Inelastic neutron scattering study of phonon density of states in nanostructured Si_1-xGe_x thermoelectrics

Inelastic neutron scattering measurements are utilized to explore relative changes in the generalized phonon density of states of nanocrystalline Si _1-xGe_x thermoelectric materials prepared via ball-milling and hot-pressing techniques. Dynamic signatures of Ge clustering can be inferred from the data by referencing the resulting spectra to a density functional theoretical model assuming homogeneous alloying via the virtual-crystal approximation. Comparisons are also presented between as-milled Si nanopowder and bulk, polycrystalline Si where a preferential low-energy enhancement and lifetime broadening of the phonon density of states appear in the nanopowder. Negligible differences are however observed between the phonon spectra of bulk Si and hot-pressed, nanostructured Si samples suggesting that changes to the single-phonon dynamics above 4 meV play only a secondary role in the modified heat conduction of this compound.