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

The phases Ca31Sn20, Sr31Pb20, and Yb31Pb20 are obtained by allowing t he 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 t he 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 pro portions of 5:5:1 within a rather distorted W5Si3-like framework. Assi gnment of classical two-center bonds to the Sn-Sn interactions shows t hat the compound is electron-precise, consistent with Zintl-Klemm conc epts for a valence compound. The diamagnetic susceptibilities of Ca31S n20 and Sr31Pb20 and the semiconducting behavior of Ca31Sn20 are in ac cord with this description.