Impurities in FeAs-based superconductor, SrFe2As2, studied by first-principles calculations

We recently report that a superconducting transition is invoked by exposure to water vapor in undoped SrFe2As2, which occurs accompanied by shrinkage of the c-axis length of the unit cell. In this paper, we calculated structures and formation energies of O-/OH-related impurities and vacancies in SrFe2As2 by first-principles calculations in order to find a model that explains the c-axis shrinkage in SrFe2As2. It was found that the incorporation of an O, OH or H2O molecule at an interstitial site cannot explain the c-axis shrinkage, and that formation of an As vacancy needs a large formation energy. While, Sr and Fe vacancies are formed with negative formation energies, and the Fe vacancy can explain the c-axis shrinkage.