High pressure synthesis and structure of a novel clathrate-type compound:: Te7+xSi20-x (x ∼ 2.5)

A novel clathrate-type compound, Te7+xSi20+ (x similar to 2.5), has been synthesized from the elements under high pressure and high temperature conditions. Its structure has been solved by Rietveld analysis of the XRD powder patterns combined with the results of an electron diffraction study. This structure is closely related to that of a type I clathrate, but with a different unit-cell parameter (a similar to 2 x a(0)) and space group (Fd-3c instead of Pm-3n). Its main feature is that it corresponds to a double clathration of a Te atom enclosed in a partially Te substituted (12.5%) Si-20 pentagonal dodecahedra, which is itself enclosed in a large Te-24 polyhedron, in form of a truncated octahedron. This outer Te network proved to be similar to the characteristic H2O host lattice of a type VII clathrate (HEF6.(H2O)(5) HF (E = P, As, Te)), the distribution of the Si/Al atoms in minerals of the sodalite group ((Na,Ca)(8)(Al6Si6O24)(Cl,S,SO4)(2)) and to the arrangement of the Ba atoms in the structure of the superconducting compound, Ba6C60. This new clathrate structure of silicon is the first one which exhibits Si-20 pentagonal dodecahedra which are only linked by inter-cluster bonds, a result which opens new prospects in the domain of the chemistry of macro-ions such as Si-20(12+) or Si-28(4+). (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.