  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.
