Investigations On Crystal Structure And Physical Properties Of TM₂B And BC₇

The superhard materials with the Vickers hardness exceeding 40 GPa have been widely applied in the traditional field of materials processing,such as cutting,polishing,and coating.Diamond and c-BN,as the traditional superhard materials,have wide applications in industries of oil extraction,geological exploration,military preparation and fine processing.Diamond is the hardest known material with the hardness about60-120 GPa.However,diamond is easy to react with iron-based materials or oxygen when the temperature reaches 700°C,which limites the application of diamond.Although c-BN is superior to diamond in thermal stability and chemical activity,its hardness?⁶3 GPa?is much lower than diamond.Moreover,c-BN can be only obtained by artificial methods and thus the complexity of the synthesis conditions and its cost are increased.Therefore,the many disciplines are dedicated to exploring and designing new superhard materials together.In general,two groups of materials are potential candidates for superhard materials:?1?the covalent simple substance or compounds formed by light elements,?2?the compounds composed of transition metals and light elements.The crystal structures and physical proprieties of 11 TM₂B-type?TM=Ti,V,Cr,Mn,Fe,Co,Ni,Nb,Mo,Ta and W?borides and BC₇ compound are systematically studied based on density functional theory.The proprieties of dynamics,thermodynamics,mechanics and electricity of TM₂B-type borides and BC₇ are studied using first-principles calculations.1.Investigations on structural determination of TM2B-type boridesWe analyze the dynamical stability of 11 TM₂B-type borides with I4/mcm symmetry.It was found that the I4/mcm phases of the?₂B??=Ti,V,Cr,Nb,Mo,Ta,and W?borides were dynamically unstable.By freezing the unstable phonon mode,thee dynamically stable I4/m structure is predicted.The metallic behavior for these borides is found according to the results of calculated electronic band structures.The calculated modulus are much higher than for the corresponding pure metals,stemming from the formation of partial covalent bonds in the compounds by the introduction of the boron atoms.Because of the significant differences of theoretical calculations and experimental data,it is further studied effect of temperature on structural stability.The calculated Gibbs free energies show that the I4/m phases of Ti₂B,V₂B,Nb₂B,Mo₂B and Ta₂B are more stable than their I4/m phases in the temperature range 0 K to 1000 K.It is interesting to note that I4/mcm structures for Cr₂Band W₂B are energetically much superior to I4/m phases above 129 K and 720 K,respectively,suggesting their I4/mcm structure can be stabilized by temperature.The results of molecular dynamics simulations show that B atoms can overcome lower energy barriers for large amplitude vibrations at limited temperature.2.The stable structures of BC7 are designed using the method of structural replacementThe I-43m and P-43m phases for BC₇ are designed using the method of structural replacement.The results of energy band and electronic density of states show that the I-43m and P-43m phases are hole conductors with strong covalent bond.Based on Chen's model,the simulated hardness for I-43m and P-43m phases reach 71.5 GPa and70.0 GPa,respectively,which indicates they are the uncompressible and superhard materials.Therefore,the two crystal structures of BC₇ belong to composites with conductive and superhard properties.