Concerning the Ca2-xMgxTt Systems, Tt=Sn, Pb

The Ca2Tt-Mg2Tt systems for tetrel Tt=Sn, Pb each contain a single Ca2-xMgxTt phase with the Co2Si structure (Pnma) over 0x1, 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, ~180 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.