First-principles Calculation Of Optical Properties Of Al₂O₃ And AlN Under High Pressure

Al₂O₃?Sapphire?is a widely used material in high pressure experiments and geosciences.For example,it is often used as an optical window for dynamic high-pressure experiments.Therefore,it is of great scientific significance and technical need to explore the variation of the optical properties of this material under high pressure.Currently,Al₂O₃ and LiF crystal materials are often used as optical windows in shock-wave experiments.Because the test samples have different impact impedances in the impact high voltage experiment,it is still an important task in the experimental research to find new window materials with different impact impedances.Under normal conditions,AlN crystals have high optical transparency and hardness,and their chemical and thermal stability are also excellent,so it is considered as a new optical window material that may be used in shock-wave experiments.Therefore,it is of great scientific significance to explore the variation of optical properties of AlN under high pressure.Thus,the research contents and conclusions are as follows:?1?The first-principle methods were used to calculate the optical properties of Al₂O₃ crystal under high pressure within 700 GPa.The results show that:1)The CaIrO₃-U₂S₃ structural phase-transition leads to the increase of main-peak intensity,the evident decrease of additional-peak intensity and the red shifts of main and additional peaks and absorption edges in Al₂O₃ absorption spectrum.2)The structural phase-transition leads to the decrease of the peak intensity of the Al₂O₃ refractive index,the increase of the peak number and the enhancement in refractive index at the wavelength range of 400-2000 nm.?2?The optical properties of AlN crystal without and with aluminum and nitrogen vacancy defects were calculated under pressure within 100 GPa.The calculated data of optical-absorption properties show that:the wurtzite-to rocksalt-structural transition in AlN will not change the optical absorbency in the visible light range?the optical-absorption coefficient of the two structural phases of AlN in the visible light region is zero?.Al-and N-vacancy defects in the rocksalt-structural phase of AlN will cause a strong optical-absorption in the visible light range.The refractive indexes at 532nm show that:the wurtzite-to rocksalt-structural transition in AlN will lead to an increase in refractive index.Al-vacancy defects in the rocksalt-structural phase of AlN will cause an enhancement in its refractive index,but N-vacancy defects will lead to the decrease in refractive index.The loss-function spectrum data indicate that:both the structural phase-transition in AlN and the Al-and N-vacancy defects in its rocksalt-structural phase cause a blue-shift of its loss-function spectrum and an increase in the main-peak intensity.The data of the reflection spectrum indicate that:the wurtzite-to rocksalt-structural transition in AlN will lead to an increase in the intensity of the main peak and the number of spectrum peaks,and the structural phase transition causes an increase of AlN reflectance in the wavelength range of 400-2000 nm;Al-and N-vacancy defects in the rocksalt-structural phase of AlN have a great influence on the reflection spectrum in the wavelength range of 200-2000 nm.