Half-antiperovskites: Structure and type-antitype relations of shandites M(3/2)AS (M = Co, Ni; A = In, Sn)

The crystallographic and electronic structures of the ternary shandite type sulfides M(3)A(2)S(2) (M = Co, Ni; A = In, Sri) were investigated by X-ray diffraction, as well as density functional theory (DFT) band structure calculations with respect to superstructure and type-antitype relations. The crystal structure of Ni3In2S2 (space group R3m, a = 5.371 angstrom, c = 13.563 angstrom) was determined from a single crystal. The shandites show type-antitype relations to oxostannates(II) M2Sn2O3 (M = K, Rb) analogously to perovskite and antiperovskite. With a perovskite superstructure a group-subgroup relation is given to antiperowskites like Ni3MgC. Because of the ordered occupation of half of the M-positions the title compounds are described as half-antiperowskites M(3/2)AS. The occupation scheme causes the formation of Kagome-nets. From bond distances covalent Ni-S bonds (< 2.20 angstrom), ionic Sn-S and In-S-interactions, as well as metallic In-Ni and Sn-Ni are concluded. The electronic band structures of the shandites show metallic characteristics similar to Ni3MgC. A band filling scheme explains the stability and properties from Co3In2S2 to Ni3Sn2S2. The highest partly occupied bands are formed by a strong mixing of Co(Ni)-3d, ln(Sn)-5p and S-3p states. A bonding description [M3S2](n-)[A2](n+) accounts for the structural and electronic properties. Band gaps for Ni3In2S2 and Co3Sn2S2 indicate metal-insulator and magnetic phase transitions.