Phonon density of states of SmS under high pressure determined by (149)Sm nuclear inelastic scattering

The impact of the pressure-induced valence transition on the phonon density of states (DOS) in SmS was studied by (149)Sm nuclear inelastic scattering of synchrotron radiation. The derived phonon DOS shows drastic differences between the divalent phase at ambient pressure and the intermediate valent phase at 1.5, 2.9, and 4.6 GPa. The phonon DOS at ambient pressure shows two peaks which can be assigned to the transverse-acoustic (TA) and longitudinal-acoustic (LA) phonon branches, while at high pressures only one broad peak is observed. The data were analyzed with a simple model for the TA and LA phonon branches, which could well describe the measured spectra. In the intermediate valent phase the pressure dependence of the cutoff energies of the TA and LA phonon branches is very different and reflects the strong modifications of the LA branches in the intermediate valent phase. This behavior is also responsible for a characteristic variation in the derived Lamb-Mossbauer factors and Debye temperatures.