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

TL;DR

This study uses Electron Paramagnetic Resonance to investigate the 105 K phase transition in SrTiO3, employing non-cubic Fe5+ as a local probe and providing new insights into Fe5+ centers and their behavior.

Contribution

It introduces a reinterpretation of Fe5+ centers in SrTiO3 and BaTiO3, proposing a model for non-cubic Fe5+ as a substitutional ion with an expanded octahedron.

Findings

Critical exponent β = 1/3 indicating second-order transition

Line broadening due to fluctuations near transition

Consistent data with Fe3+-VO pair center suggesting cooperative effects

Abstract

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.