Spin-Lattice Coupling in K0.8Fe1.6Se2 and KFe2Se2: Inelastic Neutron Scattering and ab-initio Phonon Calculations

TL;DR

This study investigates the spin-lattice coupling in K0.8Fe1.6Se2 and KFe2Se2 through inelastic neutron scattering and ab-initio phonon calculations, revealing temperature-dependent phonon behavior and the influence of Fe magnetism on phonon dynamics.

Contribution

It provides new insights into the role of Fe magnetism in phonon behavior and structural stability of K0.8Fe1.6Se2 and KFe2Se2, supported by combined experimental and theoretical analysis.

Findings

Phonon peak splitting observed below 150 K.

Magnetic calculations show different phonon spectra from non-magnetic ones.

Fe magnetism influences phonon energies and stabilizes structures.

Abstract

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 around 25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infra-red 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 as well as non magnetic 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 it 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 librational modes in K0.8Fe1.6Se2 as compared to KFe2Se2.