1.Institute of Solid State Physics, College of Physics and Electronic Engineering, Sichuan Normal University, Chengdu 610101, China 2.National Key Laboratory for Shock Wave and Detonation Physics Research, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China
Abstract:Sapphires (Al2O3) is an important ceramic material with extensive applications in high-pressure technology and geoscience. For instance, it is often used as a window material in shock-wave experiments. Consequently, understanding the behavior of its transparency change under shock compression is crucial for correctly interpreting the experimental data. Sapphire has excellent transparency at ambient conditions, but its transparency is reduced under shock loading. This shock-induced optical extinction phenomenon in Al2O3 has been studied experimentally and theoretically a lot at present, but the knowledge on the crystal-orientation effects of the extinction is still insufficient. the experimental investigations at low-pressure region (within 86 GPa) have indicated that the shock-induced extinction in Al2O3 is related to its crystal orientation, but it is not clear whether the correlation also exists at high-pressure region (~131–255 GPa). Here, to investigate this question, we have performed first principles calculations of the optical absorption properties of a-, c-, d-, r-, n-, s-, g- and m-oriented Al2O3 crystals without and with $V_{\rm O}^{ + 2}$ (the +2 charged O vacancy) defects at the pressure range of 131–255 GPa. It is found that: 1) there are obvious crystal-orientation effects of the extinction in shocked Al2O3 at high-pressure region, and they strengthen with increasing pressure; 2) shock-induced $V_{\rm O}^{ + 2}$ defects could play an important role in determining these crystal-orientation effects, but the influences of pressure and temperature factors on them are relatively weak. A further analysis shows that, at the wavelength range adopted in shock experiments, the extinction of a-orientation is the weakest (the best transparency), the extinction of c-orientation is the strongest (the worst transparency), and the extinction of s-orientation is between them; at the same time, the extinction of m-orientation is similar to that of a-orientation, the extinction of r-, n- and d-orientations is close to that of c-orientation, and the extinction of g-orientation is weaker than that of s-orientation. In view of this, we suggest that the a- or m-oriented Al2O3 is chosen as an optical window in shock-wave experiments of the high-pressure region. Our predictions could be not only helpful to understand further the optical properties of Al2O3 at extreme conditions, but also important for future experimental study. Keywords:sapphires/ high pressure/ shock-induced optical extinction/ crystal-orientation effects/ first-principles calculations
3.结果与讨论图1(a) 给出了冲击压缩下八种晶向(a, c, d, r, n, s, g和 m晶向)CalrO3-Al2O3吸收光谱的计算数据. 其中, 理想晶体数据表明, 在131—255 GPa的压力范围内, 蓝宝石吸收谱存在非常微弱的晶向效应, 而且随着压力增大, 晶向效应还进一步衰减, 同时a, m和g晶向的吸收谱还有很微小的红移(其它晶向的吸收谱几乎没有变化). 然而, $V_{\rm{O}}^{ + 2}$空位点缺陷的存在将使得蓝宝石冲击吸收谱的晶向效应显著增强, 而且该晶向效应还随着压力的升高而进一步地增强. 同时, $V_{\rm{O}}^{ + 2}$空位点缺陷还导致不同晶向的吸收曲线出现了巨大的红移现象, 且随着压力增大, 红移的程度明显加大(这与前人的理论预测相符[8]). 尽管如此, 这些缺陷模型计算的结果是否有效, 仍需要进一步验证(因为在冲击压缩下蓝宝石内部的空位点缺陷浓度具体多少尚不明确, 所以仅定性地探究空位缺陷对其性质的影响). 为此, 参照文献[8]的做法, 在131.2 GPa和255 GPa处分别又构造了一组缺陷浓度较低的晶体模型(图1(b)). 对比分析图1(a)和(b)中的缺陷晶体数据可以看出, 采用两组缺陷浓度的计算模型都得出一致的结论, 说明本文的缺陷晶体结果是可靠的[8]. 图 1 八种晶向 CalrO3-Al2O3的吸收光谱随冲击压力变化的规律(a, c, d, r, n, s, g 和 m 分别表示 a, c, d, r, n, s, g 和 m 晶向, 计算数据已做了冲击温度修正) (a) 在两个压力点分别采用较高缺陷浓度模型的计算数据(内嵌图为理想晶体数据的放大图); (b) 在两个压力点分别采用较低缺陷浓度模型的计算数据 Figure1. Shock-pressure dependence of the optical absorption spectra for CalrO3-Al2O3 with eight crystallographic orientations (a, c, d, r, n, s, g and m indicate a, c, d, r, n, s, g and m orientations, respectively. The calculated data have been corrected by shock temperature): (a) Data calculated with higher defective concentration model at 131.2 GPa and 255 GPa (the inserted figure shows perfect-crystal data); (b) data calculated with lower defective concentration model at 131.2 GPa and 255 GPa.
