Disorder and thermal transport in undoped KTaO3

The thermal conductivity of nominally pure KTaO3 exhibits an unexpected strong phonon scattering around 8 K. The question arises whether this scattering is related to the distorted microregions observed by other experiments. We have investigated a set of undoped single crystals grown by different methods. By comparison with doped specimens, it is suggested that acoustic phonons are coupled to the soft optic mode via the local disorder. From complementary dielectric permittivity and electron paramagnetic resonance experiments, oxygen vacancies and rhombic Fe3+ appear to be incontestable candidates to account for the distorted microregions.