Crystallographic study of the ternary system Pd-Ag-Te

F. Laufek¹, A. Vymazalová¹, M. Drábek¹, J. Drahokoupil², M. Dušek²

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

During the experimental study of phase relations on the Pd-Ag-Te system, the synthetic analogues of the mineral sopcheite Ag₄Pd₃Te₄ and a new phase Ag₂Pd₁₄Te₉ (also termed as Pd₆AgTe₄) were synthesised and structurally characterized. The mineral sopcheite was found in the Cu-Ni sulphide ores of the Sopcha massif, Kola peninsula, Russia by [1], where it occurs as veins with size not exceeding of 0.02 mm included in chalcopyrite. The Ag₂Pd₁₄Te₉ phase was described as small anhedral grains (<0.1 mm) by [2] from the Nadezhda deposit in Karelia, Russia. In order to understand the behaviour of these phases in the natural conditions and to clarify mechanisms of various chemical substitutions, the crystal structures of both phases have been determined.

Because of extremely low amount of natural samples and difficulties connected with their isolation, 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 350°C. After long-term annealing, the samples were quenched in a cold-water bath. The crystal structure of synthetic analogue of sopcheite Ag₄Pd₃Te₄ was solved from single-crystal X-ray diffraction data, whereas structure of Ag₂Pd₁₄Te₉ was solved from powder X-ray diffraction data.

Ag₄Pd₃Te₄: Space group Cmca, a = 12.22 Å, b = 6.14 Å, c = 12.23, V = 918 ų and Z = 4. In the layered structure of Ag₄Pd₃Te₄, the Pd atoms show a square planar coordination by the four Te atoms. The [PdTe₄] squares share two opposite Te-Te edges with adjacent [PdTe₄] squares forming layers parallel to (100). In addition, each Pd atom has four short contacts with the Ag atoms. The layers of edge-sharing [PdTe₄] squares are connected by number of Ag-Te bonds running approximately in [100] direction.

Ag₂Pd₁₄Te₉: Space group I4/m, a = 8.96 Å, c = 11.82 Å, V = 949 A³ and Z = 2. The three-dimensional framework structure of Ag₂Pd₁₄Te₉ consists of [PdTe₄] squares and [(Pd/Ag)Te₄] flattened tetrahedra. The squares and tetrahedra form slabs parallel to (001), which regularly alternate along the c-axis. The flattened tetrahedra are hallmark of this unique structure.

1. D. A. Orsoev, S. A. Rezhenova, A. N. Bogdanova, Zap. Vses. Mineralog. Obshch., 111, 114, 1982.

2. A. Y. Barkov, R. F,. Martin, M. Tarkian, G. Poirier, Y. Thibault, Can. Mineral., 39, 639, 2001.

Figure 1. The crystal structure of the synthetic analogue of sopcheite Ag₄Pd₃Te₄ in a polyhedral representation. The [PdTe₄] squares are emphasised; unit cell edges are highlighted. Note the layered character of the structure.