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

The Ca(2)Tt-Mg(2)Tt systems for tetrel Tt = Sn, Pb each contain a single Ca(2-x)Mg(x)Tt phase with the Co2Si structure (Pnma) over 0 less than or equal to x less than or equal to 1, beyond which Mg(2)Tt (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 Ca(2)Tt. Resistivity data for the tin system show changes from a semiconductor for Ca2Sn to a poor metal at CaMgSn, rho similar to 180 mu Omega 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 Tt(4-) oxidation states. (C) 2000 Academic Press.