蓝宝石冲击消光晶向效应的第一性原理

Crystal-orientation effects of the optical extinction in shocked Al2O3: a first-principles investigation

蓝宝石的冲击消光现象是高压领域中的研究热点.低压段(86 GPa范围内)的实验研究表明蓝宝石的冲击消光与晶向相关,但在高压段(压力范围:131255 GPa)是否也具有晶向相关性目前尚不清楚.为此,利用第一性原理方法,分别计算了八个不同晶向的蓝宝石理想晶体和含氧离子空位缺陷晶体在高压段的光吸收性质,结果发现:1)蓝宝石在高压段的冲击消光表现出明显的晶向效应,且该效应还随压力增大而增强;一步的数据分析可以看出,在冲击实验采用的波段内,a晶向的消光最弱(透明性最好),c晶向的消光最强与c晶向的消光接近,g晶向的消光要弱于s晶向的消光.鉴于此,如果在高压段开展加窗冲击波实验,建议选择a晶向或m晶向的蓝宝石作为其光学窗口.本文结果不仅有助于深入地认识蓝宝石在极端条件下的光学性质,而且对未来的实验研究有重要的参考作用.

Sapphires(Al2O3)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 Al2O3 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 Al2O3 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 Al2O3 crystals without and with VO^+2(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 Al2O3 at high-pressure region,and they strengthen with increasing pressure;2)shock-induced VO^+2 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 Al2O3 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 Al2O3 at extreme conditions,but also important for future experimental study.