Crystal structure and physical properties of Ti2B5 predicted by first principles calculations

Applying the advanced CALYPSO structure searching method and first principles calculations, we predict a novel superhard structure for Ti2B5, which belongs to the monoclinic C2/m space group. The previously known Hf2B5, W2B5, and δ-V2O5-type structures are also selected as the potential structures for Ti2B5. It is shown that the predicted C2/m structure is the most stable phase among these considered structures within the range of 0-100GPa. The calculations of the phonon dispersion and elastic constants affirm that the predicted C2/m phase is dynamically and mechanically stable. The predicted high shear modulus (249 GPa) and large hardness (46.9 GPa) show that it should be an underlying superhard material. In addition, the elastic anisotropy of Ti2B5 is also explored by investigating the directional dependence of the Young's modulus, shear modulus, and Poisson's ratio as well as some well anisotropy indices. The particular analyses of the density of states and electron charge density distribution verify that the strong B-B and Ti-B covalent bonds in the predicted C2/m-Ti2B5 phase play an important part in its hardness and structural stability.