Crystallographic study of Pd(Pt)-Hg-Se ternary systems

 

F. Laufek1, A. Vymazalová1, M. Drábek1, J. Drahokoupil2

 

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

2Institute of Physics of the AS CR, v.v.i., Na Slovance 2, 182 21, Praha 8, Czech Republic

frantisek.laufek@geology.cz

 

Pt-Hg-Se and Pd-Hg-Se ternary systems were experimentally studied and three new ternary phases were structurally characterised. Several phases from these systems were described as minerals or as poorly characterized naturally occurring phases. Recently, Pt2HgSe3 and Pd2HgSe3 phases were observed as small inclusions in Cauê iron-ore deposit (Itabira district, Minas Gerais, Brazil) [1] and in Hope´s Nose (Torquay, Devon, UK) [2], respectively. The Pd9Hg3Se8 compound was found in selenide-bearing deposit in Tilkerode  (Harz, Germany) and is known as a mineral tischendorfite [3]. However, tischendorfite crystal structure was not hitherto determined. 

Because of extremely low amount of natural samples and difficulties connected with their isolation, the above-mentioned phases were synthesized from elements by conventional solid-state reactions. Stoichiometric amounts of individual elements were sealed in silica glass tubes and resultant mixtures were heated at selected temperatures. Samples with Pt2HgSe3, Pd2HgSe3 and Pd9Hg3Se8 were heated at 400°C. After long-term annealing, the samples were quenched in a cold-water bath. All attempts to prepare single crystals suitable for single-crystal examinations failed; hence the crystal structures of Pt2HgSe3, Pd2HgSe3 and Pd9Hg3Se8 were determined from powder X-ray diffraction data. The programs EXPO2004 and SuperFlip were used for structure determinations; subsequent refinements were performed width FullProf program.

       Pt2HgSe3: Space group Pm1, a = 7.34 Å, c = 5.29 Å, V = 247 Å3 and Z = 2. This phase shows layered structure composed of [PtSe6] octahedra and [PtSe4] squares running parallel to (001) planes. The Se atoms are arranged in layers perpendicular to the c-axis forming the Kagomé-nets. These nets show AB stacking sequence along the c-axis. The crystal structure of Pt2HgSe3 is isostructural with Pt4Tl2X6 (X = S, Se, or Te) phases and shows many similarities to that of PtSe2. Pt2HgSe3 phase was described as a new mineral jacutingaite [4].

       Pd2HgSe3: Space group Pm1, a = 7.31 Å, c = 5.28 Å, V = 244 Å3 and Z = 2. This phase is isostructural with Pt2HgSe3.

       Pd9Hg3Se8 crystallizes in a new structure type having the space group Pmmn, a = 7.18 Å, b = 16.79,      c = 6.47 Å, V = 780 Å3 and Z = 2. Pd9Hg3Se8 crystal structure contains three distinct Pd sites, two Hg and four Se sites. Palladium atoms show square planar or octahedral coordination. The [PdSe4] squares are paired via a common Se-Se edge forming two types of [Pd2Se6] dimers. These dimers share corner and edges with Pd-based octahedra forming three-dimensional network. The Hg(2) atoms are located in channels running in [001] direction.

 

1.        A. R. Cabral, H. F. Galbiatti, R. Kwitko-Riberio, B. Lehmann, Terra Nova, 20, (2008), 32.

2.        W. H. Paar, A. C. Roberts, A. J. Criddle, D. Topa, Mineral. Mag., 62 (1998), 257.

3.        C. J. Stanley, A. J. Criddle, H. J. Förster, A. C. Roberts, Can. Mineral., 40 (2002), 739.

4.        A. Vymazalová, F. Laufek, M. Drábek, A. R. Cabral, J. Haloda, T. Sidorinová, B. Lehmann, H. F.         

            Galbiatti, J. Drahokoupil, Can. Mineral (submitted).

 

 

 

 

Figure 1. Polyhedral representation of Pt2HgSe3 and Pd2HgSe3 phases showing [Pd(Pt)Se6] octahedra and [Pt(Pd)Se4] squares. (a) Perspective view, (b) view along the c-axis.