Preparation and crystal structure of Na4Cu2S3, a thiocuprate with discrete anions

Transparent pale yellow crystals of Na4Cu2S3 were obtained from the melt of a corresponding mixture of the binary sulphides. Na4Cu2S3 is tetragonal, space group I41/a with a=9.468(1)Ä, c=36.64(2)Ä and Z=16. The crystal structure was determined from four-circle diffractometer data and refined to a conventional R of 0.021 for 1682 observed reflections with Fo(2)>3sigma(Fo(2)) and 83 variables. An outstanding feature of Na4Cu2S3 is the formation of discrete V-shaped thiocuprate anions [S-Cu-S-Cu-S](4-), with coppe in an almost linear coordination by sulphur. the bond lengths (d(Cu-S)=2.15Ä) and angles are in good agreement with those of the infinite anionic chains of KCuS. The crystal structure is based on a cubic close-packed arrangement of sulphur atoms with sodium in the centres of the tetrahedral voids, while copper atoms are shifted from the centres towards one of the edges in order to attain a linear coordination.