Inelastic neutron scattering and lattice-dynamical calculations of BaFe₂As₂

We report here extensive measurements of the temperature dependence of phonon density of states of BaFe₂As₂, the parent compound of the FeAs-based superconductors, using inelastic neutron scattering. The experiments were carried out on the thermal time-of-flight neutron spectrometer IN4 at the Institut Laue Langevin on a polycrystalline sample. There is no appreciable change in the spectra between T=10 and 200 K, although the sample undergoes a magnetic as well as a tetragonal-to-orthorhombic structural phase transition at 140 K. This indicates a rather harmonic phonon system. Shell-model lattice-dynamical calculations based on interatomic potentials are carried out to characterize the phonon data. The calculations predict a shift of the Ba phonons to higher energies at 4 GPa. The average energy of the phonons of the Ba sublattice is also predicted to increase on partial substitution of Ba by K to Ba_0.6K_0.4. The calculations show good agreement with the experimental phonon spectra and also with the specific-heat data from the literature.