Pressure-induced phonon frequency shifts in transition-metal nitrides

We report the experiments on the high-pressure phonon spectra of transition-metal nitrides HfN, ZrN, and NbN, obtained by Raman-scattering measurements. Two pronounced bands, which are related to the acoustic part at low-frequency around 200 cm(-1) and the optical part at high-frequency around 550 cm(-1) of the phonon spectrum, respectively, shift to high-frequency values with increasing pressure. An analysis of the results allows us to reproduce the experimental pressure dependence of the superconducting transition temperature T-c of ZrN and NbN. We also predict the pressure effect on T-c in HfN.