Thermoelastic properties of ReB₂ at high pressures and temperatures and comparison with Pt, Os, and Re

We have measured the phase stability and thermoelastic equation of state of ultrahard rhenium diboride at pressures up to 30 GPa and temperatures up to 2500 K using a laser heated diamond anvil cell in conjunction with synchrotron X-ray diffraction. ReB₂ is shown to be stable throughout this pressure and temperature region. The ratio of the c-axis to the a-axis provides a monitor of the annealing of plastic stresses during compression. We show that ReB₂ has a small thermal anisotropy but a large mechanical anisotropy. Combining this new data set with previously existing results from a large volume press yields a thermoelastic equation of state with a Grüneisen parameter of 2.4 (0.08) and a q of 2.7. A comparison of ReB₂ with other high electron density incompressible metals—Os, Re, and Pt—shows that ReB₂ has the lowest thermal pressure and the highest bulk modulus.