Iron-oxygen vacancy defect centers in PbTiO3 : Newman superposition model analysis and density functional calculations

The Fe3+ center in ferroelectric PbTiO3 together with an oxygen vacancy forms a charged defect associate, oriented along the crystallographic c axis. Its microscopic structure has been analyzed in detail comparing results from a semiempirical Newman superposition model analysis based on fine-structure data and from calculations using density functional theory. Both methods give evidence for a substitution of Fe3+ for Ti4+ as an acceptor center. The position of the iron ion in the ferroelectric phase is found to be similar to the B site in the paraelectric phase. Partial charge compensation is locally provided by a directly coordinated oxygen vacancy. Using high-resolution synchrotron powder diffraction, it was verified that lead titanate remains tetragonal down to 12 K, exhibiting a c∕a ratio of 1.0721