CRYSTAL STRUCTURE OF TRECHMANNITE, AGASS2

The crystal structure of trechmannite, AgAsS2, has been determined with the use of three-dimensional intensity data. The crystal is rhombohedral, having the unit-cell dimensions arh = 8.62 Å, αrh = 108° 17', ahex = 13.98 Å, Chex = 9.12 Å, space group [formula omitted]. There are 6 (rh.) or 18 (hex.) formula units in the cell. The structure was solved by interpretation of a three-dimensional partial Patterson function synthesized only with superlattice reflections and by the heavy-atom method. The crystal has a defect, distorted, galena-type structure, the metal atom positions and the S-atom positions on the threefold rotation axes being vacant. Six S atoms and six As atoms surround the vacant sites for metal and for S respectively. As has three nearest S neighbours at distances between 2,218 A and 2.3O7 A, which form the usual trigonal AsS3 pyramid. Three of these AsS3 pyramids form a threefold AS3S6 ring by sharing S atoms. This result could be expected for stereochemical reasons. If the ratio As: S is 1:2, the only possibilities are infinite chains of linked A8S3 pyramids or rings, with a threefold axis a threefold As3S6 ring is probable. Ag has three nearest S neighbours between 2.599 A and 2.646 A, with which it forms a very flat pyramid. A fourth S atom is at an intermediate distance of 2.735 A. Together these four S atoms form a distorted tetrahedral coordination around the Ag atoms. © 1969, Walter de Gruyter. All rights reserved.