Structural analysis of Sr8Ga16Ge30 clathrate compound

Crystal structural characterization of the Sr8Ga16Ge30 compound was carried out with the Rietveld refinement method using x-ray powder diffraction dat a. The structure corresponds to that of the type-I clathrate hydrate, with the Sr atoms filling the "cages" formed by the Ga and Ge atoms. Anisotropic displacement parameters were refined. The limiting figure for rms values o f the Sr(1) atoms enclosed in the dodecahedral cages is a sphere, while tha t for the Sr(2) atoms in the tetrakaidecahedral cages move is an oblate ell ipsoid. A comparison of the sizes of the cages in this structure with the o nes in other M8GaxGe46-x (M = K, Ba, or void) structures shows that once th e structure is filled (i) there is very little variation in size of the tet rakaidecahedral cages and (ii) the dodecahedral cages expand only slightly (measurably) as a function of the filler size. This is an important result from the thermoelectric point of view because it means that once the struct ure is full, the cages will not "close over" the guests, limiting their mov ement and, thus, raising the thermal conductivity. The melting point of Sr8 Ga16Ge30 is approximately 765 degrees C and the apparent optical gap is 0.0 5 eV. (C) 2000 American Institute of Physics. [S0021-8979(00)04103-7].