Computer Simulations and Experimental Studies on Transition Metal Borides to 390 GPa*

The attainment of near TPa static pressure in the laboratory has led to a renewed interest in phase transformations and shear strength measurements of incompressible transition metals rhenium (Re), osmium (Os), and their superhard diborides (ReB₂ and OsB₂) We have compressed hexagonal ReB₂, OsB_2, and Os₂B₃ to multi-megabar pressures using Focused Ion Beam machined toroidal diamond anvils. The phase transformations and equation of state were studied using micro-beam angle-dispersive x-ray diffraction at HPCAT beamline 16-BM-D. The platinum pressure marker was employed to the highest pressure of 390 GPa. First-principles simulations based on density functional theory (DFT) were utilized to model ReB₂ and OsB₂ under external pressure. The ambient-pressure lattice parameters computed by the projector augmented-wave method with generalized gradient approximation agree well with the experiment within 1% of error. We will present a comparison of experimental shear strength and equation of state with the elastic constants from DFT calculations up to the highest pressure under study.