Structural prediction of ultrahard semi-titanium boride (Ti sub(2)B) using the frozen-phonon method

The dynamical stability, crystal structure, and mechanical properties of semi-titanium boride (Ti sub(2)B) are extensively studied using the pseudopotential plane-wave method within first-principles calculations. Soft transverse acoustic phonon modes are identified in the material, which has been assumed for a long time to have an Al sub(2)Cu-type structure with a I4/mcmspace group. Using the frozen-phonon method, a tetragonal I4/mstructure is uncovered to be energetically more stable and have similar diffraction patterns compared to the previously assumed Al sub(2)Cu-type structure. Phonon dispersion and elastic constant calculations reveal the dynamical and mechanical stability of the I4/mstructure of Ti sub(2)B at ambient pressure. The calculated high elastic constants, modulus, and ideal strength show that Ti sub(2)B possesses outstanding mechanical properties.