CaTMg_2 and CaTCd_2 (T =Rh, Pd, Pt) with YPd_2Si-type Structure

摘要

The intermetallic calcium compounds CaTMg_2 and CaTCd_2 (T =Rh, Pd, Pt) were obtained by high-frequency melting of the elements in sealed niobium ampoules or through reactions in muffle furnaces. The polycrystalline samples were characterized by powder X-ray diffraction. They crystallize with a site occupancy variant of YPd_2Si, a ternary ordered version of Fe_3C. The structures of CaPdMg_2 and CaPdCd_2 were refined from single-crystal diffractometer data: Pnma, a=792.2(2), b=803.4(2), c=572.0(1) pm, wR2=0.0663, 1621 F~2 values, 24 variables for Ca_(0.94)PdMg_(2.06) and a=794.6(2), b=809.5(3), c=554.7(2) pm, wR2=0.0301, 819 F~2 values, 23 variables for CaPdCd_2. A small range of homogeneity was observed for Ca_(1-x)PdMg_(2+x). The magnesium and cadmium atoms build up three-dimensional tetrahedral substructures (306 - 327 pm Mg-Mg and 307 - 317 pm Cd-Cd) that resemble hexagonal diamond, lonsdaleite. Together with the palladium atoms one obtains three-dimensional, covalently bonded [PdMg_2] and [PdCd_2] networks which leave cages for the calcium atoms. The latter are bonded to these networks via shorter Ca-Pd contacts (298 - 319 pm in Ca_(0.94)PdMg_(2.06) and 295 - 312 pm in CaPdCd_2). The course of the interatomic distances is in line with calculated overlap populations. The CaPdMg_2, SrPdMg_2 and CaRhIn_2 structures are all derived from a CaIn_2-related subcell by an ordered filling of transition metal atoms into trigonal prisms. This leads to different herringbone patterns for the networks of puckered and elongated hexagons of magnesium and indium atoms.