  The 105 K second-order displacive phase transition of SrTiO3 has been studied
with the help of Electron Paramagnetic Resonance. The photochromic non-cubic
Fe5+ centre is used as local probe. Critical phenomena, characterized by an
exponent {\beta} = 1/3, are presented. Line broadening effects are interpreted
as stemming from time-dependent fluctuations near the phase transition point.
The data are consistent with those reported earlier on the Fe3+-VO pair centre,
indicating cooperative effects in the crystal. Also a model for the non-cubic
Fe5+ is proposed, i.e., the ion is substitutional for Ti4+ with an empty
adjacent expanded octahedron. Other Fe5+ centres in SrTiO3 and BaTiO3 are
reviewed and reinterpreted.
