Isothermal compression of bcc transition metals to 100 kbar

Abstract

Pressure‐volume relationships for the bcc transition metals in the groups VB (V, Nb, and Ta) and VIB (Cr, Mo, and W) were determined under hydrostatic pressure to 100 kbar at room temperature by means of x‐ray diffraction, employing diamond‐anvil pressure cell and the ruby (R₁ line) fluorescence calibration technique. Using the measured ultrasonic K′₀ values, where K₀ is the bulk modulus at ambient pressure, for five metals and the deduced shock‐wave K′₀ value for Cr, the values of K₀ for V, Nb, Ta, Mo, W, and Cr are calculated to be 1.54, 1.71, 1.94, 2.67, 3.07, and 1.93 Mbar, respectively, by least‐squares fit of the experimental P‐V data to the Birch‐Murnaghan equation. The K₀ values thus determined are in good agreement with the ultrasonic and shock‐wave values for V, Nb, Ta, Mo, and W. For Cr, the K₀ value (1.93 Mbar) is in agreement with the shock‐wave value (1.92 Mbar) but not with the ultrasonic value (1.65 Mbar). This discrepancy can be explained in view of the paramagnetic→antiferromagnetic transition in Cr in the temperature proximity of the ultrasonic measurements (Neel temperature T_N∼311 °K at 1 bar) and by the negative pressure dependence of T_N.