图1(a)和(b)中的数据还揭示了一个重要事实: $V_{\rm{O}}^{ + 2}$空位点缺陷的出现使得蓝宝石的冲击吸收性质及其晶向效应都发生了显著变化. 对于这些变化, 压力和温度因素的作用较弱, 而冲击诱导的$V_{\rm{O}}^{ + 2}$空位点缺陷应该是主要的贡献者(图2和图3). 图 2 八种晶向 CalrO3-Al2O3的理想晶体吸收光谱随压力变化的规律(a, c, d, r, n, s, g和m分别表示a, c, d, r, n, s, g和 m 晶向) Figure2. Pressure dependence of the optical absorption spectra for perfect CalrO3-Al2O3 with eight crystallographic orientations (a, c, d, r, n, s, g and m indicate a, c, d, r, n, s, g and m orientations, respectively).
图 3 冲击温度和空位点缺陷对八种晶向CalrO3-Al2O3高压吸收光谱的影响(a, c, d, r, n, s, g和m分别表示a, c, d, r, n, s, g和 m 晶向) Figure3. Effects of the shock temperature and vacancy point defect on the high-pressure optical absorption spectra for CalrO3-Al2O3 with eight crystallographic orientations (a, c, d, r, n, s, g and m indicate a, c, d, r, n, s, g and m orientations, respectively).
然而, 蓝宝石的冲击消光将表现出怎样的晶向相关性, 目前仍是一个问题. 由于在131—255 GPa蓝宝石的冲击消光现象主要是由吸收因素导致的[6,8], 所以可以从八个不同晶向的蓝宝石吸收光谱的数据中获得其消光性的晶向效应信息. 从图1(a)中的理想晶体数据可以看出, 在冲击实验采用的波段内(250—1000 nm [6,9]), 蓝宝石不存在吸收行为, 不能解释实测的消光现象[2,6,13], 意味着理想晶体结果不能用于推断蓝宝石冲击消光的晶向效应特征. 但是, 图1(a)中的缺陷晶体数据却能够成功地解释冲击实验在上述压力区观测到的三种现象: 1)透明性损伤现象[8-9]; 2)消光系数随波长增大而降低以及消光性随压力增大而增强的结果[6,8]; 3)随压力增大消光曲线出现了红移的现象[6,8]. 这一切都表明, 八个不同晶向的缺陷晶体数据可以用于探究蓝宝石冲击消光的晶向相关性. 而且, 冲击压力在255 GPa处, 蓝宝石缺陷晶体的计算数据表现出其c晶向的吸收性与r晶向的吸收性没有明显差别的特征(两个不同缺陷浓度模型的计算都支持这个结果), 能够解释冲击实验观测[13] (图4), 进一步地强化了本研究组的判断. 图 4 两个不同晶向 CalrO3-Al2O3在 255 GPa 处的冲击吸收光谱的计算数据和冲击消光系数的实测数据(c 和 r 分别表示 c 晶向和 r 晶向, 计算数据已做了冲击温度修正) Figure4. The calculated optical absorption spectra and the measured extinction coefficients for CalrO3-Al2O3 with two crystallographic orientations at shock pressure of 255 GPa (c and r indicate c and r orientations, respectively. The calculated data have been corrected by shock temperature).