First Principles Investigations On Borides, Carbides, Nitrides Of Lanthanides And 5d Transition Metals

Transition metal borides, carbides, and nitrides have been intensively studied both experimentally and theoretically due to their superior properties of lower diffusion coefficient, higher conductivity, hardness, and melting point. The limitation of getting enough size single crystal makes it difficult to measure the properties exactly. Thus, it is important to predict the properties of transition metal borides, carbides, and nitrides by first principles.The elastic properties and electronic structure for lanthanide nitrides with rocksalt structure were studied by pseudopotential plane wave method based on density functional theory. The calculated lattice parameters are in good agreement with the available experimental and theoretical values. The calculations indicate that NdN, PmN, SmN, EuN, and TbN are mechanically unstable. The lower B/G and Poisson's ratio of HoN and ErN show that they are brittle than other lanthanide nitrides. All the lanthanide nitrides are metallic. The hardness results indicate that they all have excellent mechanical properties. Compared with EuN and TbN, GdN has a lower hardness. The reasons for gadolinium break are also discussed.The elastic properties and electronic structure for 5d transition metal diborides with ReB₂ structure were studied by first principles. The calculations show that ReB₂ possesses the highest bulk modulus, shear modulus, and Young's modulus, following by WB₂. The calculated methods of metallity and hardness for complex multibond compounds are suggested. The hardness results indicate 5d transition metal diborides with ReB₂ structure possess excellent mechanical properties, and are also potential hard and incompressible materials.5d transition metal monocarbides with rocksalt, cesium chloride, zinc blende, and tungsten carbide structures were studied by first principles. The calculations indicates that the bulk modulus for 5d transition metal monocarbides with the same structure increase first and then decrease with increasing atomic number. The position of pseudogap for metallic compounds may be determined by density of states analysis. The pseudogap energy for the same structure shift from right to left with increasing atomic number. A valence electron number of 8 valence electrons/cell is a stable valence shell configuration for 5d transition metal monocarbides with rocksalt and cesium chloride structure, and 10 valence electrons /cell with tungsten carbide structure.5d transition metal monocarbides and mononitrides with NiAs structure and 5d transition metal carbonitrides with orthorhombic structure were studied by first principles. The elastic results indicate that Hf2CN and Ta2CN substituting one nitrogen atom for carbon atom in mononitrides have a larger shear modulus and Young's modulus, and a smaller Poisson's ratio. The elastic properties and hardness for TaC are different from other monocarbides and mononitrides in the pressure range of 20-40 GPa. As the pressure increases, the shear modulus and Young's modulus for TaC decrease first and reach a maximum value at the pressure of 28 GPa. The Poisson's ratio for TaC is just the opposite. The hardness increases with increasing pressure, and reach a minimum value at the pressure of 32 GPa. The hardness and elastic results confirm that there is no one-to-one correspondence between hardness and bulk modulus or shear modulus.