CRYSTAL-STRUCTURE AND VIBRATIONAL-SPECTRUM OF COPPER-MERCURY ORTHO-THIOPHOSPHATE CUHGPS4

The compound CuHgPS4 crystallizes in the orthorhombic system, space group Pna2(1) (No. 33), Z = 4, with lattice parameters a = 12.660(3), b = 7.3498(7), c = 6.0943(4) angstrom, and delta(calc) = 4.96 g/cm3. The title compound is stable in air and moisture and behaves as a semiconductor. The crystal structure consists of discrete tetrahedral PS43- anions joined together by Cu+ and Hg2+ cations. The arrangement of the sulfur atoms is approximately hexagonal close-packed in which P, Cu, and Hg occupy tetrahedral sites. The PS4, HgS4, and CuS4 tetrahedra are slightly distorted with mean distances d(P-S) = 2.055, d(Hg-S) = 2.524, and d(Cu-S) = 2.320 angstrom, and 8% of the Hg atoms were found to be disordered, occupying interstitial tetrahedral sites. The title compound is isotypic to AgZnPS4 and can be considered to be a defect structure of Enargite (Cu3AsS4), which is a substitution derivative of the Wurtzite (ZnS) structure. The CuHgPS4 vibrational spectrum has been recorded. The internal modes experimentally observed are in accord with the factor group prediction. A tentative assignment of the vibrational frequencies is proposed.