Equation of state of tantalum to 174 GPa

The volume compression of tantalum was measured to 174 GPa in a diamond-anvil cell using angle-resolved synchrotron x-ray diffraction. A third-order Birch-Murnaghan equation-of-state constrains the compression data rather well with the zero-pressure isothermal bulk modulus B_0T=194.7(4.8) GPa and the first pressure derivative B^′=3.4(0.1). These values are consistent with the previous ultrasonic results, B_0S=196 GPa and B^′=3.8, but are different from the previous x-ray compression results, B_0T=231 GPa and B^′=2.5. The current compression data also demonstrate that the bcc phase of Ta continues to be stable to 174 GPa, which qualifies tantalum for a pressure standard in static high-pressure experiments.