Structural disorder and thermal conductivity of the semiconducting clathrate Sr₈Ga₁₆Ge₃₀

The temperature dependence of the atomic displacement parameters for Sr₈Ga₁₆Ge₃₀ determined from refinements of neutron powder and single-crystal diffraction data shows that the anomalously large values for one of the two unique 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 remains the largest of all of the atoms in the structure. Difference Fourier maps of this Sr site show a residual nuclear density with lobes in the directions of the split-atom positions. The Ga and Ge atoms appear to be fully disordered on the three distinct framework sites. The measured atomic displacement parameters are used to derive estimates of the following thermodynamic related quantities: Debye temperature, 271 K; mean velocity of sound, 2600 m/s; temperature of the Einstein `rattler', 85 K; mean free path of heat-carrying phonons, 5.36 angstroms; and lattice thermal conductivity, 0.008 W/cm-K.