Structure and Physical Properties of the New Layered Ternary Chalcogenides Ta2NiS5 and Ta2NiSe5

Two new ternary chalcogenides have been prepared and characterized. Both Ta₂NiS₅ (D_2h¹⁷-Cmcm; a = 3.415 (1), b = 12.146 (3), c = 15.097 (4)Å) and Ta₂NiSe₅ (C_2h⁶-C2/c; a = 3.496 (1), b = 12.829 (3), c = 15.641 (4)Å; β = 90.53 (1)°) crystallize as layered structures with four formula units per cell. These layers contain octahedral tantalum atoms and tetrahedral nickel atoms each coordinated by chalcogen atoms. These polyhedra join via shared edges to form zigzag chains along the c axis. Electrical conductivity and magnetic susceptibility measurements show these materials to be diamagnetic semiconductors. The description of the structures as Ta⁴⁺Ni²⁺Q^2- (Q = S, Se) with pairing of nickel and tantalum electrons is consistent with these physical properties.