Crystal structure of the rare earth borosilicide Er8Si17B3

Journal of Alloys and Compounds 353 (2003) 233–239

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Crystal structure of the rare earth borosilicide Er 8 Si 17 B 3 ´ a , *, J. Bauer a R. Jardin a , V. Babizhetskyy a,b , R. Guerin a

´ , UMR CNRS-6511, Institut de Chimie, Campus de Beaulieu, Laboratoire de Chimie du Solide et Inorganique Moleculaire ´ ´ Avenue du General Leclerc, F-35042 Rennes Cedex, France b Max-Planck Institute, Heisenbergstrasse 1, Postfach 800665, D-70569 Stuttgart, Germany Received 25 October 2002; received in revised form 18 November 2002; accepted 18 November 2002

Abstract The ternary borosilicide Er 8 Si 17 B 3 has been prepared by tin flux. Its crystal structure has been determined from X-ray single crystal ˚ b528.8674(16) A ˚ and c53.8413(2) A; ˚ R50.029 (Rw50.034); 540 intensity data: space group Cmc21 , Z51; a54.0128(2) A, independent reflections; 44 variable parameters. The structure of a new type is derived from the AlB 2 -type with a partial order on the ¨ sublattice. The salient characteristic results from the occupancy by metalloid atoms of the tetrahedra and pyramids formed by metalloıd erbium atoms, which are arranged at the interface of the AlB 2 slabs. Finally, this structure is discussed in detail with the AlB 2 derivative structure types Ho 3 Si 4 and ErGe 1.83 .  2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Chemical synthesis; Single crystal growth; Crystal structure; X-ray diffraction

1. Introduction Many studies have been devoted to the crystal structure and physical properties of ternary compounds comprising a rare earth, boron and carbon [1–3]. In addition, theoretical calculations have been performed in order to rationalize the nature of the chemical bond within the boron–carbon sublattice [4–6]. Such an approach has been adopted for research on ternary rare earth borosilicides. Up to now, the literature is very poor concerning this topic. Some ternary phase diagrams R–Si–B when R5La, Ce, Er and Y have been built on bulk materials after arc melting and annealing the samples at 1070 K, but the occurrence of ternary phases has not been mentioned [7,8]. More recently, a few ternaries like YSi 4.6 B 17.6 and TbSi 1.2 B 41 have been found by using silicon flux or floating zone, but in the boron-rich part of the ternary phase diagram [9,10]. Such boron-rich phases stabilized by carbon have been also recently reported as for example Tb 32x C 2 Si 8 B 36 [11]. On the contrary, no silicon-rich rare earth borosilicides have been mentioned up to now in the literature. The occurrence of such phases could be scientifically and technologically important. Indeed, rare earth silicides have *Corresponding author. Tel.: 133-2-2323-6268; fax: 133-2-99635704. ´ E-mail address: [email protected] (R. Guerin).

been currently extensively studied because of their applications in semiconductor technology [12]. Especially, these binaries are of great interest since they can be formed epitaxially on monocrystalline silicon. Moreover they are characterized by the lowest known Schottky barrier heights (SBH) on n-type Si (0.3 eV for ErSi 22x ), with potential applications in infrared detectors [13–15]. In the present paper, we discuss the first new ternary borosilicide Er 8 Si 17 B 3 , which we have obtained in the silicon-rich part of the Er–Si–B phase diagram. The synthetic conditions and the crystal structure of this ternary are reported in details. The crystal structure, which is of a new type, is derived from the AlB 2 type and related to the structures of rare earth silicides R 3 Si 4 (R5Pr, Nd and Ho) [16–18] and germanides RGe 1.83 (R5Dy, Er, Ho, Tm and Y) [19–25], previously reported. Finally, structural relationships between Er 8 Si 17 B 3 , Ho 3 Si 4 and ErGe 1.83 types are widely discussed.

