Spin-phonon coupling in K0.8Fe1.6Se2 and KFe2Se2: Inelastic neutron scattering and ab initio phonon calculations

We report measurements of the temperature dependence of phonon densities of states in K0.8Fe1.6Se2 using inelastic neutron scattering technique. While cooling down to 150 K, a phonon peak splitting similar to 25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infrared measurements, indicating a lowering of symmetry of K0.8Fe1.6Se2 upon cooling below 250 K. Ab initio phonon calculations have been carried out for K0.8Fe1.6Se2 and KFe2Se2. The comparison of the phonon spectra as obtained from the magnetic and nonmagnetic calculations show pronounced differences. We show that in the two calculations, the energy range of the vibrational contribution from both Fe and Se are quite different. We conclude that Fe magnetism is correlated to the phonon dynamics and plays an important role in stabilizing the structure of K0.8Fe1.6Se2, as well as that of KFe2Se2. The calculations highlight the presence of low energy optical vibrational modes in K0.8Fe1.6Se2 compared to KFe2Se2.