It had recently been suggested that a significant amount of Xe could 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 was tested by ab initio calculations of Xe in α-quartz using DFT. Three distinct candidate sites for Xe absorption were identified—substitutional at the silicon vacancy (Xe at VSi), at the oxygen vacancy (Xe at VO) and at an interstitial site (Xe at I)—and each was shown to be mechanically stable at both P = 0 and 2 GPa. The energetics and electronic properties of these defect structures were analysed and it was shown that there was an energy barrier to the absorption at all sites at T = 0. If the Xe absorption was a single-stage process in a perfect crystal then the lowest formation energy barrier (at both P = 0 and 2 GPa) was for Xe-at-I at the interstitial site. If absorption was a two-stage process due to vacancies being already present at finite temperatures, then the subsequent barrier to Xe absorption was much lower and Xe-at-VSi had 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 was to be expected that all three candidate sites should be occupied.

An ab initio Study of Xenon Retention in α-Quartz. M.I.J.Probert: Journal of Physics - Condensed Matter, 2010, 22[2], 025501