Phase stability and physical properties of technetium borides: A first-principles study

By first-principles calculations, we establish the phase stability of technetium borides with various stoichiometries. Moreover, a new phase of TcB has been predicted using a newly developed particle swarm optimization (PSO) algorithm, which belongs to the orthorhombic Cmcm space group. The new phase is both mechanically and dynamically stable, as verified by the calculations of its elastic constant and phonon dispersion. The formation enthalpy-pressure diagrams reveal that the predicted TcB is more energetically favorable than the previously proposed WC-type structure in a pressure range from 0 to 100 GPa. Our calculations on the enthalpy-pressure relationship and convex hulls have demonstrated that the experimentally observed Tc3B, Tc7B3, and TcB2 are stable against decomposition into other components both at zero and high pressure, and the predicted TcB becomes the most stable phase above 8 GPa. An analysis of the elastic properties and Debye temperature shows that TcB2 is a potential hard material. The electronic localization function and electronic structure provide that the strong Tc-B and B-B covalent bonds are the main reason of its high hardness.