Metal Flux Growth of Complex Alkaline Earth/Rare Earth Metal Silicides with a Homologous Series of Metal Phosphide Structure Types

Abstract

Three new metal silicides were grown as competing products from reactions of silicon with barium and ytterbium in Mg/Al flux; the dominant product is determined by the heating profile as well as by the reactant ratio. The compounds Ba2Yb0.88Mg11.12Si7, Ba5Yb2.26Mg16.73Si12, and Ba20Yb4.7Mg61.3Si43 all exhibit hexagonal crystal structures that are analogous to those of ternary metal phosphides (Zr2Ni12P7, Ho5Ni19P12, and Ho20Ni66P43, respectively). The structures feature building blocks composed of silicon anions surrounded by nine metal cations; these tricapped trigonal prismatic Si@(Mg/Yb/Ba)9 polyhedra share faces to form the overall structures. Magnetic susceptibility measurements show Pauli paramagnetic behavior for all three compounds, indicating the ytterbium is divalent. The silicides have a 2:1 ratio of metal cations to silicide anions and are therefore charge-balanced. This should result in semiconducting or semimetallic behavior; the latter is supported by electronic structure calculations. The formation of these compounds indicates that a large family of complex silicides is accessible from reactions of silicon and divalent metals (Ca, Sr, Ba, Eu, Yb) in magnesium-based fluxes.