2. Experimental Single crystals of the title compound were obtained using tin as a metal flux. First, suitable amounts of silicon (purity 99.999%) and boron (99.99%) as powders and erbium (99.9%) as chips, in the nominal atomic percentage Er / Si / B53 / 1 / 2, were mixed together and cold-pressed

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0925-8388(02)01318-X

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into pellets (each sample of about 1 g). In a second time, a large excess of tin as granules was added in order to obtain a tin content of about 80% in weight. The mixtures were then sealed in silica tubes under vacuum and annealed at 1173 K during 1 week. After quenching in air, the molten buttons were slowly dissolved in diluted hydrochloric acid. Then, shiny grey truncated platelet-like single crystals were extracted for structure determination. First of all, electron probe microanalyses by energy-dispersive spectroscopy (EDS) on single crystals with the help of a scanning electron microscope (Jeol JSM-6400) confirmed erbium and silicon to be present, with a nominal overall composition in at.%: Er / Si531.15 / 68.85 (standard deviations estimated less than 1 at.%). By EDS, boron could not be detected. Several reasons can be suggested: first, the low content of boron as deducted from structure determination (vide infra), in a second time, the nearly superposition of X-ray emission spectra for boron and carbon, with in addition the Ka radiation intensity significantly lower for boron. Since the surface contamination of the sample by carbon was difficult to avoid, boron could not be detected. However, boron presence was proved unambiguously on single crystals by qualitative microanalyses performed on a Camebax SX50 wavelength dispersive spectrograph (WDS), as shown in Fig. 1. Unfortunately, no boron content by quantitative microanalysis could be determined. The structure determination was done on single crystals. First, single crystals were studied by conventional X-ray

Fig. 1. X-ray emission spectrum from WDS experiments exhibiting the presence of boron in the single crystals of Er 8 Si 17 B 3 .

photographic methods (oscillating crystal and Weissenberg). Second, X-ray intensity data collection was performed from a hemisphere of 197 images in a total exposure time of 144 min on a four-circle Nonius Kappa diffractometer, equipped with a CCD area detector employing graphite-monochromated Mo Ka radiation ( l5 ˚ The orientation matrix and the unit cell were 0.71073 A). derived from the first 10 data frames using the program DENZO [26]. Absorption correction was applied with the help of the program NUMABS [27]. Crystal structure was solved by direct methods (SIR97 [28]), and least-squares refinements and difference Fourier syntheses were run with the beta version of the JANA2000 software [29]. Crystal structure and refinement data are given in Table 1. The drawings were done using the program DIAMOND [30].

3. Results and discussion

3.1. Crystal structure From preliminary X-ray studies on single crystals, the lattice constants were found: orthorhombic symmetry, a5 ˚ b528.8674(16) A ˚ and c53.8413(2) A, ˚ Z51, 4.0128(2) A, Laue group mmm, possible conventional space groups Cmcm and Cmc21 . In a second step, after intensity data collection, the structure refinement was done with both space groups, which differ in the presence of a symmetry center. The refinement converged for both models to same reliability factors: R50.0289 (Rw50.0378) for Cmcm and R50.0292 (Rw50.0343) for Cmc21 , taking into account 540 independent reflections. On the other hand, a physically more reasonable model is in favour of the noncentrosymmetric Cmc21 group. The action of the plane m z is associated with doubling of the multiplicity of one 4a position occupied by a mixed atom (M2) in Cmc21 leading to one 8f position in Cmcm. This would result in non ˚ realistic interatomic distances d(M2–M2)50.55(2) A. Such a structural choice of noncentrosymmetric space group Cmc21 is not new since it has been previously reported for describing the structures of the binary germanides DyGe 1.85 and ErGe 1.83 [19–21]. At the opposite, the structures of other rare earth germanides, like HoGe 1.85 , TmGe 1.83 and YGe 1.85 were described in centrosymmetric space group Cmcm [22–25]. In the structure Er 8 Si 17 B 3 , all the atoms occupy the positions 4a. For metalloid sublattice, three out of the positions are fully occupied by silicon i.e., Si1, Si2 and Si3 atoms, while the other two correspond to mixed Si / B positions: M1 (63(2) at.% Si, 37(2) at.% B) and M2 (66(2) at.% Si, 34(2) at.% B). It is worthwhile to mention that our attempts to solve the structure with partially filled silicon positions without additional boron were found to be inaccurate, owing to the large increase of the isotropic displacement parameters (Ueq .0.045). This result confirms

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Table 1 Crystal data, intensity collection and refinement for Er 8 Si 17 B 3 Empirical formula Formula weight (g mol 21 ) Crystal system Space group

Er 8 Si 17.13 B 2.87 1850.14 orthorhombic Cmc21 (No.36)

Unit cell dimensions: ˚ a (A) ˚ b (A) ˚ c (A) ˚ 3) Cell volume (A Z; calculated density (g cm 23 ) Crystal shape Crystal size (mm 3 ) Linear absorption coefficient (mm 21 ) Absorption correction T min , T max u range for data collection (8)

4.0128(2) 28.8674(16) 3.8413(2) 444.97 1; 6.904 Truncated platelet 0.9530.01230.01 38.47 Gaussian integration 0.309, 0.698 1.4–35

