Mechanical properties study for new hypothetical crystalline phases of ReB2: A computational approach using density functional theory

Rhenium diboride (ReB2) in its P63/mmc crystalline structure, is widely known as a super-hard material, and has been studied many times using the Density Functional Theory (DFT) approach. In this work the same chemical composition was studied in three additional unreported hypothetical crystallographic phases by means of DFT with the hybrid functional approach, and the elastic constants of each phase were calculated. The elastic behavior of ReB2 was analyzed by means of elastic moduli calculations. Additionally, the velocities of the elastic waves of each phase were calculated, along with the Debye's temperatures, also elastic anisotropy is studied. Semi-empirical and empirical models of hardness were used to estimate qualitatively which phases are or are not hard. It has been determined that the elastic moduli of two out of the three hypothetical phases are desirable and the elastic waves move very slow (<2 km/s) in one of them. These results and the analysis of the bond critical points (bcp) of each phase allow us to conclude that one of them is soft while the other two are hard. The synthesized phase of ReB2P63/mmc was studied in order to compare and confirm the results.