.

3

2

3× 3× 3× 3×

84.5(1) 95.6(4) 179.8(2) 95.4(1) 92.5(3) 89.1(1) 177.6(4) 89.2(1) 115.9(1) 102.3(3) 107.5(1) 111.4(1) 116.8(1) 101.2(1)

Inverse of susceptibility [mol/emu]

Se(1) Se(2) Se(3) Se(4) Se(5) Cr(1) Cr(2) Ga(1) Ga(2) Ga(3)

x

23

400

500

600

T[K] Fig. 2. Temperature dependence of the molar susceptibility χ and its inverse χ−1 for Ga2 Cr1.33 Se5 .

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I. Oko´nska-Kozłowska et al. / Journal of Alloys and Compounds 366 (2004) 21–27

2.6

Ga Cr

2.4

2

2.2

1.33

Se

5

ZFC FC (H = 500 Oe)

2.0

σ [emu/g]

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0

10

20

30

40

50

Temperature [K] Fig. 3. Magnetisation in the temperature range 4.2–50 K at measuring field 500 Oe for sample cooled in zero field (ZFC) and cooled in field of 500 Oe (FC).

c-axis. The cations occupy tetrahedral and octahedral voids between the layers. Along the c-axis there is a repetition period c/3 with five different layers, which form an asymmetric unit consisting of two octahedral and three tetrahedral layers (Fig. 5a). The structure model was built on the assumption that the chromium ions have a stronger preference for the octahedral coordination than the gallium ions, which were located at the tetrahedral positions. Such a location of the cations affects the formal valence of the ions. To check this effect, we have treated the cation site occupancy factors (SOF) as free variables in the final stage of the structure calculations. The nominal full occupancy in the site 3(a) takes the value 0.3333. The refinement of (SOF) showed that there

Fig. 4. Magnetisation at 4.2 and 20 K in stationary magnetic fields and pulsed at 4.2 K.

is a considerable cation vacancy concentration in the structure and it brings about the non-centrosymmetric arrangement of the structural subunits. The formula should thus be written as Ga2 Cr1.33 Se5 . The present structure exhibits some analogy to the MgIn2 Se4 type compounds with space group R3m [32] and to MnIn2 Se4 with a = 4.051(1) and c = 39.464(2) Å, ¯ [33]. For comparison the structure of described in R3m MnIn2 Se4 is shown in Fig. 5b based on the atomic parameters given in [33]. These compounds are built of three separate slabs stacked along the c-axis. Each slab consists of four selenium layers. In the centrosymmetric structure, some positional disorder among the cations in the tetrahedral and octahedral sites was reported [33]. In pseudobinary alloys (ZnSe)3x –(In2 Se3 )1−x of the ternary system Zn–In–Se, depending on the number of Se anion layers and due to the different number of Zn and In cations, the polytype compositions with x = 0.2, 0.4 and 0.5 were reported as ZnIn2 Se4 , Zn2 In2 Se5 [26,27] and Zn3 In2 Se6 [34], respectively. In the structure of Ga2 Cr1.33 Se5 there is no separation between the close packed sequences of five Se layers. The octahedral layers alternate with single and double tetrahedral layers. The octahedra are connected by common edges, whereas the tetrahedra share the corners between themselves and edges and corners with the octahedra (Fig. 5a). The temperature dependence of the molar magnetic susceptibility χ (Fig. 2) obeys the Curie–Weiss law in the temperatures above 300 K with θC−W = 137.7 K and an effective magnetic moment equal to 3.86µB /Cr atom, which is very close to the value for the free Cr3+ ion (3.87µB /Cr3+ ). Compared with the other members of the Cd–Ga–Cr–Se4 system, for which |θC−W | was reported to be in the range

I. Oko´nska-Kozłowska et al. / Journal of Alloys and Compounds 366 (2004) 21–27

b

25

a

c

c b

a

Cr Ga Se

Fig. 5. The crystal structure of Ga2 Cr1.33 Se5 (a). For comparison the structure of MnIn2 Se4 is also shown (b), after parameters given in [33].

