Concerning the Ca_2−xMg_xTt systems, Tt=Sn, Pb

Abstract

The Ca₂Tt-Mg₂Tt systems for tetrel Tt=Sn, Pb each contain a single Ca_2−xMg_xTt phase with the Co₂Si structure (Pnma) over 0≤x≤1, beyond which Mg₂Tt (inverse CaF₂-type, x=2) appear. At x=1, Mg occupies the unique position in the tricapped trigonal prisms (TCTP) of cations about tin that is not involved in the formation of chains of edge-shared prisms, but rather it participates in interchain bonding via face-capping functions on adjoining trigonal prisms. This substitution produces notably less distortion in the TCTP units than in Ca₂Tt. Resistivity data for the tin system show changes from a semiconductor for Ca₂Sn to a poor metal at CaMgSn, ρ~180 μΩ cm at 240 K. Only very small and approximately temperature-independent magnetic susceptibilities are found over the same range, values that are only fractions of the standard diamagnetic core corrections. The systems behave as typical Zintl phases containing Ca²⁺, Mg²⁺, and Tt⁴⁻ oxidation states.