Study Of High-pressure Behavior Of Two Solid Materials(Al₂O₃,Cu)

The structural change of the solid materials under high pressures (including crystal and electronic structures) will affect their elastic, electrical and optical properties. Studying their physical properties under high-pressure is important for understanding nature. This paper is divided into two parts, studying on some physical properties of Al₂O₃ and Cu under high pressure respectively.In the first part of this thesis,using first-principles calculations, band structure and the pressure dependence of the band gap of perfect Al₂O₃, and investigate the optical properties (optical absorption coefficient, the real and imaginary parts of the dielectric function, reflection and absorption spectrum and the energy loss spectrum), and study the electronic density of states and mechanical properties of Al₂O₃ without defects at high pressures. By these calculations, some possible physical mechanisms for the transparency loss and the onset of the electrical conductivity, observed by shock-wave experiments, are presented.The main work and results of this part are as follows:1) Based on the plane-wave pseudopotential method in the frame-work of the density function theory and the local density approximation of Ceperly and Adler by the parametrization of Perdew and Zunger (LDA-CA-PZ), the author determines the pressure dependence of the band gap for the three perfect Al₂O₃ structure phases at 0 K, and the band-gap data may be obtained from the corresponding calculated energy-band structures. It is found that Corundum-Rh₂O₃(II) and Rh₂O₃(II)-CaIrO₃ transitions in alumina at 0 K cause about 7-8% and 18-20% band-gap reductions, respectively. The band gap decreases slightly with pressure in the CaIrO₃ phase region but increases in Corundum and Rh₂O₃(II) phase regions.2) While the onset point (the temperature and pressure condition) of the observed conductivity increase is compared with the phase boundary line between the Rh₂O₃(II) and CaIrO₃ structures, the conductivity increase of shocked Al₂O₃ at about 130 GPa is associated closely with this transition at about 130GPa and 1500 K. Estimations indicate that the conductivity increase (â–³lnσ), produced by the band-gap reduction due to the Rh₂O₃(II)-CaIrO₃ transition at ¹30 GPa and ¹500 K, may be estimated through a relationship:â–³lnσ～6.49 if the effect of the Rh₂O₃(II)-CaIrO₃ transition on the band gap of Al₂O₃ is considered. This information implies that the onset of the conductivity increase is attributed possibly to a band-gap decrease due to the Rh₂O₃(II)-CaIrO₃ transition at ¹30 GPa and ¹500 K. Moreover, the band gap-pressure relations may just explain the trajectory of experimental conductivity data, which shows that the predominant conduction mechanism of sapphire at shock pressures of 91-220 GPa is electronic conduction.3) Because of similarities of the crystal structures and Raman spectra of Al₂O₃-CaIrO₃ and MgSiO3 post-perovskite, the calculations of Al₂O₃ suggest that a perovskite to post-povskite transition in MgSiO3 causes perhaps a band-gap reduction as well, which makes MgSiO3 post-perovskite possess the high conductivity. This has significant implications for exploring the source of fairly large electrical conductivity at the Earth's lowermost mantle.4) Using the plane-wave pseudopotential method in the frame-work of the density function theory and the local density approximation of Ceperly and Adler by the parametrization of Perdew and Zunger (LDA-CA-PZ), the author has performed static first-principles calculations of optical absorption coefficients of perfect Al₂O₃ under high pressures. Results indicate that optical absorption coefficients of Al₂O₃ at 0-220GPa are always zero within the wavelength range adopted in shock experiments (²50-1000nm). The phase transitions in alumina at high pressure and temperature might not be responsible for its optical transparency degradation observed at shock pressures of about 130-172GPa.5) The calculated data reinforce the suggestion that the Corundum-Rh₂O₃(II) transition change the electrical conductivity of alumina but don't support the inference that this transition causes its transparency loss.In the second section of this thesis, the effect of vacancy concentration on elastic properties of copper under high pressures (from 0GPa to 35 GPa) is studied by means of MD simulation. The embedded-atom model (EAM) is employed to describeinter-atomic interaction in face-centered cubic (FCC) copper. In order to avoid the influence of surface effect, the periodic boundary conditions is employed in.simulation, so that the defect can move in the infinite space. To control the constant temperature and stress, Nose-Hoover method and Parrinello-Rahman method are.used, respectively.The main work and results of this part are as follows:1). MD method with EAM potential were used to study the elastic constants.under pressures range from 0GPa to 35GPa in the cell with size of.8×8×8.The linear relation between the elastic constants and.pressure was obtained. The data were fitted to Murnagahan isothermal equation of.state (EOS) to get the bulk modulus. The P-V/Vo relation is compared with.experimental result.2). Four cells with 5×5×5,6×6×6,7×7×7,8×8×8 chosen. Each cell has one correspond to different vacancy concentration. The simulation condition is at 0K and 0GPa. The relation between elastic constants and the vacancy concentration was calculated3). The elastic constants of the cell, containing one vacancy, with dimensions of 8×8×8 under high pressures ranging from 0GPa to 35GPa were calculated. The comparison was made for the cell with one vacancy and the perfect cell with same size. Pressure effect on the elastic properties of imperfect crystal was obtained.