Phonon band structures and resonant scattering in Na8Si46 and Cs8Sn44 clathrates

The low and glasslike thermal conductivity of metal-doped semiconductor cla thrate compounds makes them potentially high-efficiency thermoelectric mate rials. The cause of this unique and remarkable property has been postulated to be due to resonant scattering of lattice phonons by localized vibration s of the dopants. We present theoretical evidence in support of this hypoth esis through the analysis of electronic and vibrational interactions betwee n dopant atoms with the host framework. In particular, the contrasting beha vior of two clathrates: the glasslike thermal conductivity in Na8Si46 and t he normal behavior in Cs8Sn44 can be rationalized.