Ab-initio insights into the structural, elastic, bonding, and thermophysical properties of UHx (x = 1, 2, 3, 5, 6, 7, 8) under pressure: Possible relevance to high-Tc superconductivity

Abstract

Binary uranium hydrides, UHx (x = 1, 2, 3, 5, 6, 7, 8), with different crystal symmetries are potentially interesting compounds for high-Tc superconductivity and as hydrogen storage systems. In this work we have explored the structural, elastic, mechanical, bonding, and thermophysical properties of these systems under uniform pressure via density functional theory based computations. Most of the results disclosed in this work are novel. From the calculations of the cohesive energy and enthalpy of formation, we have found that the titled compounds are thermodynamically stable. The computed elastic constants at different pressures ensure elastic stability. All the binary hydrides are mechanically anisotropic. Pressure induced brittle-ductile transition takes place under high pressure. The compounds are machinable with the cubic α-UH3-Pm-3n showing very high value of the machinability index. All the compounds are fairly hard with cubic UH8 showing superhard character. The Debye temperatures and acoustic velocities of these compounds are high; the highest value is found for the cubic UH8. This is highly relevant to the predicted high-Tc superconductivity in cubic UH8 under hydrostatic pressure. The melting temperature, Grüneisen parameter, minimal phonon thermal conductivity, and the thermal expansion coefficient of these compounds have also been studied at different pressures. All these parameters show excellent correspondence with the estimated Debye temperature, elastic parameters and bonding characteristics.