  Many ferroelectric solid solutions exhibit enhanced electro-mechanical
properties at the morphotropic boundary separating two phases with different
orientations of polarization. The mechanism of properties enhancement is
associated with easy paths for polarization rotation in anisotropically
flattened free energy profile. Another mechanism of properties enhancement
related to free energy flattening is polarization extension. It is best known
at temperature-driven ferroelectric-paraelectric phase transitions and may lead
to exceedingly large properties. Its disadvantage is temperature instability of
the enhancement. In this paper a temperature-composition phase diagram is
proposed that exhibits compositionally driven-phase transitions with easy paths
for both polarization rotation and polarization extension.
