Phonon spectra of PrFeAsO _1-y via inelastic x-ray scattering

Phonons in single crystals of PrFeAsO _1-y are investigated using high-resolution inelastic x-ray scattering (IXS) and ab-initio pseudo-potential calculations. Extensive measurements show some changes in phonon spectra with temperature and/or doping. However, comparing our measurements with several ab-initio calculations we find that the experimentally observed changes are much smaller than differences between the experimental data and the calculations. Here we add to our previous work [Fukuda et al.: Phys. Rev. B 84 (2011) 064504] by showing more of the IXS spectra and details of some of the calculations. Agreement is improved if magnetism is included in the calculations, but still large disagreement remains. Several modifications of the calculations are considered and the best agreement is obtained from modifying the non-magnetic model by softening only the in-plane nearest-neighbor Fe-As force constant.