Phonon Softening in PrFeAsO$_{1-y}$ (y$\sim $0.2) by Inelastic X-Ray Scattering

We present phonon dispersion measurements from single crystals of~PrFeAsO$_{1-y}$ with T$_{c}$ (onset) of 42 to 45 K made using inelastic x-ray scattering with 1.5 meV resolution at BL35XU of SPring-8. In agreement with our previous results on powders and crystals [1] we see pronounced softening of the in-plane Fe-As modes compared to phonon calculations using pseudopotential methods in the tetragonal (non-magnetic) structure. C-axis modes are somewhat harder. No strong changes in phonon spectra across T$_{c}$ were observed at the momentum transfers investigated. We also compare our results against calculations of phonons in the magnetic parent material. [1] Fukuda\textit{, et al}, J. Phys. Soc. Japan, \textbf{77} (2008), 103715.