Anharmonic lattice dynamics in type-I clathrates from first-principles calculations

Abstract

The anharmonic lattice dynamics of ${(\mathrm{Sr}∕\mathrm{Ba})}_{8}{\mathrm{Ga}}_{16+x}{\mathrm{Ge}}_{30\ensuremath{-}x}$ type-I clathrates are studied using first-principles calculations. The guest atoms in the large cages are found to lie in strongly anharmonic potentials, resulting in off-center (disordered) equilibrium positions for the Sr rattler. The lowest energy modes for Sr form a four level system with a tunneling splitting of only $0.04\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The thermodynamical properties and the inelastic neutron scattering spectra of the anharmonic rattlers are investigated.