CaTMg2 and CaTCd2 (T =Rh, Pd, Pt) with YPd2Si-type Structure

Abstract

The intermetallic calcium compounds CaTMg2 and CaTCd2 (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 YPd2Si, a ternary ordered version of Fe3C. The structures of CaPdMg2 and CaPdCd2 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 F2 values, 24 variables for Ca0:94PdMg2:06 and a=794.6(2), b=809.5(3), c=554.7(2) pm, wR2=0.0301, 819 F2 values, 23 variables for CaPdCd2. A small range of homogeneity was observed for Ca1-xPdMg2+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 [PdMg2] and [PdCd2] networks which leave cages for the calcium atoms. The latter are bonded to these networks via shorter Ca-Pd contacts (298 - 319 pm in Ca0:94PdMg2:06 and 295 - 312 pm in CaPdCd2). The course of the interatomic distances is in line with calculated overlap populations. The CaPdMg2, SrPdMg2 and CaRhIn2 structures are all derived from a CaIn2-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.