Optical Properties Of GeO₂ And BaLiF₃ Crystals Under High Pressure

The shock temperature measurement experiment is vital in national defense and military scientific research.Due to the extreme nature of the experiment,such as instantaneous,high pressure and high temperature,an optical transparent window matching the shock-impedance is often attached to the sample to be tested.So that the light radiation propagating to the free surface can maintain the extreme state of high temperature and high pressure in a long period of time,so as to achieve the measurement of shock temperature.However,currently,there is a lack of commonly used window materials,and their shock-impedance are not high,which cannot be adapted to experimental samples with high shock-impedance.Therefore,it is a research focus to develop more optical windows with high shock-impedance.However,exploring new optical windows by experimental means consumes a lot of resources.Based on this,two potential high shock-impedance window materials,GeO₂ and BaLiF₃ crystals,are studied by first-principles method in this thesis.The optical properties of these two crystals are simulated under high pressure,and the effects of structural phase transition and vacancy defects caused by shock high pressure are considered.Theoretically,it analyzes whether they have the possibility of high shock impedance optical window which is urgently needed in dynamic high-pressure experiment,so as to do the screening work in advance for the experimenters to find new optical window.The main conclusions of this thesis are as follows:(1)The optical absorption spectrum of GeO₂ shows that the third structural phase transition caused by pressure has a very different impact on it:the first structural phase transition will lead to a weak blue shift of the absorption edge,while the second and third phase transitions will make the absorption edge red shift.In addition,the vacancy defect factor will also lead to the red shift of the absorption edge of GeO₂crystal.Nevertheless,the crystal continues to maintain its optical transparency in the visible region.At the same time,the energy band structure and density of states of the defective crystal are consistent with the variation law of the absorption spectrum.The refractive index results show that in the four phase regions of GeO₂,the refractive index decreases with the increase of pressure,but the refractive index increases due to structural phase transition and vacancy defects.The energy loss spectrum data show that the first structural phase transition has no obvious effect on the loss peak,the second structural phase transition will lead to the red shift of the loss peak and the decrease of the peak intensity,and the third phase transition will significantly enhance the blue shift of the loss peak and the peak intensity.The peak intensity of the loss peak is decreased by vacancy defects.The existence of germanium defect will make the loss peak red-shift,while oxygen defect will make it blue-shift.(2)The absorption spectrum data of BaLiF₃ crystal show that the pressure factor will not lead to the light absorption behavior of BaLiF₃ crystal in the visible region.The existence of vacancy defects will make the absorption edge of BaLiF₃red shift,but its absorption edge does not appear in the visible region.At the same time,the calculated data of the energy band structure and density of states of the defective crystal also support this result,and the energy loss function curve only appears in the band range of ultraviolet light,which is similar to the law of the absorption curve.These phenomena show that the crystal can be used as a good light transmission material in the field of dynamic high pressure experiment.In addition,the refractive index of BaLiF₃ increases with the increase of pressure.The fluorine vacancy defect increases its refractive index,but the effect of barium and lithium vacancy defects is weak.This thesis predicts that BaLiF₃ crystal has the potential to become a shock optical window,which is worthy of more in-depth experimental and scientific research by scientific researchers.