Interpretation of the softening effect in PZT ceramics near the morphotropic phase boundary

In the last two decades, Eyraud tried to interpret the soft characteristics in lead zirconate titanate ceramics (PZT) by an electron transfer between constitutive ions and/or dopants such as Ti3+/4+ or Fe2+/3+ . These electron transfers are supposed to minimize the space charges (? p ) existing at the domain walls interface due to polarization divergences (rho(p) = -div (P) over right arrow), making the domain wall movement easier (soft PZTs). Recently, the Eyraud's model has been refined and the softening mechanism is now explained by electron exchange between oxygen and lead vacancies owing to the presence of these defects in every doped PZT. Based on the analogy with BaTiO3 , the proposed correction of the model is justified using the energy levels of barium and oxygen vacancies given by Daniels and Hardtl. It is supposed that the material will exhibit soft properties if acceptor and donor sites are associated with neighbouring energy levels allowing direct electron tranfer between each other, without contributing to electron transfers from the valence band or to the conduction band. So, the present paper briefly recalls the previous results about the Eyraud's model and secondly presents the new interpretation of the softening effect. Every basic doped PZT family will be considered, that is to say, PZTs doped with an acceptor, PZTs doped with a donor and the particular (Mg, F) and (Mn, F) co-doped PZTs developed in our Laboratory.