An emerging class of superhard materials for extreme environment applications are compounds formed by heavy transition metals with light elements. In this work, ultrahigh pressure experiments on transition metal rhenium diboride (ReB2) were carried out in a diamond anvil cell under isothermal and non-hydrostatic compression. Two independent high-pressure experiments were carried out on ReB2 for the first time up to a pressure of 241 GPa (volume compression V/V0 = 0.731 ± 0.004), with platinum as an internal pressure standard in X-ray diffraction studies. The hexagonal phase of ReB2 was stable under highest pressure, and the anisotropy between the a-axis and c-axis compression increases with pressure to 241 GPa. The measured equation of state (EOS) above the yield stress of ReB2 is well represented by the bulk modulus K0 = 364 GPa and its first pressure derivative K0´ = 3.53. Corresponding density-functional-theory (DFT) simulations of the EOS and elastic constants agreed well with the experimental data. DFT results indicated that ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the c-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures.

A high-entropy transition metal boride (Hf0.2 Ti0.2 Zr0.2 Ta0.2 Mo0.2)B2 sample was synthesized under high-pressure and high-temperature starting from ball-milled oxide precursors (HfO2, TiO2, ZrO2, Ta2O5, and MoO3) mixed with graphite and boron-carbide. Experiments were conducted in a large-volume Paris–Edinburgh press combined with in situ energy dispersive x-ray diffraction. The hexagonal AlB2 phase with an ambient pressure volume V0 = 27.93 ± 0.03 Å3 was synthesized at a pressure of 0.9 GPa and temperatures above 1373 K. High-pressure high-temperature studies on the synthesized high-entropy transition metal boride sample were performed up to 7.6 GPa and 1873 K. The thermal equation of state fitted to the experimental data resulted in an ambient pressure bulk-modulus K0 = 344 ± 39 GPa, dK/dT = −0.108 ± 0.027 GPa/K, and a temperature dependent volumetric thermal expansion coefficient α = α0 + α1T + α2 T−2. The thermal stability combined with a high bulk-modulus establishes this high-entropy transition metal boride as an ultrahard high-temperature ceramic material.

High pressure study on ultra-hard transition-metal boride Os 2 B 3 was carried out in a diamond anvil cell under isothermal and non-hydrostatic compression with platinum as an x-ray pressure standard. The ambient-pressure hexagonal phase of Os 2 B 3 is found to be stable with a volume compression V/V 0 = 0.670 ± 0.009 at the maximum pressure of 358 ± 7 GPa. Anisotropic compression behavior is observed in Os 2 B 3 to the highest pressure, with the c-axis being the least compressible. The measured equation of state using the 3rd-order Birch-Murnaghan fit reveals a bulk modulus K 0 = 397 GPa and its first pressure derivative K 0 = 4.0. The experimental lattice parameters and bulk modulus at ambient conditions also agree well with our density-functional-theory (DFT) calculations within an error margin of ∼1%. DFT results indicate that Os 2 B 3 becomes more ductile under compression, with a strong anisotropy in the axial bulk modulus persisting to the highest pressure. DFT further enables the studies of charge distribution and electronic structure at high pressure. The pressure-enhanced electron density and repulsion along the Os and B bonds result in a high incompressibility along the crystal c-axis. Our work helps to elucidate the fundamental properties of Os 2 B 3 under ultrahigh pressure for potential applications in extreme environments.

Shear strength measurements have been carried out on rhenium diboride, ReB2, to a pressure of 74 GPa using a Radial X-ray Diffraction (R-XRD) technique in a diamond anvil cell using platinum as an internal x-ray pressure standard. The R-XRD result has provided a unique insight into the deformation of hexagonal crystal lattice under non-hydrostatic compression and variation of shear strength with increasing pressure. From R-XRD data, we have estimated hydrostatic component of compression to determine an equation of state of rhenium diboride yielding a bulk modulus of K0 = 366 ± 25 GPa with a pressure derivative K0′ = 4.3 ± 0.5 in good agreement with hydrostatic density functional theory calculations. The average lower bound of shear strength (τ) from various diffraction planes was then calculated using the measured interplanar d-spacing (dm) and hydrostatic component of d-spacing (dp) to be shown to approach 6.7 ± 0.4 GPa at 70 GPa. Our results show that the anisotropic compression effects observed in ReB2 under hydrostatic compression are correlated to electronic structure changes under compression as predicted by theoretical calculations.

No abstract