DISORDERED MICROTWINNED (Sc1-x)Sc4Co6Sn18
(x=0.14)
Bogdan Kotur1 and
Radovan Cerny2
1Department of Inorganic Chemistry, Ivan
Franko Lviv State University, Kyryla & Mefodiya Str.
6, UA-290005 Lviv, Ukraine. E-mail: kotur@franko.lviv.ua
2Laboratoire de Cristallographie,
Universite de Geneve, 24, quai Ernest-Ansermet, CH-1211
Geneva 4, Switzerland
Keywords: intermetallic compound, crystal structure, twinning, rare earths, stannides
During systematic investigation of phase equilibria in the Sc-Co-Sn ternary system a ternary compound with the tentative composition ScCo1.1Sn3.6 observed earlier in [1] was confirmed. Its crystal structure and composition were determined and refined by single crystal X-ray diffraction (Philips PW 1100 diffractometer, MoK-radiation, SHELXL-97 program). disordered microtwinned tetragonal, I41/acd, a = 13.341(8)Ä, c = 26.69(2)Ä , V = 4749.8 Ä3, Z = 8, Dx = 7.58 g cm-3, Mr = 2708.5, = 23.85 mm-1, 73 refined parameters, R=0.0638 for 1639 observed unique reflections with Fo > 4s(Fo). The title compound belongs to a large series of stannides with formula (R1-xSnx) R4M6Sn18 (R = rare earth, M = Co, Ru, Rh, Ir, Os) and two related structures (tetragonal phase II with cell parameters a~13.7Ä, c~27.4Ä and pseudo-cubic phase II' (earlier sometimes called phase III) with a~13.7 which was explained as microtwinned phase II [2, 3]). The structure of (Sc1-x)Sc4Co6Sn18 (x=0.14) is isotypic with (Er1-xSnx)Er4Rh6Sn18 (x=0.67) [2]. Refined atomic parameters:
| Atom | Site | x | y | z | Ueq [Ä2] |
| Sc1* | 8(b) | 0 | 1/4 | 1/8 | 0.023(2) |
| Sc2 | 32(g) | 0.1314(5) | 0.1178(5) | 0.3082(3) | 0.0073(7) |
| Sn2 | 32(g) | 0.0897(2) | 0.1605(2) | 0.4199(1) | 0.0083(3) |
| Sn31 | 16(f) | 0.1731(3) | 0.4231(3) | 1/8 | 0.019(1) |
| Sn32 | 16(f) | 0.3265(3) | 0.5765(3) | 1/8 | 0.013(1) |
| Sn33 | 32(g) | 0.1726(4) | 0.2531(2) | 0.0393(2) | 0.0128(9) |
| Sn34 | 32(g) | 0.0033(2) | 0.0762(4) | 0.0384(2) | 0.0133(9) |
| Sn4 | 16(e) | 0.2819(2) | 0 | 1/4 | 0.0156(8) |
| Co1 | 16(d) | 0 | 1/4 | 0.2525(5) | 0.005(2) |
| Co2 | 32(g) | 0.0008(3) | 0.0084(5) | 0.3749(4) | 0.006(1) |
* Occupation factor = 0.86(2)
The crystal was microtwinned according to the model described
in [2, 3]. The refined volume fractions of three twin domains
were 0.381(7), 0.287(7) and 0.332(7) respectively. Analysis of
interatomic distances in the structure indicates to the mixed
type of bonding. Sn-Co and some Sn-Sn interatomic distances are
remarkably contracted indicating to the covalent bonding between
these atoms.
1. G.P. Espinosa, A.S. Cooper and H. Barz, Mat.
Res. Bull. 17 (1982) 963-969.
2. J.L. Hodeau, M. Marezio and J.P. Remeika, Acta Cryst. B40
(1984) 26-38.
3. S. Miraglia, J.L. Hodeau, F. de Bergevin, M. Marezio and G.P.
Espinosa, Acta Cryst. B43 (1987) 76-83.