Interstitial and substitutional zirconium in SrTiO3

We investigate Zr in SrTiO3 (STO) as a model for nuclear waste forms in which the fission product 90Sr eventually decays to stable Zr through beta emission. The transformation of a divalent into a tetravalent constituent is expected to affect the long-term structural and chemical stability of this solid. Computational methods of electronic structure theory, specifically the density functional theory (DFT) within the supercell model, are used to predict the thermodynamic stability and electronic states of interstitial and Sr- or Ti-substituted Zr atoms in the STO lattice. Native defects such as vacancies and antisites are also considered. When Zr replaces Sr, its most stable configuration is to simply occupy the Sr site. For Zr added to the lattice, its most stable configuration is to replace a Ti, making a ZrTi impurity plus a Ti interstitial. ZrSr is predicted to be a double electron donor, ZrTi is electrically inactive and interstitial Zr and Ti are predicted to be quadruple donors, with all donor levels in the conduction band. The interstitials are all predicted to increase the crystal volume, and lead to a tetragonal distortion of the lattice. Experiments with injection of Zr and O atoms into STO qualitatively confirm these predictions of crystal structural changes.