Many ferroelectric solid solutions exhibit enhanced electromechanical 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. © 2010 American Institute of Physics. ͓doi:10.1063/1.3479479͔The properties of materials that are defined by polarization change, such as dielectric permittivity and piezoelectric coefficients, may be enhanced in phase transition regions where there is a significant polarization variation. The examples are divergence of the dielectric and piezoelectric properties near points of temperature-, 1 ͑see Fig. 1͒, stress-, 2 electric field-, 3,4 and compositionally-driven 5,6 structural phase transitions. The compositionally induced structural change, the so-called morphotropic phase transition, 6,7 is of a great practical interest because the variable that drives the transition ͑i.e., composition͒ is inherent to the material and the transition point is maintained at operating conditions without an external influence ͑e.g., temperature, electric field, or stress͒. The morphotropic phase transition is the origin of large piezoelectric properties in Pb͑Zr 1−x Ti x ͒O 3 ͑PZT͒ solid solution, the most extensively used piezoelectric material. The enhancement of the properties in PZT occurs in the region of the composition-temperature phase diagram where crystal structure changes from tetragonal ͑T͒ to rhombohedral ͑R͒ via an intermediary monoclinic ͑M͒ phase as the Zr/Ti ratio becomes greater than ϳ52/ 48 ͑see Fig. 2͒. 8 This region is known as the morphotropic phase boundary ͑MPB͒. 6,7 Search for new piezoelectrics ͑for example, leadfree͒ is naturally focused on materials exhibiting a PZT-like MPB. In this paper the general concept of properties enhancement in the phase transition regions is used to propose a modified type of the phase diagram with compositionallydriven phase transitions.Ab initio and phenomenological calculations have shown that the intrinsic mechanism of the properties enhancement in phase transition regions is flattening of a free energy profile. 2,3,[9][10][11][12][13] This mechanism appears to be common to most phase transitions, whether compositionally-, stress-, electric field-, or temperature-driven. The anisotropy of the free energy profile determines easy paths for polarization change and corresponding properties enhancement. 11 Consider first a familiar example of the temperature-driven phase ...