Magnesium and Cadmium in Covalently‐Bonded Lonsdaleite Networks: Synthesis, Structure, and Bonding of <i>AET</i>Mg₂ and Sr<i>T</i>Cd₂ (<i>AE</i> = Ca, Sr; <i>T</i> = Pd, Ag, Pt, Au)

Abstract

Abstract The alkaline earth metal compounds AET Mg 2 and AET Cd 2 ( 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 SrPdMg 2 and SrPdCd 2 were investigated by X‐ray diffraction on single crystals: MgCuAl 2 type, Cmcm , a = 436.42(4), b = 1130.1(1), c = 820.54(7) pm, wR 2 = 0.0115, 511 F 2 values for SrPdMg 2 and a = 443.5(2), b = 1063.0(2), c = 810.2(2) pm, wR 2 = 0.0296, 386 F 2 values for SrPdCd 2 with 16 variables for each refinement. The magnesium and cadmium atoms build up [ T Mg 2 ] and [ T Cd 2 ] 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 SrPdMg 2 with a larger cohesive energy than SrPdCd 2 . This is illustrated by larger bonding Pd–Mg interactions, opposite to compensating Pd–Cd between bonding and antibonding states.