空位缺陷和相变对冲击压缩下蓝宝石光学性质的影响

为了探究冲击压缩下蓝宝石光学性质的变化行为,本文采用第一性原理方法,在180 GPa的压力范围内计算了蓝宝石理想晶体和含空位点缺陷晶体的光学性质.吸收光谱数据表明,仅考虑压力和温度因素不能解释冲击消光实验的结果,而冲击诱导的氧离子空位点缺陷应该是导致该结果的一个重要原因.波长在532 nm处的折射率数据表明:1)蓝宝石的两个高压结构相变将导致其折射率明显上升;在Corundum和Rh_2O_3相区,其折射率将随冲击压力增大而降低;在CalrO_3相区,压力小于172 GPa时,其折射率随冲击压力增大而缓慢地降低,但172 GPa以上时折射率却随冲击压力增大而逐渐增大;2)空位点缺陷对折射率随冲击压力的变化规律有明显的影响.本文结果不仅有助于增强用空位点缺陷的物理机理来解释蓝宝石冲击透明性损伤现象的可靠性,而且对未来进一步的实验研究以及发展新型窗口材料有重要的参考作用.

Effects of vacancy point defects and phase transitions on optical properties of shocked Al2O3

The velocity interferometer system for any reflector (VISAR) and pyrometric measurements in dynamic highpressure experiments require the use of an optical window, and Alumina (Al₂O₃) or sapphires is often considered as a window material due to its high shock impedance and excellent transparency. Consequently, understanding the characteristics of its transparency and refractive index change under shock loading is crucial for explaining such experimental data. Experimental studies indicate optical transparency loss in shocked Al₂O₃. The mechanisms for the phenomenon are some interesting issues. A first-principles study suggests that shock-induced V_O⁺² (the +2 charged O vacancy) defects in Al₂O₃ could be an important factor causing the transparency loss. Recently, the red shift of the extinction curve (i.e., the wavelength dependence of the extinction coefficient) with increasing shock pressure has been observed. It is needed to ascertain whether this behavior is also related to shock-induced vacancy point defects. In addition, up to now, information about Al₂O₃ refractive index at a wavelength of 532 nm under strong shock compression (the optical source wavelength in VISAR measurement is usually set at 532 nm) has been unknown, and neither the effects of structural transitions nor vacancy point defects on the refractive index of shocked Al₂O₃ are determined. Here, to investigate the above-mentioned questions, we perform first principles calculations of optical absorption and refractive index properties of Al₂O₃ crystal without and with V_O⁺² and V_Al³ (the -3 charged Al vacancy) defects in a pressure range of 180 GPa (the calculations in CASTEP are carried out by the plane-wave pseudo potential method in the framework of the density functional theory). Our absorption data show that the observed optical extinction in shocked Al₂O₃ cannot be explained by only considering pressure and temperature factors, but shock-induced V_O⁺² should be an important source for this behavior. On the basis of these results, we may judge that 1) the transparency loss explanation for shocked Al₂O₃ in the view of vacancy point defects is reasonable; 2) the absorption extinction should dominate the extinction phenomenon observed in shocked Al₂O₃. Our calculations find that high-pressure structural transition in Al₂O₃ causes an obvious enhancement of its refractive index. The refractive index decreases with increasing shock pressure in corundum and Rh2O3 regions, and decreases slightly below 172 GPa and increases slowly above 172 GPa with increasing shock pressure in CalrO3 region. The V_O⁺² and V_Al³ defects in Al₂O₃ have apparent influences on the shock pressure dependence of its refractive index. These results mean that the information about Al₂O₃ refractive index under strong shock loading cannot be obtained simply by extrapolating its low pressure data. Our prediction could be of importance for future experimental study and new window-material development.