Pressure-induced novel structure with graphene-like boron-layer in titanium monoboride

Abstract

The recent discovery of the novel boron-framework in boron-rich metal borides with complex structures and intriguing features under high pressure has stimulated the search into the unique boron-network in the metal monoborides or boron-deficient metal borides at high pressure. Herein, based on the particle swarm optimization algorithm combined with first-principles calculations, we thoroughly explored the structural evolution and properties of TiB up to 200 GPa. This material undergoes a pressure-induced phase transition of Pnma → Cmcm → Pmmm. Besides of two known phases Pnma and Cmcm, an unexpected orthorhombic Pmmm structure was predicted to be energetically favored in the pressure range of 110.88–200 GPa. Intriguingly, the B covalent network eventually evolved from a one-dimensional zigzag chain in Pnma-TiB and Cmcm-TiB to a graphene-like B-sheet in Pmmm-TiB. On the basis of the microscopic hardness model, the calculated hardness (H v) values of Pnma at 1 atm, Cmcm at 100 GPa, and Pmmm at 140 GPa are 36.81 GPa, 25.17 GPa, and 15.36 GPa, respectively. Remarkably, analyses of the density of states, electron localization function and the crystal orbital Hamilton population (COHP) exhibit that the bonding nature in the three TiB structures can be considered as a combination of the B–B and Ti–B covalent interactions. Moreover, the high hardness and excellent mechanical properties of the three TiB polymorphs can be ascribed to the strong B–B and Ti–B covalent bonds.