Compressibilities and phonon spectra of high-hardness transition metal-nitride materials

We report compressibilities measured by synchrotron X-ray diffraction and phonon spectra from Raman scattering at high pressure in the diamond anvil cell (DAC) for cubic transition metal nitrides TiN1−x , γ-Mo2N and VN x . The high-hardness metal nitride compounds have large values of the bulk modulus. B1-structured nitrides normally have no allowed first-order Raman spectra. However, they exhibit broad bands that reflect the vibrational density of states g(ω) associated with breakdown of q=0 selection rules because of the presence of N3− vacancies on anion sites. Peaks in g(ω) at low frequency are identified with the longitudinal and transverse acoustic (TA) branches. The maximum in the TA band is correlated with the superconducting transition temperature in these materials (T c). In situ Raman scattering measurements in the DAC thus permit predictions of the T c variation with pressure for cubic nitrides and isostructural carbide materials.