Structural disorder and thermal conductivity of the semiconducting clathrate Sr8Ga16Ge30

The temperature dependence of the atomic displacement parameters for Sr8Ga1 6Ge30 determined from refinements of neutron powder and single-crystal diff raction data shows that the anomalously large values for one of the two uni que Sr atoms persist from 295 to 11 K. Its position is better described by a fractionally occupied four-fold split site, but the rms displacement rema ins the largest of all of the atoms in the structure. Difference Fourier ma ps of this Sr site show a residual nuclear density with lobes in the direct ions of the split-atom positions. The Ga and Ge atoms appear to be fully di sordered on the three distinct framework sites. The measured atomic displac ement parameters are used to derive estimates of the following thermodynami c related quantities: Debye temperature, 271 K; mean velocity of sound, 260 0 m/s; temperature of the Einstein "rattler", 85 K; mean free path of heat- carrying phonons, 5.36 Angstrom; and lattice thermal conductivity, 0.008 W/ cm-K. (C) 2000 Elsevier Science S.A. All rights reserved.