First-Principles Calculations For Properties Of CaO And SiC Under High Pressure

Structural properties, electronic properties, elastic properties and thermalproperties of CaO and SiC have been evaluated by first-principles plane-wavepseudopotential density functional theory. The calculated properties are in goodagreement with experimental measurements and other theoriral results. Meanwhile,the phase transition of CaO and SiC at zero temperature and high pressure is studiedas a start point to its thermal properties.The structural B1-B2 phase transition of CaO and the elastic properties of theB1 phase of CaO are investigated by first-principles plane-wave pseudopotentialdensity functional theory method. The dependences of the elastic constants c_ij, theaggregate elastic modulus B_s and G, the elastic anisotropic parameter A, and theDebye temperature (?)_D on pressure are successfully discussed. From the usualcondition of equal enthalpies, we find that the structural B1-B2 phase transition ofCaO occurs at 62.8GPa, consistent with the experimental value 63 GPa. From ourelastic constants of CaO under pressure, we find that the B1-B2 structuraltransformation occurs at about 64GPa, which be consonant with the front describedtransition pressure 62.8 GPa and experimental measurements.The structural ZB-RS phase transition of SiC and the electronic properties ofSiC are investigated by first-principles plane-wave pseudopotential densityfunctional theory method, and the lattice constant a, the bulk modulus B₀, the firstorder pressure derivative of bulk modulus B′₀ and elastic constants c_ij are obtained.According to the usual condition of equal enthalpies and commontangentconstruction of energy-volume curve, we find that the transition from the ZB structure to the RS structure occurs at the pressure of 70.5 GPa and 75.4 GPa,respectively. Compared with experimental values 100GPa, our obtained the phasetransition pressures are better in agreement with the theoretical values 65GPapredicted in ab initio density-functional calculations. The thermodynamic propertiesof the ZB structures including the thermal expansion coefficient, specific heat,equation of state have been evaluated through the quasi-harmonic Debye model, wenote that at very low temperatures (below 100K), Cv is almost insensitive to P,within the considered P range. Above this temperature, Cv is found to decrease withincreasing P, which is due to the anharmonic approximations of the Debye modelused here. However at high temperatures T＞1800 K, it becomes again insensitive toP because of the anharmonic effect on Cv suppressed, and Cv is very close to theDulong-Petit limit. Our calculated Cv at ambient condition is 28.87 J mol^-1 K^-1.