First-Principles Calculations For Properties Of AlB₂ Compound Under High Pressure

Recently, properties of material at high pressures and high temperatures have been the objects of intensive experimental and theoretical investigations. The metal boride has many unique properties, especially makes one arouse the interest. The thermal stability and semiconductor properties is eximious and it has the fairish neutronabsorption ability, so it has been used to make the semiconductor and the atomic pile material. Despite the technological developments of AlB₂ , the high pressure behavior of AlB₂ has been the subject of considerable attention. Many fundamental problems for AlB₂ under high pressure condition, such as the structural, electronic and bonding mechanisms, still remain unsolved.Firstly, we employ the first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of density functional theory (DFT) to calculate the equilibrium lattice parameters and the thermodynamic properties of hcp structure AlB₂ compound, and the lattice constant a and c, the five independent elastic constants and the bulk modulus B₀ and the first order pressure derivative of bulk modulus B₀′are obtained. The equilibrium lattice parameters obtained are 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.Secondly,. the thermodynamic properties of the AlB₂ 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 temperatureÃŽËœincreases. 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 anharmonic effect on CV is suppressed, and CV is very close to the Dulong-Petit limit 3 k_B (≈49.90 J mol^-1 K^-1), which is followed to all solids at high temperature.The heat capacity CV in this work is 34.39714 J mol-1 K-1 at zero pressure and ambient temperature. On the other hand, through the quasi-harmonic Debye model, the dependences of the normalized lattice parameters a/a₀ and c/c₀ on pressure P, the normalized primitive volume V/V₀ on pressure P, as well as the variation of the thermal expansionαwith temperature and pressure are obtained successfully.Finally, using the elastic constants, we obtain the adiabatic bulk modulus BS, shear modulus G, aggregate acoustic velocities and Debye temperature. Unfortunately, no theoretical or experimental data are yet available for our comparison.