Soft and isotropic phonons in PrFeAsO1-y

Phonons in single crystals of PrFeAsO1-y are investigated using high-resolution inelastic x-ray scattering and ab initio pseudopotential calculations. Extensive measurements of several samples (y similar to 0, 0.1 and 0.3) at temperatures spanning the magnetic ordering temperature (T-N similar to 145 K for y similar to 0) and the superconducting transition temperature (T-c = 36 K for y similar to 0.1 and T-c = 45 K for y similar to 0.3) show that there are some changes in phonon spectra with temperature and/or doping. We compare our measurements with several ab initio pseudopotential models (nonmagnetic tetragonal, oxygen-deficient O-7/8 supercell, magnetic orthorhombic, and magnetic tetragonal) and find that the experimentally observed changes are much smaller than the differences between the experimental data and the calculations. Agreement is improved if magnetism is included in the calculations via the local spin density approximation, as the Fe atomic motions parallel to the ferromagnetic ordering direction are softened. However, the antiferromagnetically polarized modes remain hard, and in disagreement with the experimental data. In fact, given the increasing evidence for anisotropy in the iron pnictide materials, the phonon response is surprisingly isotropic. We consider several modifications of the ab initio calculations to improve the agreement with the experimental data. Improved agreement is found by setting the matrix to zero (clipping the bond) between nearest-neighbor antiferromagnetically aligned Fe atoms in the magnetic calculation, or by softening only the in-plane nearest-neighbor Fe-As force constant in the nonmagnetic calculation. We discuss these results in the context of other measurements, especially of phonons, for several FeAs systems. Fluctuating magnetism may be a partial explanation for the failure of the calculations, but seems incomplete in the face of the similarity of the measured phonon response in all the systems investigated here including those known to have static magnetism.