Clathrate guest atoms under pressure

Abstract

Powder inelastic neutron scattering (INS) has been used to determine the guest atom “rattling” energy in thermoelectric clathrates Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8,Ga16) under different applied conditions. Chemical pressure was exerted by the atomic substitution, and a physical pressure of 9 kbars was applied using a clamp cell. The volume reduction induced by the physical pressure increases the energy of the guest atom rattling mode, but the local chemical environment in the cage also appears to have a similar effect. The guest atom energies were investigated as function of temperature, and softening of the guest atom modes was observed upon cooling the sample. Ba8Ga16Ge30 with holes (p-type) and electrons (n-type) as charge carriers reveal similar temperature behavior, suggesting anharmonic potentials of similar shape for the Ba guest atom independent of the charge carrier type. For Sr8Ga16Ge30 a much stronger anharmonic potential was observed compared with Ba8Ga16Ge30. The guest atom energies for Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8) extracted from powder INS were compared with Einstein energies obtained from atomic displacement parameters determined from multitemperature single crystal x-ray diffraction. Additionally, the Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8) samples were characterized with respect to their thermoelectric properties.