CA31SN20 AND RELATED-COMPOUNDS - NOVEL ZINTL PHASES CONTAINING DIMERS AND PENTAMERS OF TIN OR LEAD

The phases Ca31Sn20, Sr31Pb20, and Yb31Pb20 are obtained by allowing the pure elements to react in sealed Ta containers at 1100-1180-degrees-C followed by slow cooling. All are isostructural with Pu31Pt20. The structure of Ca31Sn20 was redetermined (14/mcm, Z = 4, a = 12.5267(5) angstrom, c = 39.912(7) A, R/R(w) = 3.2%/3.7%). A new description of the structure is given in terms of isolated tin atoms plus well-bonded tin dimers and linear pentamers (d(Sn-Sn) = 3.06-3.16 angstrom) in proportions of 5:5:1 within a rather distorted W5Si3-like framework. Assignment of classical two-center bonds to the Sn-Sn interactions shows that the compound is electron-precise, consistent with Zintl-Klemm concepts for a valence compound. The diamagnetic susceptibilities of Ca31Sn20 and Sr31Pb20 and the semiconducting behavior of Ca31Sn20 are in accord with this description.