Magnesium and Cadmium in Covalently‐Bonded Lonsdaleite Networks: Synthesis, Structure, and Bonding of AETMg2 and SrTCd2 (AE = Ca, Sr; T = Pd, Ag, Pt, Au)

Abstract

AbstractThe alkaline earth metal compounds AETMg2 and AETCd2 (AE = Ca, Sr; T = Pd, Ag, Pt, Au) were synthesized by induction‐melting (or in muffle furnaces) of the elements in sealed niobium ampoules. The new phases were characterized by powder X‐ray diffraction. The structures of SrPdMg2 and SrPdCd2 were investigated by X‐ray diffraction on single crystals: MgCuAl2 type, Cmcm, a = 436.42(4), b = 1130.1(1), c = 820.54(7) pm, wR2 = 0.0115, 511 F2 values for SrPdMg2 and a = 443.5(2), b = 1063.0(2), c = 810.2(2) pm, wR2 = 0.0296, 386 F2 values for SrPdCd2 with 16 variables for each refinement. The magnesium and cadmium atoms build up [TMg2] and [TCd2] polyanionic networks, which leave cavities for the calcium and strontium atoms. The bonding variations within the polyanions, which are mainly influenced by the length of the b axis are discussed. Ab initio calculations of electronic structure, charge densities, and chemical bonding, characterize SrPdMg2 with a larger cohesive energy than SrPdCd2. This is illustrated by larger bonding Pd–Mg interactions, opposite to compensating Pd–Cd between bonding and antibonding states.