An ab initio study of xenon retention in α-quartz

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.