An ab initio study of xenon retention in α-quartz

TY - JOUR

T1 - An ab initio study of xenon retention in α-quartz

AU - Probert, M. I. J.

PY - 2010/1/20

Y1 - 2010/1/20

N2 - It has recently been suggested that a significant amount of Xe can be absorbed in alpha-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 alpha-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.

AB - It has recently been suggested that a significant amount of Xe can be absorbed in alpha-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 alpha-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.

KW - MOLECULAR-DYNAMICS

KW - NOBLE-GASES

KW - DEFECTS

KW - CASTEP

KW - ORIGIN

KW - MANTLE

KW - ICE

UR - http://www.scopus.com/inward/record.url?scp=72249119246&partnerID=8YFLogxK

U2 - 10.1088/0953-8984/22/2/025501

DO - 10.1088/0953-8984/22/2/025501

M3 - Article

SN - 0953-8984

VL - 22

SP - 1

EP - 10

JO - Journal of physics : Condensed matter

JF - Journal of physics : Condensed matter

IS - 2

M1 - 025501

ER -