Range in hkl: h k l Number of collected reflections Number of unique reflections, R int Independent reflections with I0 .1s (I0 ) Number of variables Weighting scheme Final R indices [I0 .1s (I0 )]

0,h,6 245,k,46 26,l ,6 3727 621, 0.0604 540 44 w51 /(s 2 (Fobs )1(0.0004 Fobs )2 ) R50.029 Rw50.034 1.03 3.76 and 23.41

Goodness-of-fit ˚ 3) Largest diff. peak and hole (e / A

the occurrence of boron in the structure, as observed by WDS. Nevertheless, the Si3 atom maintains a too large displacement parameter but attempts to refine the occupancy factor did not indicate any significant deviation from full occupancy. Such a consideration has been previously reported for some germanium atoms in the crystal structure of binary germanides, like Y 3 Ge 4 [25]. The final difference Fourier synthesis did not reveal any significant electron density peaks. The final formula of this borosilicide, as deduced from the structure refinement, is therefore Er 8 Si 17.13 B 2.87 , leading to the atomic proportions Er / Si / B528.57 / 61.18 / 10.25. The corresponding Er / Si ratio of 31.84 / 68.16 fits nicely with the nominal composition given by electron probe microanalysis: 31.15 / 68.85. Atomic positional, displacement parameters are

listed in Tables 2 and 3, while the selected interatomic distances are given in Table 4.

3.2. Structural description The representation of the structure Er 8 Si 17 B 3 is shown in Fig. 2a. It can be described from slabs of three AlB 2 blocks, connected through erbium polyhedra occupied by the M1 and Si3 atoms, in order to generate a threedimensional stacking. The alternating sequence of the slabs occurs along the [010] direction. The inner AlB 2 block within each slab is filled by the M2 atoms, while the two outer blocks are occupied by the Si1 and Si2 atoms. Consequently, the metalloid atoms Si1, Si2 and M2 present the expected trigonal prismatic metal coordination. The

Table 2 Atomic positional and isotropic displacement parameters for Er 8 Si 17 B 3 Atom

Wyckoff site

Occ.

x

y

z

a ˚ 2) Ueq (A

Er1 Er2 Si1 Si2 Si3 M1 M2

4a 4a 4a 4a 4a 4a 4a

1 1 1 1 1 0.63(2) Si10.37(2) B 0.66(2) Si10.34(2) B

0 0 0 0 0 0 0

0.55894(2) 0.67112(2) 0.1424(2) 0.0930(2) 0.7505(2) 0.7786(2) 0.0125(2)

0.7753(9) 0.27207 0.775(6) 0.288(6) 0.782(7) 0.278(10) 0.344(3)

0.0067(2) 0.0072(2) 0.011(1) 0.007(2) 0.031(2) 0.015(2) 0.021(4)

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

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Table 3 Anisotropic displacement parameters for Er 8 Si 17 B 3 Atom

U11

U22

U33

U12

U13

U23

Er1 Er2 Si1 Si2 Si3 M1 M2

0.0068(3) 0.0062(3) 0.007(2) 0.005(2) 0.034(2) 0.016(2) 0.004(3)

0.0081(3) 0.0091(3) 0.015(2) 0.017(2) 0.014(2) 0.016(3) 0.016(3)

0.0051(4) 0.0061(3) 0.011(2) 0.000(4) 0.043(4) 0.013(4) 0.044(11)

0 0 0 0 0 0 0

0 0 0 0 0 0 0

20.0002(9) 0.0031(10) 20.012(6) 0.003(6) 0.018(8) 0.003(12) 20.016(4)

occurrence of metalloid–metalloid bonds induce a twodimensional network formed by distorted hexagons, developing by sharing edges in an infinite manner along the [001] direction. Within these hexagons, the interatomic ˚ Si–M2 (2.34 A) ˚ and distances Si–Si (2.35 and 2.43 A), ˚ M2–M2 (2.06 A) (Table 3) are in good accord with the expected ones from theoretical covalent radii of silicon ˚ and boron (0.86 A) ˚ [31]. (1.17 A) Since AlB 2 slabs are translated from each other by a / 2 along the [010] direction (Fig. 3), linkage of two slabs is obtained through the M1 and Si3 atoms that are distributed in such a way that infinite zigzag –M1–Si3–M1–Si3– chains develop along the [001] direction (Fig. 3a). In addition, these chains are connected together by additional Si3–M1 bonds to generate also infinite zigzag chains along the [100] direction (Fig. 3b). The Si3–M1 distances ˚ are in good accord comprised between 2.07 and 2.17 A ˚ given for Si–B with the theoretical ones 2.03 and 2.34 A Table 4 ˚ for Er 8 Si 17 B 3 and estimated standard Main interatomic distances (A) deviations Er1

