Novel thorium borides and their hardness and superconductivity under pressure: Ab initio calculations

With the motivation of searching for new superconductors with better hardness in the Th-B system, we have performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0-200 GPa. From our enthalpy of formation calculations, it is found that only ThB4 (Space group No. P4/mbm), ThB6 (Space group No. Pm (3) over bar 3m), and ThB12 (Space group No. Fm (3) over barm) compositions are preferred at zero pressure, in accordance with the experimental reports. Under pressure, our calculations predict the thermodynamically stable compositions, ThB (Space group No. R (3) over barm), Th3B4 (Space group No. R (3) over barm), ThB2 (Space group No. Amm2), and ThB8 (Space group No. R (3) over barm) at 54, 19, 35, and 51 GPa, respectively. Furthermore, although the thermodynamical stability of these compounds is predicted to be at high pressures, the possibility of occurrence at 0 GPa due to metastability still exists. The detailed calculations and analysis of the electronic structure and mechanical and thermal properties have been carried out on these materials using the ab initio tool. The P4/mbm phase of ThB4 and the Fm (3) over barm phase of ThB12 exhibit high hardness of 30.5 and 35.2 GPa under ambient conditions, which is comparable the hard material, B4C. On the other hand, among all the thorium borides, ThB2 shows a high electron-phonon coupling constant of 0.804 and is predicted to have Tc around 12.72 K. In addition, it has been found that the predicted high-pressure compositions, Th3B4 and ThB, display superconductive behavior under pressure. Additionally, various thermophysical quantities such as lattice thermal conductivity, bulk modulus, thermal expansion coefficient, specific heat capacity, and Gibbs free energy under ambient conditions have been determined from these calculations.