Effects Of Vacancy Point Defects,Phase Transitions And Crystallographic Orientations On Optical Properties Of Shocked Al₂O₃

Laser interferometric and pyrometric measurements in shock-wave dynamic highpressure experiments require use of an optical window.Sapphires?Al₂O₃?is usually considered as an optical-window material due to its high strength,high melting point,high shock impedance and excellent transparency.For explaining above experimental data,we need to understand the characteristics of transparency and refractive index change of Al₂O₃ under shock loading.Thus,acquiring information of its optical-transparency and refractive-index behaviors under shock loading is of great scientific and technical interests.At present,although the behaviors and physical mechanisms of the transparency and refractive-index change in shocked Al₂O₃ have been studied theoretically and experimentally,the knowledge on the crystal-orientation effects of these optical properties is still insufficient.In addition,the dynamic response characteristics of different crystals are disparate in the velocity measurement of the added window.Thus,various sapphire samples may be used in the experiment.Therefore,it plays an important role to explore the crystal-orientation effects of impact transparency and refractive index for understanding the material impact dynamic response characteristics.Meanwhile,the crystal-orientation effects of other optical properties of sapphire?for example,energy loss spectrumas,etc.?also need to be studied,and the results may have important reference for future experimental investigation.According to the above analysis,the main research work and conclusions of this subject are as follows:?1?We perform first principles calculations of optical absorption and refractive index properties of Al₂O₃ crystal without and with vacancy point defect in the 180 GPa pressure range.The calculation results show that:1)it connot explain the observed optical extinction in shocked Al₂O₃ if only considering pressure and temperature factors,but shock-induced oxygen ion vacancy point defect should be an important source for this behavior;2)the refractive index data of Al₂O₃ indicate that the high-pressure structural transition will lead to an obvious increase of its refractive index.The refractive index of Al₂O₃ decreases with enhancing shock pressure in corundum and Rh₂O₃ regions,and it decreases slightly below 172 GPa and increases slowly above 172GPa with increasing shock pressure in CalrO₃ region;3)the vacancy point defects in Al₂O₃ have obvious influence on the shock pressure dependence of its refractive index.?2?The optical-absorption properties of a-,c-and r-oriented sapphire crystals without and with?the+2 charge oxygen vacancy?defects in the pressure range of 255GPa were calculated by using the first-principles method,and the obtained data are mainly to investigate the crystal-orientation effects of the optical-absorption properties of shocked sapphire.The optical-absorption data in sapphire show that:1)the crystal-orientation effects are relatively weak in Corundum region;2)there are certain differences in the absorption properties among three crystal orientations?a,c and r?in Rh₂O₃ region;3)in CalrO₃ region,the c-and r-oriented absorption properties are basically consistent,but there are significant discrepancies for the absorption properties between a and c?or r?orientations,and interestingly,shock-induced oxygen vacancy in sapphire will lead to an appearance in differences of the absorption properties between c and r orientations as well as the further enlargement of its discrepancies between a and c?or r?orientations.These results suggest:1)the changes of crystal-orientation effects of shock transparency in sapphire,observed experimentally at²6 GPa,should be mainly caused by the scattering extinction;2)the disappearance in differences of shock transparency between c and r orientations,observed experimentally at²55 GPa,could be related to the Rh₂O₃-CalrO₃ structural transition in Al₂O₃,and thus the shock-pressure point,at which this phenomenon occurs initially,should be about 163GPa.?3?The optical properties of a-,c-and r-oriented Al₂O₃ crystals with and without oxygen ionic vacancy point defect in the shock-pressure range of 255 GPa were calculated by using first principles methods?Refractive-index data of the perfect Al₂O₃at a wavelength of 1550 nm show that there are strong,weak and strong crystal-orientation effects in Corundum,Rh₂O₃ and CalrO₃ regions,respectively.The loss-function spectrum data of the perfect Al₂O₃ within the wavelength range of 250 nm indicate that its crystal-orientation effects are not obvious in Corundum and Rh₂O₃regions;there are some crystal-orientation effects near the main peak in CalrO₃ region:the intensity of the main peak for c and r orientations are basically consistent,but intensity of the main peak for a orientation is higher than that in c and r orientations.Defective calculations show that the presence of defects increases the refractive-index crystal-orientation effects of sapphire in the Rh₂O₃ structure at 131.2 GPa,but it almost vanishing in the CalrO₃ structure at 255 GPa.In addition,oxygen ionic vacancy point-defects influences weakly the crystal-orientation effects in Al₂O₃ loss-function spectra.