2Si1 2Si2 2Si2 2M2 2M2 2M2

3.135(3) 2.92(2) 2.98(2) 2.926(8) 3.255(9) 2.890(5)

Si2

2Er1 2Er1 2Er2 1Si1 1Si1 1M2

2.98(2) 2.92(2) 3.020(3) 2.43(3) 2.35(3) 2.331(8)

Er2

2Si1 2Si1 2Si2 1Si3 1Si3 2Si3 1M1 2M1 2M1 2Er2

2.90(2) 2.91(2) 3.020(3) 2.97(2) 3.02(2) 3.025(4) 3.11(1) 3.12(2) 3.15(2) 3.12(2)

Si3

1Er2 1Er2 2Er2 1M1 1M1 2M1

3.02(2) 2.97(2) 3.025(4) 2.10(4) 2.07(4) 2.174(3)

M1

1Er2 2Er2

3.101(6) 3.15(2)

2Er1 2Er2 2Er2 2Si1 1Si2 1Si2 1M1

3.135(3) 2.91(2) 2.90(2) 3.84(3) 2.35(3) 2.43(3) 2.29(1)

1Si1 1Si3 1Si3 2Si3

2.283(7) 2.07(4) 2.10(4) 2.174(3)

2Er1 2Er1 2Er1 1Si2 2M2

3.255(9) 2.926(8) 2.890(5) 2.331(8) 2.06(2)

Si1

M2

and Si–Si bonds, respectively. Contrary to the trigonal prismatic metal coordination adopted by Si1, Si2 and M2 atoms, the Si3 and M1 ones are surrounded by four and five erbium atoms in a tetrahedral and square-based pyramidal coordination, respectively (Fig. 4). Alternating sequence of tetrahedra and pyramids sharing faces occurs along the [100] and [001] directions (Fig. 2a).

3.3. Structural relationship The crystal structure Er 8 Si 17 B 3 is closely related to the ones of Ho 3 Si 4 (a54.1911(6), b523.785(4), c5 ˚ and ErGe 1.83 (a54.0527(1), b529.4964(5), 3.8013(6) A) ˚ previously reported [18,21]. Despite the c53.8871(6) A), choice of centrosymmetric space group Cmcm for Ho 3 Si 4 , all three structures exhibit very close atomic coordinates. As a consequence, structural relationships can be established through the same AlB 2 -type slabs which are separated from each other by metal tetrahedra and pyramids. The main differences result from the occupancy of these polyhedra. Indeed, the pyramids and tetrahedra are completely empty for Ho 3 Si 4 (Fig. 2b), while the tetrahedra are filled by germanium atoms for ErGe 1.83 (Fig. 2c). Therefore, a two-dimensional square-like network of germanium atoms, which develops perpendicularly to the long axis, occurs in the structure of ErGe 1.83 (Fig. 2c). The structure of the ternary borosilicide Er 8 Si 17 B 3 corresponds to the final step of filling-up the voids, since in addition to the tetrahedra occupied by the Si3 atoms, the pyramidal sites are now also completely filled by the M1 atoms (Fig. 2a). As a result, the metalloid / metal ratio is widely modified, since it increases from 1.33 for Ho 3 Si 4 to 2.5 for Er 8 Si 17 B 3 through 1.83 for ErGe 1.83 . One could expect values equal to 1.5 for the silicide and 2 for the germanide ¨ deficient, since in the but both binaries are metalloıd AlB 2 -type slab, there is some occupancy defect on one silicon (Si3) and one germanium (Ge4) position (t 567%), respectively. From a bonding point of view, it is worth noting that in Er 8 Si 17 B 3 the covalent metalloid atoms adopt three different bonding modes. The three-connected Si1, Si2 and M2 atoms can be described as sp 2 -hybridized atoms. Assuming the octet rule being satisfied, these atoms are formally in the oxidation state 21 (if M2 is considered as a silicon atom). A similar approach applied to the tetrahedrally

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237

Fig. 2. Crystal structures given in projection on the (100) plane: (a) Er 8 Si 17 B 3 , (b) Ho 3 Si 4 (space group Cmcm) and (c) ErGe 1.83 . Large, medium and small spheres represent erbium (or holmium) atoms, silicon (or germanium) in full occupancy, silicon (or germanium) in partial occupancy as well as mixed (Si, B) atoms, respectively. Black and white spheres are translated from each other by half a period of the projection direction. The metalloid sublattice as well as the metal polyhedra between the AlB 2 -type slabs are emphasized. For more clarity, two unit cell have been drawn.

