Mixed Cations for the Stabilization of New Features. Ca 6.2 Mg 3.8 Sn 7 , a Phase with Planar Zigzag Chains of Interlinked Square-Planar Tin Units

The structure of orthorhombic Ca 6.24(6) Mg 3.76(6) Sn 7 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 [Sn 5 12- ] chains are built of interconnected square-planar ( D 2 h ) Sn 5 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-Sn 0 ) 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 Sn 5 consists of normal s, p x, p y (σ) bonding, a single 4c-2e nonbonding MO on the outer tins, and a p z 2 lone (π) pair. This new intermediate degree of hypervalency makes the unit isoelectronic with XeF 4 2+ . 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.