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Crystal structures of chalcostibite (CuSbS₂) and emplectite (CuBiS₂): Structural relationship of stereochemical activity between chalcostibite and emplectite

Divison of Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan

Correspondence: ^* E-mail: kyono{at}arsia.geo.tsukuba.ac.jp

The crystal structures of chalcostibite CuSbS₂ (orthorhombic, space group Pnma, a = 6.018(1), b = 3.7958(6), and c = 14.495(7) Å, V = 331.1(1) ų, Z = 4, R1 = 0.040, wR2 = 0.155 for 533 reflections) and emplectite CuBiS₂ (orthorhombic, space group Pnma, a = 6.134(1), b = 3.9111(8), and c = 14.548(8) Å, V = 348.8(2) ų, Z = 4, R1 = 0.037, wR2 = 0.112 for 492 reflections) were redetermined using a four-circle diffractometer and graphite-monochromatized MoK radiation. These two crystal structures are composed of MS₅ square pyramids (M = Sb and Bi) and nearly regular CuS₄ tetrahedra. The five M-S bond distances in the SbS₅ square pyramid in chalcostibite are always shorter than corresponding distances in the BiS₅ square pyramid in emplectite because the Sb atom is smaller than the Bi atom. The a cell parameter increases appreciably from chalcostibite to emplectite not only because of increasing M-S bond distances in the MS₅ square pyramid, but also because of increasing Cu-S2-Cu bond angles along a. The increase in the b cell parameter is caused mainly by increasing M-S bond distances along b. In contrast, the slight increase of the c cell parameter is largely brought about by decreasing Cu-S2-Cu bond angles ascribed to weakened stereochemical activity of Bi 6s² lone-pair electrons. The anisotropic change of unit-cell parameters from chalcostibite to emplectite is strongly associated with the positions of the lone-pair electrons in the unit cell.