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Fig. 3. 3D representation of the –M1–Si3–M1–Si3– zigzag chains at the interface of two successive AlB 2 -type slabs in the structure Er 8 Si 17 B 3 : (a) along the [001] direction, (b) along the [100] direction. Black and white atoms are translated from each other by a / 2.

Fig. 4. Coordination polyhedra formed by the erbium atoms between the AlB 2 -type slabs: (a) tetrahedron and (b) square-based pyramid occupied by the Si3 and M1 atoms, respectively. In addition, near neighboured ¨ atoms are also presented. metalloıd

4. Conclusion coordinated Si3 atom (sp 3 -hybridized) leads to suggest an oxidation state equal to zero for this atom (Fig. 4). The coordination mode of M1 is less straightforward, even if considered as a silicon atom. Indeed, M1 is connected to five silicon atoms (4Si311Si1) in a square base pyramidal arrangement. This hypercoordination mode is reminiscent of the one of the hypervalent (12 electrons) bromine atom in BrF 5 [32,33]. Assuming that M1 is a silicon atom isoelectronic to Br in BrF 5 , one is left to suggest the oxidation state 23 for this atom. From the above hypothesis, it follows that the overall metalloid formal charge is equal to 224 per Er 8 Si 17 B 3 formula unit. It is in a perfect balance with the corresponding 124 formal cationic charge assuming Er 31 cations. This result however does not take into account the presence of boron in the mixed M1 and M2 positions. In order to check the above qualitative hypothesis and to get a better insight into the bonding of Er 8 Si 17 B 7 as well as on related Si and Ge compounds [18,21], a theoretical analysis is currently in progress in our laboratory in order to rationalize the structure and properties of these phases.

We have reported on the first silicon-rich ternary borosilicide Er 8 Si 17 B 3 which has been obtained in the Er–Si–B system using tin as a metal flux. Since no binary erbium silicide with a metalloid / metal ratio larger than 2 is known (ErSi 22x , disordered AlB 2 -type), the ternary compound presented here is a boron-inserted silicide. The structure determined on single crystals shows a partially ordered structure which is derived from the AlB 2 type. The salient characteristic of this new structure type results from the occupancy by metalloid atoms of the metal tetrahedra and pyramids arranged at the interface of the AlB 2 slabs. Structural relationships with the defective and AlB 2 -related structures R 3 Si 4 (R5Pr, Nd and Ho) and RGe 1.83 (R5Dy, Ho, Er, Tm, Y) have been widely discussed. Recently, we have synthesized by tin flux a similar borosilicide with holmium. The unit cell parameters have been found: a54.0285(2), b529.0188(17), c53.8601(3) ˚ The X-ray structure determination on single crystal A. confirmed the isotypy with Er 8 Si 17 B 3 , although there is a very slight deviation in the boron and silicon content in the mixed M1 and M2 positions, leading to the final formula

R. Jardin et al. / Journal of Alloys and Compounds 353 (2003) 233–239

Ho 8 Si 18 B 2 . It is very likely that this series of ternary borosilicides exists for all the other heavy rare earths (R5Gd–Lu).

Acknowledgements ´ R. Jardin wishes to thank the Conseil Regional de Bretagne for financial support through a doctoral fellowship (1999–2002). V. Babizhetskyy is grateful to the Centre National de la Recherche Scientifique for a research grant (2001–2002). The authors also thank T. Roisnel (Centre ´ de Diffractometrie, Universite´ de Rennes 1) for X-ray intensity data collections, J.C. Jegaden, O. Rastoix and J. Le Lannic (CMEBA, Universite´ de Rennes 1) as well as M. Bohn (IFREMER, Brest) for their assistance in electron probe microanalyses.

