Department of Physics, University of York, Heslington, York YO10 5DD, UK.
J Phys Condens Matter. 2010 Jan 20;22(2):025501. doi: 10.1088/0953-8984/22/2/025501. Epub 2009 Dec 10.
It has recently been suggested that a significant amount of Xe can be absorbed in α-quartz and that this might be a significant process in the recycling of Xe from the atmosphere to the interior of the Earth. This suggestion is tested by ab initio calculations of Xe in α-quartz using DFT. Three distinct candidate sites for Xe absorption are identified-substitutional at the silicon vacancy (Xe@V(Si)), at the oxygen vacancy (Xe@V(O)) and at an interstitial site (Xe@I)-and each is shown to be mechanically stable at both P=0 and 2 GPa. The energetics and electronic properties of these defect structures are analysed and it is shown that there is an energy barrier to the absorption at all sites at T=0. If the Xe absorption is a single-stage process in a perfect crystal then the lowest formation energy barrier (at both P=0 and 2 GPa) is for Xe@I at the interstitial site. If absorption is a two-stage process due to vacancies being already present at finite temperatures, then the subsequent barrier to Xe absorption is much lower and Xe@V(Si) has the lowest formation energy. However, it should be expected that there will be a much higher density of oxygen vacancies available for Xe absorption under realistic Earth core conditions and so in this scenario it is to be expected that all three candidate sites should be occupied.
最近有人提出，α-石英中可以大量吸收氙，而且这可能是将氙从大气循环到地球内部的一个重要过程。这项建议通过使用 DFT 对α-石英中的氙进行从头算计算来验证。确定了三种不同的氙吸收候选位置——硅空位处的替位（Xe@V(Si)）、氧空位处的替位（Xe@V(O)）和间隙位置处的替位（Xe@I）——并且在 P=0 和 2 GPa 下，每个位置都表现出机械稳定性。分析了这些缺陷结构的能量学和电子性质，表明在 T=0 时，所有位置的吸收都存在能量障碍。如果 Xe 的吸收在完美晶体中是一个单阶段过程，那么在 P=0 和 2 GPa 下，最低的形成能垒（formation energy barrier）是间隙位置处的 Xe@I。如果由于在有限温度下已经存在空位，吸收是一个两阶段过程，那么随后的 Xe 吸收障碍要低得多，Xe@V(Si) 具有最低的形成能。然而，应该预期在实际的地球核心条件下，会有更高密度的氧空位可用于 Xe 吸收，因此在这种情况下，预计所有三个候选位置都将被占据。
