Acoustic impedance and interface phonon scattering in Bi2Te3 and other semiconducting materials

We present first-principles calculations of the phonon dispersions of Bi2Te3 along with calculations of the sound speed anisotropy for a number of materials, and we discuss these in relation to acoustic phonon interface scattering in ceramics. The Bi2Te3 phonon dispersions show agreement with what is known from neutron scattering for the optic modes, while we find a difference between the generalized gradient approximation and local density results for the acoustic branches. This is a consequence of an artificial compression of the van der Waals bonded gaps in the Bi2Te3 structure when using the generalized gradient approximation. As a result, local density approximation calculations provide a better description of the phonon dispersions in Bi2Te3. A key characteristic of the acoustic dispersions in several materials studied is the existence of a strong anisotropy in the velocities. Such an anisotropy may be a significant consideration in the reduction of lattice thermal conductivity by nanograin boundary scattering. This is a well-known technique commonly employed to improve thermoelectric performance. We develop a model to quantify the effect of this anisotropy for this interface scattering in ceramics, and we apply this to Bi2Te3 and compare with PbTe and several other semiconductors. DOI: 10.1103/PhysRevB.87.045317