Lattice instability coupled with valence degrees of freedom in valence fluctuation compound YbPd

Inelastic x-ray scattering (IXS) was used to examine the valence fluctuation compound YbPd at 300 K in the $\ensuremath{\Gamma}\ensuremath{-}X$, $\ensuremath{\Gamma}\ensuremath{-}M, X\ensuremath{-}M$, and X-R directions. The obtained spectra and determined phonon dispersion relations demonstrate lattice instabilities related to the successive transitions and correlations between phonon spectra and valence degrees of freedom in YbPd. The low-lying branches in the phonon dispersion relations as well as the broadened linewidths of the IXS spectra observed around the longitudinal $X$ point provide supporting evidence of correlation between a valence-lattice interaction and the successive transitions in YbPd. X-ray absorption spectroscopy was also performed in order to discuss correlations between lattice instabilities and Yb valence degrees of freedom in YbPd.