The nature of iron-oxygen vacancy defect centers in PbTiO3
  The iron(III) 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 semi-empirical Newman superposition model analysis based on finestructure 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. 
H. Mestric, R.-A. Eichel, T. Kloss, K.-P. Dinse, So. Laubach, St.   Laubach, P.C. Schmidt, K.A. Schoenau, M. Knapp, H. Ehrenberg