EPR studies of the 105 K phase transition of SrTiO3 with the non-cubic Fe5+ as local probe and a reinterpretation of other Fe5+ centres

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.