Magnesium and cadmium in covalently-bonded lonsdaleite networks : synthesis, structure, and bonding of AETMg2 and SrTCd2 (AE = Ca, Sr ; T = Pd, Ag, Pt, Au)

The 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