Crystal structures, atomic vibration, and disorder of the type-I thermoelectric clathrates Ba8Ga16Si30, Ba8Ga16Ge30, Ba8In16Ge30, and Sr8Ga16Ge30 -: art. no. 144107

Temperature dependent synchrotron powder diffraction and single crystal neutron diffraction data are used for probing the vibrational states and disorder in type I clathrates Ba8Ga16Si30, Ba8Ga16Ge30, Ba8In16Ge30, and Sr8Ga16Ge30. If an empirical disorder term is included, the temperature dependence of the atomic displacement factors (ADPs) of the framework and guest atoms can be described by a Debye and Einstein model, respectively. None of the guest atoms in the large cages are located in the center and the vibrational frequencies (theta(E)) are of the order 80 K or larger for all structures, in good agreement with theoretical predictions. Even though the Sr ADPs are larger than the Ba ADPs in all the clathrates, the data show that theta(E) of Sr in Sr8Ga16Ge30 is larger than for the Ba atoms. This is due to stronger guest-host chemical bonding in Sr8Ga16Ge30. Since theta(E) of Sr has been reported to be much smaller in the literature we have also measured the specific heat of Sr8Ga16Ge30 with Ba8Ga16Ge30 as a reference. It is found that localized excitations with a characteristic energy of approximately 35 K exist in both compounds, however, the total number of states is too low to be associated with either tunneling states or localized vibration of each of the guest atoms.