High-pressure behavior and thermoelastic properties of niobium studied by in situ x-ray diffraction

In situ synchrotron energy dispersive x-ray diffraction (XRD) experiments on Nb have been conducted at pressures up to 6.4GPa and temperatures up to 1073K. From the pressure-volume-temperature measurements, thermoelastic parameters were derived for the first time for Nb based on the thermal pressure (ΔP_th) equation of state (EOS), modified high-T Birch-Murnaghan EOS, and Mie-Grüneisen-Debye EOS. With the pressure derivative of the bulk modulus KT′ fixed at 4.0, we obtained the ambient isothermal bulk modulus K_T0=174(5) GPa, the temperature derivative of bulk modulus at constant pressure (δKT/δT)P=-0.060(8) GPa K^-1 and at constant volume (δKT/δT)V=-0.046(8)GPa K^-1, the volumetric thermal expansivity αT(T)=2.3(3)×10-5+0.3(2)×10-8T (K^-1), as well as the pressure dependence of thermal expansion (δα/δP)T=(-2.0±0.4)×10-6 K^-1GPa ^-1. Fitting the present data to the Mie-Grüneisen-Debye EOS with Debye temperature Θ₀=276.6K gives γ₀=1.27(8) and K_T0=171(3) GPa at a fixed value of q=3.0. The ambient isothermal bulk modulus and Grüneisen parameter derived from this work are comparable to previously reported values from both experimental and theoretical studies. An in situ high-resolution, angle dispersive XRD study on Nb did not indicate any anomalous behavior related to pressure-induced electronic topological transitions at ∼5GPa as has been reported previously.