166–250 K [21], the relatively low value of θC−W in the present compound points to weaker effective couplings. The sign of θC−W is positive, which indicates that the interactions between the magnetic moments of chromium are ferromagnetic. A deviation from the Curie–Weiss law is clearly seen in Fig. 2. However, cooling down to 4.2 K and subsequent heating to room temperature gave no evidence of

a magnetic transition. In the zero field cooled- and field cooled runs no thermomagnetic effect was observed (Fig. 3). It might be concluded that the sample at 4.2 K was still in the paramagnetic state. It is seen in Fig. 4 that the magnetisation at 4.2 K in a stationary field of 140 kOe did not show saturation and reached the value of µ = 2 ␮B /Cr ion. Applying a pulsed magnetic field of 350 kOe, the saturation

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I. Oko´nska-Kozłowska et al. / Journal of Alloys and Compounds 366 (2004) 21–27

magnetisation µ = 3µB /Cr was obtained which corresponds to that expected for the Cr3+ ion. This means that in high external magnetic fields the magnetic interactions are strong enough to saturate the magnetic moment at 4.2 K but they are not sufficient to cause any magnetic transition. These observations are consistent with the proposed structural model built of the ferromagnetic clusters of CrSe6 , separated by the non-magnetic GaSe4 layers. Statistically distributed Cr3+ vacancies bring about perturbations in the ferromagnetic interactions between the nearest-neighbour Cr–Cr ions. Besides, if we assume that the Cr3+ moments are aligned along the c-axis, the GaSe4 layers introduce a type of dilution of the ferromagnetic octahedral domains. The magnetisation µsat , systematically increasing in a strong field as shown in Fig. 4, evidences that our measurements refer to a magnetisation spatially averaged over the sample. In this structure the Ga layers enable a paramagnetic state at 4.2 K, much lower than in the CdCr2−x Gax Se4 system [21]. Thus, a high Ga concentration in the mixed chromium selenides causes not only a break down of the spinel structure, characteristic for x up to about 0.13, but it also limits the magnitude of the ferromagnetic interactions. The electrical resistivity measurements show that the single crystals of this alloy are semiconductors. The temperature coefficient of resistivity, αTCR , which has been calculated from the formula: ρ(T) = ρ0 (1 + αTCR T), is −12.8 × 10−3 K−1 . The activation energy, EA , in the extrinsic and intrinsic regions, which has been determined from the formula:

 −E1 −E2 σ = σ1 exp + σ2 exp , kT kT is E1 = 0.038 eV in the extrinsic region, and E2 = 0.111 eV in the intrinsic region. The conductivity σ(T) observed in the

Fig. 6. Electrical conductivity ln σ(T) vs. temperature.

Fig. 7. Temperature dependence of the Seebeck coefficient.

low temperature region results from the thermal activation of electrons from the valence band (VB) to the acceptor level (Fig. 6). At high temperature the increasing conductivity is also connected with thermal activation of electrons from the valence band to the conduction band (CD). The cationic vacancies and cross-related anionic non-stoichiometry give raise to the formation of additional local energy levels close to the conduction and valence bands. Donor atoms (Se) contribute electrons to the conduction band, and acceptor atoms (Cr, Ga) remove electrons from the valence band, leaving holes behind. The measurements of the Seebeck coefficient (Fig. 7) indicate that the sign of the thermoelectric power is positive, thus confirming the hole conductivity as dominating.

4. Conclusions The present paper, being a continuation of our studies on the influence of the Ga3+ ions on the crystal structure and properties in the system CdCr2−x Gax Se4 , established that with high Ga concentration the spinel structure Ga2x/3 Cr2 Se4 did not form. The new single crystals, showing a layered structure, were described with the formula Ga2 Cr1.33 Se5 . This result indicated that there is a considerable vacancy concentration, which has strong impact on the electrical transport and magnetic properties of the crystal, compared to the other members of the system. The experimental data indicate that magnetic interactions in the present compound are of ferromagnetic nature but the effective magnetic couplings are much weaker than in CdCr2 Se4 . In the latter, having the cubic spinel structure, the metal atoms are well ordered in their positions, while the cation vacancies in the present compound cause spin defects, giving rise to the formation of the ferromagnetic clusters diluted by non-magnetic regions of GaSe4 tetrahedra. The electrical transport depends on the Ga/Cr vacancies acting as acceptors and leading to p-type conductivity.

