Synthesis and crystal structures of Pd₃Pb₂Te₂ and Pd₃AgSe

F. Laufek¹, A. Vymazalová¹, M. Drábek¹, J. Drahokoupil², Chareev D.A.³, Kristavchuk A.V. ³

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

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

³Institute of Experimental Mineralogy RAS, Chernogolovka, Russia

frantisek.laufek@geology.cz

This presentation is a part of systematic research on crystal structures, phase relations and selected physical properties of phases from Pd-Pb-Te and Pd-Ag-Se ternary systems. These systems comprise many platinum-group minerals or phases, which can be found in various ore deposits as potential new minerals. Consequently, phases from these systems are of interest in mineralogy and geology of ore deposits.

The ternary compounds Pd₃Pb₂Te₂ and Pd₃AgSe were synthesised from elements by conventional solid-state reactions. Stoichiometric amounts of individual elements were sealed in evacuated silica glass tubes and resultant mixtures were heated at selected temperatures. Samples with Pd₃Pb₂Te₂ and Pd₃AgSe were heated at 400°C and 350°C, respectively. After long-term annealing, the samples were quenched in a cold-water bath. All attempts to prepare single crystals of studied phases suitable for single-crystal analysis failed, the crystal structure determinations of Pd₃Pb₂Te₂ and Pd₃AgSe were performed from conventional powder X-ray diffraction data. The structure solutions of title compounds by direct methods were accomplished using the program EXPO2004 [1]; subsequent refinements were performed with FullProf program.

Pd₃Pb₂Te₂: Space group Pmmn, a = 8.59, b = 5.93, c = 6.31 Å, V = 322 ų and Z = 2. Its crystal structure can be described as a layered structure formed by face-shared [PdPb₄Te₂] octahedra running parallel to b axis. Two independent palladium atoms are surrounded by four lead and two tellurium atoms showing distorted octahedral coordination with tellurium atoms in trans positions with respect to one another. Two independent lead atoms are coordinated by six palladium atom in two different ways. The crystal structure of Pd₃Pb₂Te₂ shows many structural similarities to the structure of shandite (Ni₃Pb₂S₂, Rm) and parkerite (Ni₃Bi₂S₂, C2/m). The phase Pd₃Pb₂Te₂ was described as a new mineral pašavaite [3].

Pd₃AgSe: Space group Pa, a = 8.63 Å, V = 642 ų and Z = 8. The silver atom is surrounded by 12 palladium atoms, 3 selenium atoms and 1 silver atom. 12 of these atoms (9 Pd and 3 Se) form a truncated tetrahedron (Fig. 1). The other atoms (3Pd + 1 Ag) are located slightly above the centers of hexagonal faces of truncated tetrahedron. Two neighboring truncated tetrahedra share the hexagonal faces and thus form a basic structural unit of the Pd₃AgSe structure. Similarly, as was mentioned for the isostructural Au₃CaGa compound [4], the Pd₃AgSe structure show comparable local structural motives as quasicrystals. Nevertheless, Pd₃AgSe is a conventional crystalline compound. Therefore, it can be viewed as an approximant of quasikrystal [5].

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

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

3. A. Vymazalová, F. Laufek, M. Drábek, J. Haloda, T. Sidorinová, Can. Mineral., 47, (2009), 53.

4. Q. Lin, J. D. Corbett, Inorg. Chem., 47, (2008), 3462.

5. Q. Lin, J. D. Corbett, Struc. Bonding, 133 (2009), 1.

This study was supported by the internal project of the Czech Geological Survey (Project No. 332300).

Figure 1. Coordination of Ag atoms in crystal

structure of Pd₃AgSe. Note the truncated

tetrahedron formed by Pd and Se atoms. .