First-principles Calculations Of Phase Stability And Physical Properties Of Transitional Metals Boride

Transition metal borides have been extensively investigated by both theories and experiments. The crystal structural, thermodynamic, mechanical and dynamic properties of transition metal borides are studied by first-principle calculations based on density functional theory. The main research results are as follows:（1）The crystal structure, thermodynamic, mechanical and dynamic properties of chromium borides are systematically investigated by first-principle based on density functional theory. The calculated lattice parameters are in good agreement with previous theoretical and experimental results. Among the considered borides, the hardness of hP12-CrB2, oI10-CrB4 and oP10-CrB4 are 42.7 GPa, 45 GPa and 46.1 GPa, respectively. This indicates that they are potential superhard materials. For hP3-CrB2, it is thermodynamically stable and becomes dynamically stable at much higher pressure（e.g. 100 GPa）. The synthesis of hP3-CrB2 could be conducted at high pressures. At 0 GPa, the ground state phases are t I12-Cr2 B, t I32-Cr5B3, tI16-Cr B, oP10-CrB4 and hP12-CrB2; At 50 GPa, the most stable phases are t I12-Cr2 B, tI16-Cr B, o I14-Cr3B4, oC20-Cr2B3 and oP10-CrB4. At 100 GPa, t I12-Cr2 B, t I16-Cr B, oI14-Cr3B4, oC20-Cr2B3, hP3-CrB2 and oP10-CrB4 are the most stable phases. For CrB3, the phase transition mechanism was discussed.（2）The structural, mechanical and thermodynamic properties of WB3 are investigated by means of evolutionary algorithm and first-principles calculations. For WB3, twelve possible phases have been considered: seven hypothetical crystal structures（WB3_m-Al B2, WB3₆u, WB3_OsB3, WB3_MoB3, WB3_FeB3, WB3_TaB3 and WB3_TcP3）, and five predicted structures（WB3-187₁u, WB3187₂u, WB3-187₃u, WB3-164₂u, WB3-150₄u）. Our calculations indicated that WB3₄u, WB3₆u, WB3₁87₃u and WB3_OsB3exhibit good thermodynamic stability. At 0GPa, the most stable phase is WB3₆u. Above 12 GPa, WB3_OsB3 becomes the most stable phase.The volume and axis Compressibility for WB3₄u, WB3₆u, WB3₁87₃u and WB3_OsB3 compared with diamond have been studied. For OsB3-WB3 and WB3₁87₃u, they demonstrate remarkable incompressibility along c direction.（3）The effect on the stability and mechanical properties of nonstoichiometric tungsten borides with W vacancies defects has been studied by first-principles calculations. A series of supercell were built based on W2B9, WB3₄u and WB3_OsB3 with vacancies on the tungsten sub-lattice. The enthalpy of formation becomes smaller, when concentration of W vacancy increases. It is found that with the increasing of concentration of W vacancy defects, bulk modulus, shear modulus and hardness of tungsten borides become smaller. The presence of W-vacancies in considered tungsten borides compounds reduces their thermodynamically stable and mechanical strength.