Abstract
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.
Highlights
Transition metal borides have shown intriguing mechanical and structural properties combining the attractive features of metallic bonding with rigid covalent boron-boron bonding [1,2,3]
Rhenium diboride (ReB2 ) has shown desirable mechanical properties with a high average hardness of 30–60 GPa [4,5,6,7] and bulk modulus of 334–360 GPa [4,5], comparable to that of diamonds (442 GPa) [8]. Such materials are useful for their applicability under extreme conditions requiring a combination of high-temperature chemical stability and resistance to plastic deformation. Many superhard materials such as diamonds are prone to oxidation in high-temperature environments and have a propensity for chemical reactivity with transition metals
The equation of state determined from the non-hydrostatic for the first time to a pressure of 241 GPa
Summary
Transition metal borides have shown intriguing mechanical and structural properties combining the attractive features of metallic bonding with rigid covalent boron-boron bonding [1,2,3]. Rhenium diboride (ReB2 ) has shown desirable mechanical properties with a high average hardness of 30–60 GPa [4,5,6,7] and bulk modulus of 334–360 GPa [4,5], comparable to that of diamonds (442 GPa) [8]. Such materials are useful for their applicability under extreme conditions requiring a combination of high-temperature chemical stability and resistance to plastic deformation.
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