First-principles Study For YBa₂Cu₃O₇Physical Roperties Under High Temperature And Pressure

The research of high-temperature superconducting materials has20years of history. The high-temperature superconducting materials have a variety of significant features, such as zero-resistance effect, the Meissner effect (perfect diamagnetism), Joseph effect. YBa2Cu3O7is a typical high-temperature superconducting material, and there are lots of studies on this material, including theoretical and experimental research, but there are few studies under extreme conditions (high temperature and pressure).Firstly, based on the first prinples of plane wave pseudopotential density functional theory, we calculated the lattice constant of YBa2Cu3O7. By calculating the CASTEP software, we have calculated a series of the different lattice constant b between3.900and4.000with the increment0.001at fixed c/a, which different c/a ratio changes between3.035to3.125with the increment of0.01, a series of different values of lattice constant a are set to calculate the total energies E and the corresponding volumes V. It is found that the solid YBa2Cu3O7with c/a of3.066has the minimum energy, and the corresponding equilibrium lattice parameters a, b, and c are3.900,3.958and11.959, respectively. The lattice constant a,b and c clearly decrease with pressure increasing, the c axis is the most compressible.Secondly, we calculate the thermodynamic properties of different temperature and pressure for YBa2Cu3O7using the quasi-harmonic Debye model, including the constant pressure heat capacity, constant volume heat capacity, Debye temperature and thermal expansivity. We find that under constant pressure, the constant pressure and constant volume heat capacity increase with the increasing of temperature. The constant volume heat capacity approaches the Dulong-Petit maximum (9NAkB) when the temperature is above700K, and the value is324.285J mol-1K-1for YBa2Cu307. The Debye temperature increases with the increasing pressure. under the higher pressure, the effect of temperature on the physical quantity of Debye temperature is weaker. In addition, at the constant temperature, the thermal expansivity decreases sharply before the pressure is up to the30GPa, when P>30GPa the rate of declining trend becomes slow.Finally, using CASTEP software, we calculate the9elastic constants for under different pressures in order to research the elastic properties of7. Using the9elastic constants, we obtained the bulk modulus, shear modulus and Young’s modulus and Poisson’s coefficient and velocity of sound. We find that the bulk modulus, shear modulus and Young’s modulus increase linearly with the pressure increasing. Furthermore, the value of the Poisson’s coefficient for YBa2Cu3O7is0.331, which indicates the material is metal materials and has the properties of metals.