Splitting of guest atom sites and lattice thermal conductivity of type-I and type-III clathrate compounds in the Ba-Ga-Ge system

The crystal structures of type-I and type-III clathrate compounds in the Ba-Ga-Ge system have been investigated by synchrotron X-ray powder diffraction at room temperature with single- and split-site models to describe thermal vibration of the Ba atoms that are believed to 'rattle' in the encapsulating cages. The split-site model is verified to be more plausible when judged from the changes of the extent of thermal vibration of the Ba atoms and the shape changes of the encapsulating cages with the Ga content for both types of compound. When described with the split-site model, the value of lattice thermal conductivity at room temperature decreases with increasing split distance of the Ba atom site for both types of compound, indicating that the split distance can be used as a parameter to describe the extent of thermal vibration (rattling motion) of the Ba guest atoms in the encapsulating cages.