Physical Properties Of LaF₃ And SrF₂ Crystals Under High Pressures

In shock wave high-pressure experiments,transparent windows are often attached behind non transparent materials such as metals with matching impedance,so as to measure the thermal radiation temperature,interface particle velocity and other physical quantities.Under strong impact compression,the optical and elastic properties of optical window materials will change,and these physical properties are related to the correct interpretation of experimental signals.Therefore,it is very important to study the physical properties of window materials under high pressure in shock wave high-pressure experiments.At present,the window materials that can be used are very sparse and have low impact impedance values,covering a discrete and very limited range.Therefore,there is an urgent need to develop high-impedance window materials to accommodate the relevant physical properties measurements of high-density sample materials.Due to the large number of initially transparent materials,it would be extremely time-consuming and labor-intensive to explore them experimentally.Therefore,the first-principles calculation methods is used to screen out optical window materials with further experimental research value in advance.Under normal conditions,LaF₃and SrF₂crystals have high energy gap value,density and wide transparent band range,which have the potential to become high density candidate window materials.The optical properties and elastic properties of LaF₃and SrF₂were calculated under high pressure,and factors such as pressure,phase transition and vacancy site defects were taken into account,the following conclusions are obtained:(1)The absorption spectrum data of LaF₃show that the absorption edge will blue shift with the increase of pressure in the two structural phases.Phase transition and vacancy point defects make the absorption edge of LaF₃red shift,but do not make it enter the visible region,LaF₃can still maintain good transparency.The refractive index data at 532 nm show that the refractive index of LaF₃perfect crystal increases with the increase of pressure.The defect of lanthanum vacancy will lead to a slight decrease in the refractive index of LaF₃,while the defect of fluorine vacancy will slightly increase it.In addition,the density values of LaF₃show a single increasing trend with increasing pressure,and the data related to elastic properties also show that LaF₃have good compression resistance.(2)In the pressure range of 100 GPa,considering the influence factors of pressure,phase transition and vacancy defects on SrF₂,it is found that these factors do not cause SrF₂to absorb light in the visible region,and SrF₂can still maintain good transparency.The structural phase transition at 6.8 GPa and 29.5 GPa increases the refractive index at 532 nm,and the vacancy defects have little effect on the refractive index.The density values of SrF₂crystals in the pressure range of 100 GPa increase monotonically with increasing pressure,and the two kinds of vacancy defects decrease the density values slightly.The above results indicate that LaF₃and SrF₂have the potential to become candidate optical window materials with higher density,which is valuable for further experimental investigation.