相变及空位缺陷对CaF2在高压下光学性质的影响

本文采用第一性原理方法, 计算了CaF2的理想晶体和含钙、氟空位点缺陷晶体在100 GPa压力范围内的光吸收谱和折射率. 吸收谱数据表明, 压力因素诱导的两个结构相变对 CaF2的吸收谱均有影响: 第一个相变将导致其吸收边蓝移, 而第二个相变却引起其吸收边红移. 钙空位点缺陷会使得CaF2的吸收边微弱蓝移, 但氟空位点缺陷却导致其吸收边有显著的红移. 然而, 这些红移的行为并未使得CaF2晶体在250-1000 nm的波段范围内出现光吸收的现象（是透明的）. 532 nm处的折射率数据显示, 在 CaF2的三个结构相区（Fm3m、Pnma、P63/mmc相区）, 其折射率均随压力增大而增加; 同时, 高压结构相变以及氟空位点缺陷也使得CaF2的折射率增大, 但钙空位点缺陷却导致其折射率减小. 数据分析表明, CaF2晶体有成为冲击窗口材料的可能, 本文所获得的信息对未来的实验研究有参考价值.

Influences of phase transitions and vacancy defects on optical properties of CaF2 crystal at high pressure

The optical properties of CaF2 crystal with and without VCa-2 and VF+1 vacancy defects within pressure 100 GPa were calculated using the first-principles method. The absorption data show that the two pressure-induced structural transitions in CaF2 have influences on its absorption spectra: the first structural transition leads to a blue-shift of its absorption edge, and the second one results in a red-shift of its absorption edge. VCa-2 will cause a slight blue-shift in its absorption edge, but VF+1 leads to an obvious red-shift in its absorption edge. However, these red-shift behaviors do not give rise to its light absorption within 250-1000 nm. The refractive-index data at 532 nm indicate that the refractive indexes in Fm3m, Pnma and P63/mmc regions of CaF2 crystal increase with increasing pressure, and its high-pressure structural transitions and VF+1 also cause an enhancement in refractive index, but VCa-2 reduces its refractive index. Our analysis suggests that CaF2 is likely to be a shock window material, and the calculated results would provide some important information for further experimental study.