  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.
