Phonon density of states in NdFeAsO1-xFx

We report measurements of the phonon density of states in iron oxypnictide superconductors by inelastic x-ray scattering. Good agreement with ab initio calculations that do not take into account strong electronic correlations is found, and an unpredicted softening of phonon branches under F doping of these compounds is observed. Raman-scattering experiments lead us to conclude that this softening is not related to zone-center phonons, and consequently imply an important softening of the relevant phonon branches at finite-momentum transfer Q.