Mixed cations for the stabilization of new features. Ca₆.2Mg₃.8Sn₇, a phase with planar zigzag chains of interlinked square-planar tin units

Abstract

The structure of orthorhombic Ca_6.24(6)Mg_3.76(6)Sn₇ contains two types of chains built from edge- or face-sharing trigonal prisms of Ca and Mg atoms that are centered by Sn atoms. In one type, bonded zigzag ¹_∞ [Sn₅¹²⁻] chains are built of interconnected square-planar (D_2h) Sn₅ units. Alternating chains of condensed trigonal prismatic cations centered by isolated Sn atoms are interbonded to the first type through Sn-Ca,Mg interactions. The chain unit is two electrons richer than that for a simple Zintl-Klemm formalism for 1−, 2−, and 4-bonded Sn (4b-Sn⁰) because of the square-planar tin bonding. Band structure calculations within the extended Hückel formalism indicate the infinite tin chain alone is closed shell, but a metallic behavior (via open π-bonding) is produced by cation mixing in the full structure. Local bonding within square-planar Sn₅ consists of normal s, p_x, p_y (σ) bonding, a single 4c-2e nonbonding MO on the outer tins, and a p_z² lone (π) pair. This new intermediate degree of hypervalency makes the unit isoelectronic with XeF₄²⁺. Property studies on powdered samples reveal a good metallic conduction but a small diamagnetism that is not uncharacteristic of heavy p-elements and their compounds.