Iodine defect energies and equilibria in ZrO2

Abstract

Incorporation energies and defect equilibria in monoclinic, tetragonal and cubic phases of ZrO2 are predicted, using density functional theory calculations, for iodine dopant concentrations between 10−5 and 10−3 atoms per formula unit of ZrO2. Data are presented for monoclinic and tetragonal polymorphs, in the form of Brouwer diagrams, to show the defect response at oxygen pressures ranging from 10−35 to 100 atm. The oxygen pressure required for stoichiometry in monoclinic ZrO2 is approximately 10−7.5 atm, at both low and high iodine concentrations, whereas for tetragonal ZrO2, it increases from 10−10 to 10−6.5 atm as the iodine concentration is increased from 10−5 to 10−3 atoms/formula unit. The dominant defects in monoclinic ZrO2 are IO• charge-compensated by I‴Zr at low oxygen pressures, and a combination of I‴Zr, IO••• and Ii• defects at high oxygen pressures. In tetragonal ZrO2, the dominant defects at low oxygen pressures are e′, VO•• and IO•. At high oxygen pressures, h• and I‴Zr are dominant, with additional charge-compensation from V″″Zr defects when iodine concentrations are low. The concentration of IO defects in the tetragonal phase decrease with increasing oxygen pressure above stoichiometry, demonstrating competition between iodine and oxygen for occupancy of the anion site. This has implications for fuel and cladding designs that are resistant to iodine-SCC.