Crystal-orientation effects of the optical extinction in shocked Al<sub>2</sub>O<sub>3</sub>: a first-principles investigation
Authors: Li Tian-Jing , Cao Xiu-Xia , Tang Shi-Hui , He Lin , Meng Chuan-Min ,   Institute of Solid State Physics, College of Physics and Electronic Engineering, Sichuan Normal University, Chengdu 610101, China,   National Key Laboratory for Shock Wave and Detonation Physics Research, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

Sapphires (Al<sub>2</sub>O<sub>3</sub>) is an important ceramic material with extensive applications in high-pressure technology and geoscience. For instance, it is often used as a window material in shock-wave experiments. Consequently, understanding the behavior of its transparency change under shock compression is crucial for correctly interpreting the experimental data. Sapphire has excellent transparency at ambient conditions, but its transparency is reduced under shock loading. This shock-induced optical extinction phenomenon in Al<sub>2</sub>O<sub>3</sub> has been studied experimentally and theoretically a lot at present, but the knowledge on the crystal-orientation effects of the extinction is still insufficient. the experimental investigations at low-pressure region (within 86 GPa) have indicated that the shock-induced extinction in Al<sub>2</sub>O<sub>3</sub> is related to its crystal orientation, but it is not clear whether the correlation also exists at high-pressure region (~131–255 GPa). Here, to investigate this question, we have performed first principles calculations of the optical absorption properties of a-, c-, d-, r-, n-, s-, g- and m-oriented Al<sub>2</sub>O<sub>3</sub> crystals without and with <inline-formula><tex-math id="M1">\begin{document}$V_{\rm O}^{ + 2}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20190955_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20190955_M1.png"/></alternatives></inline-formula> (the +2 charged O vacancy) defects at the pressure range of 131–255 GPa. It is found that: 1) there are obvious crystal-orientation effects of the extinction in shocked Al<sub>2</sub>O<sub>3</sub> at high-pressure region, and they strengthen with increasing pressure; 2) shock-induced <inline-formula><tex-math id="M2">\begin{document}$V_{\rm O}^{ + 2}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20190955_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20190955_M2.png"/></alternatives></inline-formula> defects could play an important role in determining these crystal-orientation effects, but the influences of pressure and temperature factors on them are relatively weak. A further analysis shows that, at the wavelength range adopted in shock experiments, the extinction of a-orientation is the weakest (the best transparency), the extinction of c-orientation is the strongest (the worst transparency), and the extinction of s-orientation is between them; at the same time, the extinction of m-orientation is similar to that of a-orientation, the extinction of r-, n- and d-orientations is close to that of c-orientation, and the extinction of g-orientation is weaker than that of s-orientation. In view of this, we suggest that the a- or m-oriented Al<sub>2</sub>O<sub>3</sub> is chosen as an optical window in shock-wave experiments of the high-pressure region. Our predictions could be not only helpful to understand further the optical properties of Al<sub>2</sub>O<sub>3</sub> at extreme conditions, but also important for future experimental study.