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

Abstract

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 TiN_1−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.

Keywords:

• Superhard materials
• High pressure
• Nitrides
• Compressibility
• Phonon spectra

Acknowledgements

This work was supported by UK Engineering and Physical Sciences Research Council grant GR/R65206. P.F.M. is a Wolfson-Royal Society Research Merit Award Fellow. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), and Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-0217473), Department of Energy—Geosciences (DE-FG02-94ER14466) and the State of Illinois. Use of the APS was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. W-31-109-ENG-38.