High pressure synthesis and crystal structure of two forms of a new tellurium-silicon clathrate related to the classical type I

Two new clathrate-type structures have been identified in the samples obtained by high pressure-high temperature treatment of appropriate mixtures of elemental silicon and tellurium at 5 GPa and 1200 degreesC for 60 min reaction time. They are both related to the classical type I silicon clathrate, G(8)Si(46) (G = guest species). The corresponding structures have been solved by X-ray single crystal diffraction analysis. They proved to correspond to a cubic and rhombohedral forms of the same compound, Te16Si38 (or more precisely Te-8@(Si38Te8)), in which eight extra tellurium atoms are substituted for silicon ones in the 16i crystallographic sites of the parent structure. In the cubic form, the space group is reduced from Pm-3n to P-43n, and the formation of strong bonds between the Te atoms at the centre of the tetrakaidecahedral cages and one or two silicon atoms of the surrounding cage is clearly observed, which is followed by a decrease of the coordination number of the Te atoms in substitutional position from 4 to 3. In the more distorted rhombohedral form, the 16i and 24k sites of the parent structure are both split in four sites. The formation of strong bonds involving the Te atoms at the centre of the tetrakaidecahedral cages is confirmed, but the main characteristic comes from the formation of another kind of strong bonds involving the Te atoms at the centre of the dodecahedral cages. These bonds are at the origin of the elongation of the structure along the [I I I] direction, which corresponds to the polar axis. (C) 2004 Elsevier SAS. All rights reserved.