Optical Properties Of Al₂O₃ And YAG Crystals Under High Pressure

Sapphire(Al₂O₃ crystal)is an important ceramic material with extensive applications in high-pressure science.For instance,it is often used as an optical window in shock-wave experiments.Consequently,acquiring information of the behavior of its transparency change under shock compression is crucial for correctly interpreting these experimental data.Under ambient conditions,sapphire has excellent optical transparency,but its transparency is significantly reduced under shock loading.Currently,the experimental and theoretical investigations on the shock-induced optical extinction phenomenon in sapphire have been made a lot,but most of them are based upon Al₂O₃ crystal with a certain crystal-orientation.In fact,the knowledge on the crystal-orientation effects of the sapphire shock-extinction is also essential for following reason:experimental researchers need to find the direction of shock propagation for which shocked sapphire is most transparent as an optical window.In addition,although Al₂O₃ and LiF crystals are often considered as optical windows in dynamic high-pressure experiments at present,people are also actively looking for a new shock window materials.In summary,the study details and conclusions of this paper are indicated as follows:(1)The optical absorption properties of eight crystallographic orientations(a-,m-,g-,s-,n-,r-,d-and c-oriented)Al₂O₃ crystals without and with oxygen-vacancy defects at high-pressure region(¹31-255 GPa)were calculated using the first-principles method.The results show that:1)there are the obvious crystal-orientation effects of the extinction in shocked sapphire at high-pressure region,and these effects strengthen with increasing pressure;2)Shock-induced+2 charged oxygen-vacancy defects could play an important role not only in explaining the phenomena of the transparency loss in shocked sapphire,but also in determining the crystal-orientation effects in its optical extinction;3)A further analysis of these crystal-orientation effects shows that,at the wavelength range adopted in shock experiments(²50-1000 nm),the a-orientation has the weakest extinction(the best transparency),the c-orientation has the strongest extinction(the worst transparency),and the extinction of s-orientation is between them,meanwhile,the extinction of m-orientation is similar to that of a-orientation,the extinction of n-,r-and d-orientations is close to that of c-orientation,and the extinction of s-orientation is stronger than that of g-orientation.Therefore,we recommend that the a-or m-oriented Al₂O₃ crystal is chosen as the optical window in shock-wave experiments of the above-pressure region.(2)To investigate whether yttrium aluminum garment(YAG)crystal could be a shock window material,based upon the first-principles method,the optical properties of YAG crystal without and with oxygen-vacancy defects were calculated within pressure 120 GPa.1)The absorptive data show that:within 120 GPa,there is no light absorption(keep transparent)for the perfect crystal and defective crystal with+2 charged oxygen vacancy(the lowest formation energy)in the visible region;2)The refractive-index data at 532 nm indicate that:within 20 GPa,the refractive index of YAG crystal decreases with increasing pressure;above20 GPa,its refractive index rises gradually with increasing pressure.Furthermore,oxygen vacancy point defect will lead to its refractive index an obvious decrease;3)The reflective data of YAG show that:the pressure loading will cause the peak intensity of its reflection spectrum a decrease.at the same time,oxygen vacancy point defect in YAG further weaken the peak intensity;4)The energy loss spectrum data show that:The pressure loading will also cause a decrease in the peak intensity of its energy loss spectrum.Meanwhile,oxygen vacancy point defect in YAG further weaken the peak intensity.Analyzing the above conclusions,it is found that yttrium aluminum garment crystal is likely to be a shock window material,and the obtained results in this paper may provide some useful guidance for further experimental study.