Lattice dynamics in Bi2Te3 and Sb2Te3: Te and Sb density of phonon states

The lattice dynamics in Bi2Te3 and Sb2Te3 were investigated both microscopically and macroscopically using Sb-121 and Te-125 nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi2Te3. A prominent peak in the Te specific density of phonon states at 13 meV, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a similar to 20% force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at 20 K in Bi2Te3, 1.75(1) km/s, compared to Sb2Te3, 1.85(4) km/s, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at 295 K, 2.4 Wm(-1)K(-1) for Sb2Te3 and 1.6 Wm(-1)K(-1) for Bi2Te3, cannot be explained by anharmonicity alone given the rather modest Gruneisen parameters, 1.7(1) for Sb2Te3 and 1.5(1) for Bi2Te3, without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy.