Compressibility measurements and phonon spectra of hexagonal transition-metal nitrides at high pressure: ε-TaN, δ-MoN, and Cr2N

We report compressibility measurements for three transition metal nitrides (ε-TaN, δ-MoN, Cr2N) that have structures based on hexagonal arrangements of the metal atoms. The studies were performed using monochromatic synchrotron x-ray diffraction at high pressure in a diamond anvil cell. The three nitride compounds are well-known high hardness materials, and they are found to be highly incompressible. The bulk modulus values measured for ε-TaN, Cr2N, and δ-MoN are K0=288(6) GPa, 275(23) GPa, and 345(9) GPa, respectively. The data were analyzed using a linearized plot of reduced pressure (F) vs the Eulerian finite strain variable f within a third-order Birch-Murnaghan equation of state formulation. The K0' values for ε-TaN and δ-MoN were 4.7(0.5) and 3.5(0.3), respectively, close to the value of K0'=4 that is typically assumed in fitting compressibility data in equation of state studies using a Birch-Murnaghan equation. However, Cr2N was determined to have a much smaller value, K0'=2.0(2.0), indicating a significantly smaller degree of structural stiffening with increased pressure. We also present Raman data for ε-TaN and δ-MoN at high pressure in order to characterize the phonon behavior in these materials. All of the Raman active modes for ε-TaN were identified using polarized spectroscopy. Peaks at low frequency are due to Ta motions, whereas modes at higher wave number contain a large component of N motion. The high frequency modes associated with Ta-N stretching vibrations are more sensitive to compression than the metal displacements occurring at lower wave number. The mode assignments can be generally extended to δ-MoN, that has a much more complex Raman spectrum. The x-ray and Raman data for ε-TaN show evidence for structural disordering occurring above 20 GPa, whereas no such change is observed for δ-MoN