Crystal structure and atomic vibration of Ba-Ge based type-III clathrate compounds

Crystal structures of type-III clathrate compounds Ba₂₄B^III XGe_100-X, where B^III is Al and In have been investigated by synchrotron X-ray powder diffraction as functions of Al and In content and temperature, paying special attention to the changes of the cage structure and thermal vibration behavior of framework and guest atoms. The Debye and Einstein temperatures of type-III clathrate compounds are respectively determined from the temperature dependence of atomic displacement parameters (ADPs) of framework and guest atoms. The localized vibration of guest atoms in type-III clathrate compounds is evidenced by the low Einstein temperature for Ba(2) atom in open-dodecahedron. The lattice thermal conductivity of type-III clathrate compounds is discussed in terms of the rattling frequency of guest atom with comparing to that reported for type-I clathrate compounds.