First-Principles Calculations For Properties Of ReB₂ Compound Under High Pressure

Ultrahard materials are used in many applications, from abrasives and cutting tools to scratch-resistant coatings. Unfortunately, almost all superhard materials (diamond, cubic BN, etc.) are expensive because they either occur naturally in limited supplies or have to be made at high pressure synthetically. Recently, Chung et al. successfully synthesized the hard material ReB₂ under ambient conditions, for which the incorpotation of B into the interstitial sites of Re to form ReB₂ brings only 5% expansion of the Re lattice. At a certain direction, the incompressibility of ReB₂ is the same as the diamond, at another direction, the compressibility of ReB₂ is only slighty higher than the diamond. At low forces, the hardness of ReB₂ is comparable to that of cubic BN. At higher forces, the hardness of ReB₂ is only slighty lower than the cubic BN.ReB₂ is a new superhard material. Compared with other materials, both the theoretical research and development of ReB₂ are not very sophisticated. In that case, it can not be applied on a large scale. Consequencely, it is impossible for ReB₂ to replace the diamond in a short time. But we are sure that it will have a prosperous future tommrrow. Many fundamental problems for ReB₂ under high pressure condition, such as the structural, electronic and bonding mechanisms, still remain unsolved. Main contents studied in this thesis is divided into 3 parts, their main contents are summarized below: Firstly, we employ ab initio plane-wave pseudopotential density functional theory to calculate the equilibrium lattice parameters, and the lattice constant a and c , the five independent elastic constants, the bulk modulus B0 and the first order pressure derivative of bulk modulus B₀′are obtained. Our results are in good aggreement with the experimental data and other calculations. At T=300K, P=0, using the single-crystal elastic constants of ReB₂, we obtainΘ_D= 712.70K, which agrees with the valueΘ_D= 716K through the quasi-harmonic Debye model..Secondly, the thermodynamic properties of the ReB₂ are obtained through the quasi-harmonic Debye model. we calculate the heat capacities and the Debye temperatures at different temperatures and different pressures, and find that as pressure increases, the heat capacity CV decreases and the Debye temperatureΘ_D increases. It is shown that when T < 1500 K, the heat capacity CV is dependent on both the temperature T and the pressure P. However, at higher pressures and/or higher temperatures, the an harmonic effect on C_V is suppressed, and CV is very close to the Dulong-Petit limit 9N_A k_B (≈74.83 J mol^-1 K^-1), which is followed to all solids at high temperature.Finally, the band structure,density of state have been calculated.The calculated results indicate a metalic characteristic in ReB₂. We use the CASTEP to discuss total, valence band ,and conduction band density of state of ReB₂ at different pressures based on band theory. It shows relative intensity of density of state on each section has a slight decrease when the pressure increases. Moreover, as pressure increases gradually, density of state of valence bands and conduction bands shift towards the high energy, the width of valence bands and conduction bands increase slightly. Furthermore, we have calculated Charge Transfer, Bond Length, Bond Population. We find that as pressure increases, the charge transfer increases, but the bond length decreases ,and population has also changed .