Comparison of Positron Lifetimes Across Oxide Chemistry, Structure and Charge

We conducted density functional theory based positron lifetime calculations for cation and oxygen monovacancies in a range of oxides -- hematite, magnetite, hercynite, and alumina -- to compare the impact of defect chemistry and crystal structure on the predicted lifetimes. We also examined the role of defect charge state. A comparison across the same type of crystalline structure but different composition shows us that oxygen vacancies only induce a slight increase in the positron-electron overlap and thus barely modify the positron lifetime as compared to the bulk. A much more substantial increase of positron lifetime is observed for cation monovacancies, regardless of crystal structure or the elemental nature of the vacancy, which we ascribe to an enhanced localization of charge density around the vacant site. The structural and compositional richness of the oxide leads to higher defect positron lifetimes, with defected hercynite exhibiting the longest positron lifetimes. The charge state of cation monovacancies modifies only by a small percentage the positron localization, relegating to secondary importance the metal defect's oxidation state in describing the lifetime of positrons within vacancy traps.