Anharmonic phonons in the high-temperature phase of KNiCl3

The high-temperature phase of the hexagonal halide perovskite KNiCl3 is investigated using time-of-flight single crystal neutron diffraction at 633 K (360 °C). Phonons are captured through thermal diffuse scattering, integrated in energy but resolved in momentum. Harmonic phonon calculations based on density functional theory yield imaginary phonon frequencies for this phase, indicating the presence of structural instabilities at this level of theory. It is shown that the inclusion of anharmonic phonon–phonon interactions removes these instabilities, leading to good qualitative agreement with the experimental diffuse scattering. These results demonstrate that the high-temperature phase of KNiCl3 is stabilized by anharmonic phonon–phonon interactions.