POLARON CORRELATIONS IN D-WAVE PEROVSKITES REVISITED

We investigated the conduction at a ferroelectric phase transition using the model of small nonadiabatic polaron hoppings with special reference to the cubic-tetragonal transition in Barium Titanate. It has been proved for this material that electron-phonon couplings and dispersion of the L.O. vibrational frequencies are both sufficiently low, so that correlated hopping polarons can activate the corresponding DRIFT and HALL mobilities with an ab initio activation energy of about 6 10~2 eV. The various values given in previous works are probably due to electron detrappings at oxygen vacancies rather than to polaron self-trappings at the intrinsic Ti4+ sites. Comparison between theory and experiments shows that oxygen stoichiometry plays no sensible role on the activation process of mobility. Our model is in favour of a conduction band minimum, at the I” point, rather than at the X3 point. Furthermore, no critical effects take place in the pretransitional region. © 1991, Taylor & Francis Group, LLC. All rights reserved.