Optical And Elastic Properties Of CaO And KMgF₃ Crystals Under High Pressure

The shock temperature measurement of non-transparent samples requires the use of transparent window materials with matching shock impedance for auxiliary measurement(shock impedance Z=?*D,?is the initial density,D is the shock wave velocity).At present,the impedance values provided by common window materials(single crystal LiF and Al₂O₃)are limited and discontinuous.Therefore,in order to better adapt to experimental samples with different shock impedances and impedance values between LiF and Al₂O₃,it is necessary to constantly explore new transparent windows.Furthermore,it is costly and time-consuming to explore new shock windows from various materials directly through experiments.Therefore,this paper will preliminarily screen out potential window materials through theoretical calculation to prepare for further experimental exploration.Based on the above,the high-pressure transparency of CaO and KMgF₃ crystals were calculated using first-principles method,and the influence factors of shock compression such as pressure,structural transformation and vacancy defects were taken into account,so as to explore whether they have the potential to become optical windows in dynamic physical properties experiments.At the same time,the changes of refractive index and elastic properties under high pressure were calculated,so as to provide some theoretical support for future scientific research.The calculation results are as follows:(1)The data of light absorption properties of CaO crystal show that the pressure,structural phase transformation and defects all make the absorption edges red shift in the high pressure range of 120 GPa,but they does not red shift to the visible band all the time,indicating that the crystal may have good high pressure transparency.The refractive index data at 532 nm and 1550 nm showed that pressure,phase transition and oxygen vacancy defect would increase the refractive index of CaO crystal,while calcium vacancy would slightly decrease its refractive index,and the increase of defect concentration caused by pressure would further increase its refractive index.The high pressure elastic properties of CaO crystal show that all the structural phases(B1 and B2)in the range of 120 GPa are thermodynamically stable.The elastic value of CaO crystal increases except poisson's ratio due to pressure and structural transformation.In addition,the changes of vacancy defects and their concentration also affect the elastic properties of CaO crystal.(2)It can be seen from the calculated data of absorption spectrum of KMgF₃crystal that:in the high pressure range of 100 GPa,the absorption edges of KMgF₃crystal are blue shifted(that is,the range of transparent band becomes wider)under the influence of pressure factors,but red shifted due to the influence of vacancy point defects,and the effect of fluorine vacancy is the most obvious.Even so,its absorption edges does not enter the visible region.The calculated results of refractive index(at532 nm and 1550 nm)show that the refractive index value of KMgF₃ crystal increases gradually with increasing pressure,The presence of potassium vacancy and magnesium vacancy will slightly decrease the refractive index,but the fluorine vacancy will increase the refractive index.The calculation results of elastic properties are as follows:the elastic constant and elastic modulus of KMgF₃ perfect crystal increase with the increase of pressure,and the stability criterion shows that KMgF₃crystal maintains thermodynamic stability in the high pressure range of 100 GPa.The existence of vacancy defects will change the elastic properties,but the effect is not significant.These results indicate that CaO and KMgF₃ crystals have the potential to be candidate optical windows for shock wave experiments,and further experimental investigation is recommended.