Soft phonon mode as the origin of the reduced thermal conductivity in Ta-based trirutiles

Ta-based trirutiles of the series ATa2O6 (A = Ni, Co) reportedly present suppressed thermal conductivity (κ) values compared to their Sb-based counterparts. Particularly, the κ values at room temperature for Ta-based samples are in the range observed in materials already employed as thermoelectric devices, suggesting they are potential candidates as a starting point for thermoelectric applications. Here, we investigate their phonon dynamics through a combination of Raman scattering measurements with density functional theory (DFT) calculations. For the Ta-based compounds only, our results reveal the presence of an Eg low-energy optical phonon that softens by ∼10 cm−1 upon cooling from 300 to 15 K, indicating this is a zone-center soft mode associated with an unrealized structural phase transition. The soft mode enhances the phonon density of states at low energies, as directly manifested in the second-order Raman scattering data and also captured by DFT phonon calculations. These results provide insights into the low κ-values of Ta-based trirutiles and place zone-center soft phonons as a key ingredient for the development of novel thermoelectric materials.