First-Principles Calculations For Properties Of ZrB₂ Compound Under High Pressure

Transition-metal borides ZrB₂ has several unique properties. For example, both metallic and ceramic property, high melting point 3245oC, high hardness, fine conductivity and thermal conduction, chemical stability and so on .They are excellent refractory ceramics material with chemical inertness for molten metal. Recently, because of its unique high strength, temperature, conductivity, stability, and other fine properties, it has demonstrated the formidable vitality.under the environment of high temperature. Especially it plays an unique role in many fields which metal and ceramics can not play.Despite the technological developments of ZrB₂, some behaviors of ZrB₂ under the high pressure have not been paid enough attention. Many fundamental problems for ZrB₂ under high pressure condition, such as the structural, electronic and bonding mechanisms, has not been solved. The main contents studied in this thesis are divided into 3 parts, which are summarized below:Firstly, we have employed 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′has been obtained. The equilibrium lattice parameters obtained have been shown to be in good agreement with available experimental data and other theoretical results. No theoretical or experimental data for elastic constants are yet available for our comparison. At T=300K, P=0, using the single-crystal elastic constants of ZrB₂, we obtainΘ_D= 908.787K, which agrees well with the valueΘ_D= 910K by Wiley D.E.et al.Secondly, the thermodynamic properties of the ZrB₂ have been obtained through the quasi-harmonic Debye model. We have calculated the heat capacities and the Debye temperatures at different temperatures and different pressures, it is found 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 C_V is dependent on both the temperature T and the pressure P. However, at higher pressures and/or higher temperatures, the harmonic effect on C_V is suppressed, the calculated C_V is very close to the Dulong-Petit limit 9NA kB (≈74.85 J mol^-1 K^-1), which is obeyed by to all solids at high temperature.Finally, we have use the CASTEP package of Materials Studio to discuss total, valence band, and conduction band density of state of ZrB₂ at different pressures based on band theory. It has been noted that 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 .