SYNTHESIS AND STRUCTURE OF CoGeTe AND CoSn_1.5Te_1.5

F. Laufek¹, J. Navrátil², M. Plášil³

¹Czech Geological Survey, Geologická 6, Praha 5, 152 00, Czech Republic

²Joint Laboratory of Solid State Chemistry of IMC AS ČR and University of Pardubice, Studentská 84, 532 10 Pardubice, Czech Republic

³Charles University, Faculty of Science, Albertov 6, Praha 2, 128 43, Czech Republic

laufek@cgu.cz

This presentation is a part of systematic investigations on crystal structures and physical properties of M-X-Ch compounds of cobalt-group metals (M=Co, Ir, Rh) and main group IV and VI elements (X = Si, Ge, Sn; Ch = S, Se, Te). These phases are of interest in materials science because of their possible thermoelectric applications.

The ternary compounds, CoGeTe and CoSn_1.5Te_1.5 were synthesised from the elements by high temperature solid state reactions. Co powder was at first heated at 800 °C for 1 hr in H₂ atmosphere to remove possible oxides. Stoichiometric amounts of Co (99.99%), Ge (99.99%), Sn (99.99 %) and Te (99.99%) were sealed in evacuated quartz tubes and heated at 1150 °C for 4 hrs. Following this, the samples were ground using the agate mortar and pestle, and sealed again under vacuum in quartz tubes. The mixtures were heated at 550 ˚C for three days. The resultant material was once again ground and heated at 670 °C and 600 ˚C for CoGeTe and CoSn_1.5Te_1.5, respectively. After long-term annealing, the samples were quenched in cold water.

Here we report a detailed structural study on the two ternary compounds CoGeTe and CoSn_1.5Te_1.5. As single crystals of sufficient quality were not available, the analyses were performed on powder samples. The crystal structure of CoGeTe was solved by direct methods by means of EXPO2004 [1] program package, while the inicial structure model for CoSn_1.5Te_1.5 was derived from data published for CoGe_1.5Te_1.5 [2]. Both structures were refined by Rietveld method by means of FullProf program [3].

CoGeTe: space group Pbca, a = 6.1892(4), b = 6.2285(4), c = 11.1240(6) Å, Z = 8, R_wp = 0.083, R_B = 0.065. The crystal structure of CoGeTe can be viewed as a ternary ordered variant of α-NiAs₂ (also known as a mineral pararammelsbergite), which is transitional between the marcasite-type and the pyrite-type structures. Each Co atom is surrounded by three Ge and Te atoms showing a distorted octahedral coordination. One octahedral edge is shared with an adjacent octahedron, compared to two shared edges in the marcasite structure and none in the pyrite structure. Other vertices of the [CoGe₃Te₃] octahedron are connected by corners sharing. (Fig. 1). Similar description has been described for PtSiSb [4].

CoSn_1.5Te_1.5: space group R-3, a = 12.9062(2), c = 15.7837(3) Å, Z = 12, R_wp = 0.106, R_B = 0.046. The structure of CoSn_1.5Te_1.5 can be described as a modification of a cubic structure of CoSb₃ [5] (skutterudite type). The weak superstructure reflections found in powder diffraction pattern reveal the ordering between Sn and Te atoms. This ordering reduces the symmetry from cubic to rhombohedral. Each Co atom is surrounded by three Te and Sn atoms forming a distorted octahedral coordination. The [CoSn₃Te₃] octahedra share only corners with six neighbouring octahedra. One of the most characteristic features of the CoSn_1.5Te_1.5 structure is the presence of two distinct four-member rings [Sn₂Te₂] (Fig 2). The schematic representation of the CoSn_1.5Te_1.5 structure is shown in the Fig. 2. A compound with this composition was mentioned at the conferences [6, 7] but no structural details were given.

1. A. Altomare, R. Caliandro, M. Camalli, C. Cuocci, C. Giacovazzo, A. Moliterni. R. Rizzi, J. Appl. Cryst., 37, (2004), 1025.

2. P. Vaqueiro, G.S. Sobany, A.V. Powell, K.S. Knight, J. Solid State Chem., 179, (2006), 2055.

3. J. Rodríguez-Carvajal, FullProf.2k, Laboratoire Léon Brillouin, France, 2006.

4. M. Wang, M.G. Morgan, A. Mar, J. Solid State Chem., 175, (2003), 231.

5. Th. Schmidt, G. Kliche, H. D. Lutz, Acta Crystallogr.. C 43, (1987), 1978.

6. Y. Nagamoto, K. Tanaka, T. Koyanagi, Proceeddings of the 16th International Conference on

Thermoelectrics, Dresden, Germany.

7. A. Smaleez, Q. Lin, D. C. Johnson, J. Mertin, Material Research Meeting 2005, p. 137., Boston, USA..

This study was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Project No. KJB 300130612), by the internal project of the Czech Geological Survey (Project No. 3230) and by the Czech Science Foundation (Project No. 203/07/0267).

Figure 1. Polyhedral representation of CoGeTe Figure 2. (a) Polyhedral representation of CoSn_1.5Te_1.5

structure showing the [CoGe₃Te₃] octahedra. Black structure showing the corner sharing arrangement of

and white circles are Ge and Te atoms, respectively. the [CoSn₃Te₃] octahedra. (b) Four-member Sn₂Te₂

rings.