Iodine defect energies and equilibria in ${\\text{ZrO}_2}$


Incorporation energies and defect equilibria in monoclinic, tetragonal and cubic phases of ${\text{ZrO}2}$ are predicted, using density functional theory calculations, for iodine dopant concentrations between ${10^{-5}}$ and ${10^{-3}}$ atoms per formula unit of ${\text{ZrO}2}$. 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 ${10^0}$ atm. The oxygen pressure required for stoichiometry in monoclinic ${\text{ZrO}2}$ is approximately ${10^{-7.5}}$ atm, at both low and high iodine concentrations, whereas for tetragonal ${\text{ZrO}2}$, 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 ${\text{ZrO}2}$ are ${\text{I}\{\text{O}}^•}$ charge-compensated by ${\text{I}\{\text{Zr}}^{´´´}}$ at low oxygen pressures, and a combination of ${\text{I}\{\text{Zr}}^{´´´}}$, ${\text{I}\{\text{O}}^{•••}}$ and ${\text{I}\{\text{i}}^{•}}$ defects at high oxygen pressures. In tetragonal ${\text{ZrO}2}$, the dominant defects at low oxygen pressures are ${\text{e}^{´}}$, ${\text{V}\{\text{O}}^{•}}$ and ${\text{I}\{\text{O}}^{•}}$. At high oxygen pressures, ${\text{h}^{•}}$ and ${\text{I}\{\text{Zr}}^{´´´}}$ are dominant, with additional charge-compensation from ${\text{V}\_{\text{Zr}}^{´´´}}$ 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.

Journal of Nuclear Materials
Mark Wenman
Mark Wenman
Senior Lecturer in the Department of Materials