The stannides AuNiSn₂ and AuCuSn₂ - Bulk synthesis and superstructure determination

The stannides AuNiSn₂ and AuCuSn₂ were prepared by melting of the elements in silica ampoules at 1300 K followed by slow cooling to room temperature. The structures of both compounds were refined on the basis of single crystal X-ray data: P3̄m1, a = 412.41(14), c = 529.24(11) pm, wR2 = 0.0268, 159 F² values, 10 variables for AuNiSn₂ and a = 425.97(17), c = 526.88(15) pm, wR2 = 0.0507, 159 F² values, 11 variables for AuCuSn² (twinned crystal, BASF = 0.305(4)). These stannides crystallize with a superstructure of the NiAs type with a complete ordering of the transition metal atoms. They derive from a AuSn subcell structure, where every other layer of octahedral voids in the hexagonal closest packing of the tin atoms is filled by nickel in AuNiSn₂ and by copper in AuCuSn₂. Due to the symmetry reduction smaller NiSn_6/6 (CuSn_6/6) and larger AuSn_6/6 octahedra alternate along the c axis. The crystal chemistry is discussed on the basis of a group-subgroup scheme.