  The morphotropic phase boundary (MPB) in the magnetoelectric multiferroic
BiFeO3-PbTiO3 is shown to be uniquely dependent on crystallite size. The system
exhibits the phenomenon of abnormal grain growth (AGG) during sintering with
abrupt increase in the grain size from ~ 1 micron to ~ 10 microns. The large
sized free crystallites (10 microns) exhibit pure tetragonal phase. Phase
coexistence sets in after reducing the size to ~ 1 micron. These results prove
that size, and not the internal stress, as the controlling mechanism for
stabilizing phase coexistence in this system.
