Crystallographic study of pyrite related phases: PtSnS, PtSnSe and PtSnTe

F. Laufek¹, J. Plášil²

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

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

frantisek.laufek@geology.cz

This presentation is a continuation of our systematic investigations on crystal structures and selected physical properties of M-X-Ch compounds of nickel-group metals (M = Ni, Pd, Pt) and main group IV. and VI. elements (IV = Si, Ge, Sn; VI = S, Se, Te). These phases are of interest in materials science because of their possible thermoelectric applications. Moreover, as was mentioned by [1], many of these compounds show interesting structural features of the pyrite (FeS₂, Pa3) type structural family. This is because the presence of X-X or X-Ch pairs and related ordering phenomena [1].

The ternary compounds PtSnS, PtSnSe and PtSnTe were synthesised from elements by conventional high temperature solid state reactions. Stoichiometric amounts of Pt (99.9%), Sn (99.99%), S (99.995%), Se (99.99%) and Te (99.99%) were sealed in evacuated silica tubes and heated for 800 °C for 1 day. Following this, the samples were ground using agate mortar and pestle, and sealed again and heated at 800 °C for one week. The resultant material was once again ground and heated at 800 °C for two weeks. Finally, the samples were quenched in cold water.

The existence of PtSnS, PtSnSe and PtSnTe compounds is given in [1]. Also a relationship of these phases to the pyrite structural family (more specifically to the cobaltite type) is proposed [1]. However, no structural details including atomic coordinates are given. Here we report a detailed structural study of title phases. As single crystals of sufficient quality were not available, the structural analyses were performed on powder samples. The structures of title compounds can be derived from the pyrite structure (FeS₂) replacing of S-S dumbbells by X-Ch anion pair. For similar structures three arrangements of the ordering of anionic atoms were proposed [1, 2]. One possibility corresponds to the ullmanite type structure (NiSbS, P2₁3), which retains cubic symmetry. Another option of ordering of anionic atoms represents the cobaltite type structure (CoAsS, Pca2₁). Also an intermediate possible structure model was described in space group R3 [1]. To determine which ordering scheme can be applied for PtSnS, PtSnSe and PtSnTe careful analysis of powder diffraction patterns was done. The powder diffraction patterns of title compounds and pyrite are very similar. However, the presence of additional diffractions indicating the ordering with respect to lowering symmetry and splitting of specific diffractions demonstrating deviations from cubic lattice, revealed the CoAsS structure model for PtSnS, PtSnSe and PtSnTe. Final refinement was done by Rietveld method using FullProf program [3].

PtSnS, PtSnSe and PtSnTe display orthorhombic symmetry, space group Pca2₁. In these three compounds, Pt is surrounded by three Sn and X atoms showing distorted octahedral coordination. These [PtSn₃X₃] octahedra are connected by corner-sharing. An important feature presents in the structure of title compounds is the existence of Sn-X pairs (Figure 1).

1. R. Weihrich, D. Kurowski, A.C. Stűckl, S. Matar, F. Rau, T. Bernert, J. Solid State Chem., 177, (2004), 2591.

2. A.J. Foecker, W. Jeitschko, J. Solid State Chem., 169, (2001), 69.

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

This study was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Project No. KJB 300130612) and by the internal project of the Czech Geological Survey (Project No. 323000).

Figure 1. (a) Polyhedral representation of PtSnX (X = S, Se, Te) structures (space group Pca2₁) showing the [PtSn₃X₃] octahedra and Sn-X pairs. (b) Structure of pyrite (FeS_2, space group Pa3) is shown for comparison.