Structural investigation of cooperite (PtS) crystals
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CTURE OF INORGANIC COMPOUNDS
Structural Investigation of Cooperite (PtS) Crystals V. I. Rozhdestvinaa, A. A. Udovenkob, S. V. Rubanovc, and N. V. Mudrovskayaa aInstitute
of Geology and Nature Management, Far East Branch, Russian Academy of Sciences, ul. Zeiskaya 239, Blagoveshchensk, 675000 Russia b Institute of Chemistry, Far East Branch, Russian Academy of Sciences, pr. 100-letiya Vladivostoka, Vladivostok, 690022 Russia c Bio21 Institute, University of Melbourne, Flemington Road 30, Melbourne, Victoria 3052, Australia e-mail: [email protected] Received June 3, 2014
Abstract—The single-crystal structure of cooperite, a natural platinum sulfide PtS, is studied by X-ray diffraction supported by high-resolution scanning transmission electron microscopy and X-ray spectrum microanalysis. It is found that, in addition to the main reflections corresponding to the known tetragonal cell (a = 3.47 and c = 6.11 Å; space group P42/mmc), many weak reflections with intensities I ≤ 60σ(I) are clearly observed. These reflections fit the tetragonal cell (space group I4/mmm) with doubled parameters. In structures with small (P42/mmc) and large (I4/mmm) cells, the S atoms occupy statistically two special positions. It is shown that the chemical composition of the cooperite crystals deviates from the stoichiometric composition: sulfur-deficient specimens predominate. DOI: 10.1134/S1063774516020176
INTRODUCTION Cooperite is a rarely occurring platinum sulfide mineral named after R. Cooper, who was the first to characterize this mineral in 1928 [1]. In 1932, Bannister and Hey solved the structure of cooperite (PtS) and corrected its formula based on Debye powder patterns obtained by the method of rotating specimen [2]. They found that the mineral crystallizes in the tetragonal crystal system with lattice parameters a = 3.47 and c = 6.10 Å, space group P42/mmc (no. 131). Each Pt atom forms four coplanar bonds, and each sulfur atom forms four tetrahedral bonds. The S ions form a tetragonally distorted (c/2a = 0.88) simple cubic packing (the height of the unit cell is twice the height of its pseudocell) [3]. For synthetic PtS, the cell parameters a = 3.4700 and c = 6.1096 Å (Z = 2) were determined using X-ray powder diffraction patterns [4]. The PtS structure belongs to degenerate structures of the CsCl type. The bivalent Pt2+ has a square environment formed by S atoms. Squares centered by large Pt2+ cations that are perpendicular to the (100) and (010) planes form separate layers alternating along the [001] direction [5]. The squares around Pt are parallel to the fourfold axis, not perpendicular to it. Due to this geometry of the atomic packing, the structure contains large open channels (Fig. 1). At a pressure of 3 GPa, a PtS crystal undergoes a phase transition to the structure isomorphic to that of palladium monosulfide (PdS) [7] with space group P42/m (no. 84) and 16 atoms in the unit cell. It has lower symmetry and
higher density than cooperite. In this structure, platinum and sulfur retain their square and tetrahedral coordina
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