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References [1] P.K. Baltzer, P.J. Wojtowicz, M. Robbins, E. Lopatin, Phys. Rev. 151 (1966) 367. [2] C. Haas, A.M.J.G. van Run, P.F. Bongers, W. Albers, Solid State Commun. 5 (1967) 657. [3] H.W. Lehmann, Phys. Rev. B. 163 (1967) 488. [4] H.W. Lehmann, G. Harbecke, J. Appl. Phys. 38 (1967) 946. [5] A.R. von Neida, L.K. Shick, J. Appl. Phys. 40 (1969) 1013. [6] H. von Philipsborn, J. Cryst. Growth 9 (1971) 296. [7] H. Hahn, K. Schröder, Z. anorg. allg. Chem. 269 (1952) 135. [8] N. Menyuk, K. Dwight, J. Arnott, W. Wold, J. Appl. Phys. 37 (1966) 1387. [9] J. Tang, T. Matsumoto, T. Naka, T. Furubayashi, S. Nagata, N. Matsumoto, Physics B 259–261 (1999) 857. [10] N.K. Belskij, G.G. Shabunina, L.I. Ochertjanova, T.G. Aminov, Izv. Akad. Nauk. SSSR, Neorg. Mater. 12 (1976) 1279. [11] F.K. Lotgering, R.P. Van Stapele, G.H.A.M. Van der Steen, J.S. Van Wiesingen, J. Phys. Chem. Solids 30 (1969) 799. [12] K. Wakamura, T. Arai, S. Onari, K. Kudo, T. Takahashi, J. Phys. Soc. Jpn. 35 (1973) 1430. [13] T. Watanabe, J. Phys. Soc. Jpn. 37 (1974) 140. [14] M. Hamedoun, F. Mahjoubi, F.Z. Bakkali, A. Hourmatallah, M. Hachimi, A. Chortmiti, Physica B 299 (2001) 1. [15] J. Alvarez, V. Sagredo, J. Mantilla, J. Magn. Magn. Mater. 196–197 (1999) 407. [16] A. Menth, A.R. Van Neida, L.K. Shick, D.L. Malm, J. Phys. Chem. Solids 32 (1972) 1338. [17] P. Gilbart, M. Robbins, V.G. Lambrecht, J. Phys. Chem. Solids 34 (1973) 1363. [18] I. Oko´nska-Kozłowska, H.D. Lutz, T. Gro´n, J. Krok, T. Mydlarz, Mater. Res. Bull. 19 (1984) 1.

27

[19] I. Oko´nska-Kozłowska, U. Koch, W. Schmidt, H.D. Lutz, Z. anorg. allg. Chem. 510 (1984) 88. [20] A. Winiarski, I. Oko´nska-Kozłowska, J. Heimann, M. Neumann, J. Alloys Comp. 232 (1996) 63. [21] I. Oko´nska-Kozłowska, E. Malicka, A. Wa´skowska, J. Heimann, T. Mydlarz, J. Solid State Chem. 158 (2001) 34. [22] I. Oko´nska-Kozłowska, E. Malicka, R. Nagel, H.D. Lutz, J. Alloys Comp. 292 (1999) 90. [23] I. Oko´nska-Kozłowska, E. Malicka, A. Wa´skowska, T. Mydlarz, J. Solid State Chem. 148 (1999) 215. [24] H.D. Lutz, B. Engelen, M. Fischer, M. Jung, Z. anorg. allg. Chem. 566 (1988) 55. [25] M. Fischer, Thesis, University of Siegen, Germany, 1988. [26] F. Lappe, A. Niggli, R. Nitsche, J.G. White, Z. Kristallogr. 117 (1962) 146. [27] F.G. Donika, G.A. Kiosse, S.I. Radautsan, S.A. Semiletov, V.F. Zhitar, Kristallografija 12 (1967) 854. [28] Kuma Diffraction Instruments Gmbh, User Guide, 1995–2000. [29] G.M. Sheldrick, SHELXL’97, Program for the Crystal Structure Solution and Refinement, University of Göttingen, 1997. [30] L. Haupt, R. von Helmolt, U. Sondermann, K. Bärner, Y. Tang, E.R. Giessinger, E. Ladzinsky, R. Braunstein, Phys. Lett. A 165 (1992) 473. [31] U. Neitzel, K. Bärner, J. Phys. C 11 (1978) 4975. [32] P. von Dotzel, H. Schäfer, G. Schoen, Z. anorg. allg. Chem. 426 (1976) 260. [33] K.-J. Range, U. Klement, G. Doell, E. Bucker, J.R. Baumann, Z. Naturforsch. 46B (1991) 1122. [34] S.I. Radautsan, F.G. Donika, G.A. Kiosse, I.G. Mustya, V.F. Zhitar, Phys. Stat. Sol. 34 (1969) K129.