Effect of nanocrystallinity on lattice dynamics in Bi₂Te₃ based thermoelectrics

The lattice dynamics in as-cast and nanocrystalline thermoelectric Bi₂Te₃ based p-type and n-type material were investigated using inelastic neutron scattering. Generalized densities of phonon states show substantial agreement between the lattice dynamics in as-cast samples and previous studies. The lattice dynamics in the nanocrystalline materials differ significantly from its as-cast counterparts in the acoustic phonon regime. In nanocrystalline p-type and n-type compounds, the average acoustic phonon group velocity was found to be reduced to 80(5)% and 95(2)% of the value in as-cast material. It is argued that point-defect and strain contrast scattering may play an important role for the understanding of lattice thermal conductivity in (nanocrystalline) Bi₂Te₃ based thermoelectrics beside the observed decrease of sound velocity.