STRUCTURAL CHEMISTRY AND VALENCE OF INDIUM SULFIDE - RECONSIDERATION OF ITS CRYSTAL-STRUCTURE

The known crystal structures of the mixed valence compounds In6S7 and Sn0.5In5S7 are characterized by the existence of indium atoms in different valence states: In+-ions (Sn2+), dumbbell shaped In-2(4+) groups and In3+-ions. The In-2(4+) and In3+ species are embedded in a characteristic coordination of sulfur atoms found separately in comparable simpler nonmixed-valence chalcogenides of indium. In6S7 however is so far the only example with monovalent indium in a surrounding of sulfur. The question whether P2(1) or P2(1)/m is the correct space group for the description of In6S7 (or Sn0.5In5S7) cannot be answered clearly on the basis of the existing experimental data because it only affects a small possible deviation of the ''lone pair cation'' In+ (Sn2+) from a mirror plane. This may however be of great importance for futural studies of the physical properties. We prove the crystal structure of Sn0.5In5S7 and In6S7 to be actually isotypic. Moreover we show, that the pure tin containing compound does not exist, but is indeed In6S7 in which In+-ions are partly (max. 50%) substituted by Sn2+.