References [1] P. Rogl, in: Phase Diagrams of Ternary Metal–Boron–Carbon Systems, MST International Services, ASM, Stuttgart, Germany, 1998. ¨ [2] E. Bidaud, K. Hiebl, R.D. Hoffmann, R. Pottgen, C. Jardin, J. Bauer, R. Gautier, P. Gougeon, J.Y. Saillard, J.F. Halet, J. Solid State Chem. 154 (2000) 286. [3] O. Oeckler, C. Jardin, H. Mattausch, A. Simon, J.F. Halet, J.Y. Saillard, J. Bauer, Z. Anorg. Allg. Chem. 627 (2001) 1389. [4] F. Wittkar, J.F. Halet, J.Y. Saillard, P. Rogl, J. Bauer, Inorg. Chem. 33 (1994) 1297. [5] J. Bauer, J.F. Halet, J.Y. Saillard, Coord. Chem. Rev. 178–180 (1998) 723. [6] C. Jardin, O. Oeckler, H. Mattausch, A. Simon, J.F. Halet, J.Y. Saillard, Inorg. Chem. 39 (2000) 5895. [7] N.F. Chaban, Yu.B. Kuz’ma, Inorg. Mater. 36 (9) (2000) 882. [8] N.F. Chaban, Yu.B. Kuz’ma, Dopov. Akad. Nauk Ukr. RSR 11 (1971) 1048. [9] N.M. Belyavina, V.Ya. Markiv, M.V. Speka, J. Alloys Comp. 283 (1999) 162. [10] F.X. Zang, A. Satoj, T. Tanaka, J. Solid State Chem. 164 (2002) 361.

239

[11] J.R. Salvador, D. Bilc, S.D. Mahanti, M.G. Kanatzidis, Angew. Chem. 41 (5) (2002) 844. [12] S.P. Murarka, in: Silicides for VLSI Applications, Academic Press, New York, 1983, and Refs. within. [13] E.-F. Bertaut, P. Blum, C.R. Acad. Sci. Paris 231 (1950) 626. [14] R.T. Tung, J.M. Poate, J.C. Bean, J.M. Gibson, D.C. Jacobson, Thin Solid Films 93 (1982) 77. [15] E. Rosencher, S. Delage, Y. Campidelli, F. Arnaud d’Avitaya, Electronic Lett. 20 (1984) 762. ¨ J. Alloys [16] N. Pinguet, F. Weitzer, K. Hiebl, J.C. Schuster, H. Noel, Comp. 348 (2003) 1. ¨ J. Alloys Comp. 315 (2001) [17] P. Boulet, F. Weitzer, K. Hiebl, H. Noel, 75. [18] I. Ijjaali, G. Venturini, B. Malaman, J. Alloys Comp. 269 (1998) L6. [19] P. Schobinger-Papamantellos, D.B. De Mooij, K.H.J. Bushow, J. Less-Common Met. 163 (2) (1990) 319. [20] I.R. Mokraya, V.K. Pecharskii, Z.M. Shpyrka, O.I. Bodak, V.K. Bel’skii, I.E. Pats, Dopl. Akad. Nauk. Ukr. SSR, Ser. B 3 (1989) 48. [21] O. Oleksyn, P. Schobinger-Papamantellos, C. Ritter, C.H. de Groot, K.H.J. Bushow, J. Alloys Comp. 252 (1997) 53. [22] O. Zaharko, P. Schobinger-Papamantellos, C. Ritter, J. Alloys Comp. 280 (1998) 4. [23] G. Venturini, J. Alloys Comp. 308 (2000) 200. [24] I. Ijjaali, G. Venturini, B. Malaman, J. Alloys Comp. 284 (1999) 237. [25] N.M. Belyavina, V.Ya. Markiv, M.V. Speka, J. Alloys Comp. 283 (1999) 162. [26] Nonius Kappa CCD Program Package COLLECT, DENZO, SCALEPACK, Nonius BV, Delft, The Netherlands, 1998. [27] P. Coppens, in: F.R. Ahmed, S.R. Hall, C.P. Huber (Eds.), Crystallographic Computing, Munksgaard, Copenhagen, 1970, pp. 250–270. [28] G. Cascarano, A. Altomare, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, D. Siliqi, M.C. Burla, G. Polidori, M. Camalli, Acta Crystallogr. A52 (1996) C79. [29] V. Petricek, M. Dusek, JANA2000, Program for Crystal Structure Refinement, Institute of Physics, Academy of Sciences of the Czech Republic, Praha. [30] K. Brandenburg, ‘DIAMOND’, version 2.0, 1998. [31] L. Pauling, in: Nature of the Chemical Bond, 3rd Edition, Cornell University Press, Ithaca, NY, 1960. [32] S. Noury, S. Bernard, R.J. Gillespie, Inorg. Chem. 41 (8) (2002) 2164. [33] R. Ponec, A.J. Duben, J. Computational Chem. 20 (8) (1999) 760.