| Cadmium Sulfide (CdS) and Zinc Oxide (ZnO) are both semiconductor materialswith wide application in the field of optoelectronics. The light elements B-C-Nsuperhard material has a wide potential application in the material processing field,especially in the metal processing field where the diamond cannot be applied. With thecontinuous expansion of their application range, they will face the extreme conditions ofhigh temperature and high pressure. Therefore, it is extremely significant to investigatetheir properties under extreme conditions for understanding the properties and scope ofapplication of these types of materials. Because of the limitation of experimentalconditions in high temperature and high pressure, the scope of the study is often verylimited. Thus, the theoretical research has increasingly become an importantlyindispensable way to understand the properties of these materials in a wider range ofpressure and temperature conditions.In recent years, under the density functional theory, the method of usingquasi-Debye model combined with first principles pseudopotential and plane wave basisset, provides an effective mean for investigating the material phase stability, phasetransition, mechanics and thermodynamic properties under high pressure and hightemperature. Althogh the theoretical study is restricted by the model and the method andcondition of calculation, the obtained results and experience show that we still canobtain more accurate results, so as to predict the material properties, guide theexperimental research, reduce the experimental blindness and save the people resources.Mechanical and thermodynamic properties of solid are dependent on theirsymmetry, atom structure and chemical bond property. Due to the different symmetry,solid has an obvious anisotropy in different crystal structure, such as elastic modulus,thermal expansion coefficient of different direction, thermal conductivity, resistivity,electric, electric polarization, magnetic susceptibility and refractive index are different.Anisotropy is one of the important characteristics of the crystal has very importantresearch value. In this work, the study of three kinds of materials (CdS, ZnO and BC2N)belonging to the cubic, hexagonal and tetragonal structure, representatively, at roomtemperature and atmospheric pressure, has some representativeness and typicalsignificance for the first principles study of anisotropy on the mechanics andthermodynamic properties. The detailed work has been given in the following:(1) We have obtained phase stability, phase transition conditions, thermodynamicproperty and electronic structure for CdS with WZ, ZB and RS phase under highpressure and temperature. It turns out that the cubic ZB phase has a larger enthalpy thanthat of WZ phase with value0.1eV and thus the WZ phase is more stable. In highpressure the phase transition pressure is2.18GPa for CdS from hexagonal to tetragonalphase. However, heat capacity is barely influenced by pressure under high temperature.The expansion coefficient increases very obviously with temperature under300K butnearly stable with temperature increase to higher than300K. In pressure lower than30GPa, expansion coefficient has a clear variation with temperature. The increase ofpressure prevents the variation of expansion coefficient. Moreover, in our calculation,we found that there is no asymmetry in expansion coefficient. This is just thecharacteristic of Cubic system.(2) Contrastive study has been carried for the mechanical and thermodynamicproperties of hexagonal WZ and cubic RS structure of II-VI material ZnO under highpressure and high temperature. The results show that the WZ-ZnO phase at0Ktemperature will change into cubic RS structure at the pressure of8.83GPa, and thehexagonal WZ structure of ZnO exhibits some anisotropy. Moreover, we have discussedthe inherent mechanism of the ZnO anisotropy.(3) For the crystal systems beyond cubic phase, we have carried a contrastive studyon the tetragonal crystal BC2N belonged to the space group of P2221. By theoreticalcalculation we have obtained the state equations, thermal capacity, linear expansioncoefficient along crystal axis, Debye temperature in BC2N under high temperature andpressure with P2221structure. The calculated results of crystal structure and bulkmodulus are in good agreement with experiment. Moreover, the B-M state equation hasbeen obtained with parametersB0and B being393GPa and3.72. The curves ofthermal capacity and Debye temperature have been given in temperature range from0to3000K and pressure range from0to200GPa. And we calculated the linear expansioncoefficients along a, b and c axis and found that in high temperature the coefficientsalong a and b are in agreement but in c axis it is larger. Obviously, the tetragonal BC2Nexhibits anisotropy. The reasons for this have been obtained by calculating the bondpopulation, charge density in crystal structure and bond length with temperature.(4) By introducing a method for obtaining expansion coefficient under hightemperature in anisotropic material based on first principle, the P-V and E-V has been calculated in solids. Crystal constant function has been obtained by fitting EOS equation,by which volume can be calculated. Moreover, anisotropic thermodynamic property hasbeen anticipated for cubic, hexagonal and tetragonal phase materials in high pressureand temperature.This paper systematically studies the properties of materils belonging to threedifferent lattice symmetry under high temperature and high pressure, focusing on thestructural stability and phase transition, and the problem of anisotropic thermalexpansion. We have obtained some valuable new results. These results have importantmeaning for understanding the physcial properties at high temperature and high pressureof the three kinds of materials, also for guiding the experimental work and theapplication of these materials. |
PDF Full Text Request