Abstract
The structural state near the morphotropic phase boundary (MPB): a composition-driven phase
transition observed often in ferroelectric solid solutions, has been for a long time in the highlights
of materials science because of the strong enhancement of properties at MPB. Although there have
been extensive studies on MPB to understand the underlying structure-property relations, there
is still no rigorous structural model at the atomic level to depict categorically the state of local
polarization and the roles of individual cations. Here we report our results from pair distribution
function (PDF) and Raman scattering analyses on xBiMg1=2Ti1=2O3-(1-x)PbTiO3 exhibiting MPB
at xMPB = 0.63, as a model solid solution for the relatively novel and technologically promising
perovskite-type (ABO3) ferroelectric systems xBiMeO3-(1-x)PbTiO3. Our study reveals that the
MPB occurs at the composition where a strong orientation disorder of polar displacements for A-site
cations as well as for B-site Ti is developed and at the same time, the average magnitude of o -centre
displacements of the ferroelectrically active B-site cations equalizes that of the ferroelectrically active
B-site cations, which allows for a strong dynamic coupling between the A-site and the B-site cations.
The composition dependence of the cation-displacement orientation disorder resembles that of the
structural tetragonality, describing the macroscopic strain, as well as of the Curie temperature Tc and
correlates reversely with the composition dependence of the longitudinal piezoelectric coecient d33.
Similar local structural phenomena should be the atomistic driving force for enhanced properties near
MPB in other solids solutions with ferroelectrically active elements in two di erent crystallographic
positions.
