M 2Ta 9S 6 (M ≡ Fe, Co, Ni): A metal-rich channel structure with condensed tetrakaidecahedral tantalum clusters

Abstract

Fe 2Ta 9S 6, Co 2Ta 9S 6, and Ni 2Ta 9S 6 have been prepared by high temperature techniques. The crystal structure has been determined from single crystal X-ray diffraction data for Fe 2Ta 9S 6 ( R = 0.042) and Ni 2Ta 9S 6 ( R = 0.031). The hexagonal cell contains one formula unit. The tantalum substructure can be viewed as condensed clusters consisting of tetrakaidecahedra sharing faces along the c axis and vertices in the hexagonal plane. The tantalum clusters are additionally bridged by low coordinated sulfur atoms sheathing channels of about 400 pm diameter which run along the c axis. The iron-group metal occupies centers of the tricapped trigonal tantalum prisms. Anomalies in the lattice parameters of Ni 2Ta 9S 6 are discussed.