Crystallographic study of ternary systems with platinum-group metals

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

Within the research of phase relations of the Pd-Hg-Se and Ag-Pd-Se systems, two new ternary phases Pd₅HgSe and (Ag,Pd)₂₂Se₆ were discovered. Several phases from these systems are known as minerals or as poorly characterized naturally occurring phases. Pd₅HgSe and (Ag,Pd)₂₂Se₆ phases have not yet been described as minerals. However they can be expected to be found in nature, probable in association with palladium selenides or another platinum-group minerals.

Both 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 Pd₅HgSe and (Ag,Pd)₂₂Se₆ 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; consequently the crystal structures of Pd₅HgSe and (Ag,Pd)₂₂Se₆ were determined from powder X-ray diffraction data. The Rietveld refinements were performed width the FullProf program.

Pd₅HgSe: Space group P4/mmm, a = 4.01 Å, c = 7.04 Å, V = 113 ų and Z = 1. The crystal structure of this compound is composed of alternating layers of the AuCu₃-type [HgPd₁₂] cuboctahedra and the PtHg₂-type [SePd₈] rectangular prism. These rectangular prisms share the four Pd-Pd edges running parallel to c direction, with four adjacent rectangular prisms forming a layer parallel to (001) plane. The layers of [SePd₈] rectangular prism and [HgPd₁₂] cuboctahedra are connected by the common edges formed by the Pd atoms. This phase represents the structural antitype to the HoCoGa₅ structure type [1].

(Ag,Pd)₂₂Se₆: Space group Fm3m, a = 12.31 Å, V = 1868 ų and Z = 4. This structure can be viewed as a 3a.3a.3a superstructure of cubic Pd (a = 3.89 Å), where only 4 from 108 octahedral holes are occupied. The Ag and Pd atoms are expected to substitute each other and form mixed sites. The main structural building unit are the [Pd/Ag]₁₂ cuboctahedra and [Pd/Ag]₈ cubes, which alternate in the NaCl fashion. Both polyhedra are centred by (Pd/Ag) atoms. The (Ag,Pd)₂₂Se₆ crystal structure shows close structural relation to the Cr₂₃C₆ and Mn₂₃Th₆ type structures, nevertheless it represents a distinct structural type.

1. F. Laufek, A. Vymazalová, M. Drábek, J. Navrátil, T. Plecháček, J. Drahokoupil, S. State Chemistry,

(submitted) (2012).

Figure 1. (a) Polyhedral representation of Pd₅HgSe structure emphasizing the [HgPd₁₂] cuboctahedra and [SePd₈] rectangular prisms. Unit cell edges are highlighted. (b) Schematical view of the (Ag,Pd)₂₂Se₆ crystal structure showing the [Pd/Ag]₁₂ cuboctahedra and [Pd/Ag]₈ cubes.