Ferroelectric solid solutions with morphotropic boundary: Rotational instability of polarization, metastable coexistence of phases and nanodomain adaptive states

Ferroelectric solid solutions with different symmetry phases at opposite sides of the perovskite-based pseudo-binary phase diagram are considered. An analysis of the concentration dependence of polar anisotropy shows conditions under which the anisotropy changes sign, assuming zero value. The orientational stability of a polarization was investigated near this point, which is also a position of so-called morphotropic boundary (MB). The symmetry conditions resulting in the high-symmetry ferroelectric phases with finite temperature-composition stability field as well as the thermodynamic conditions providing their metastable two-phase coexistence are formulated. A loss of orientation stability around MB results in a divergence of fluctuations of polarization direction and a peak of dielectric susceptibility. The calculated energy of a 90 degrees domain wall demonstrates that this energy is drastically reduced or even vanishes at the MB. The latter leads to a miniaturization of the domain structure to the nano-scale level and the formation of a mixed adaptive state. The miniaturization explains the observed special properties of ferroelectrics near MB that are extrinsic for the nanodomain adaptive state. It also explains the observed diffraction patterns generated by nanodomains of the mixed state. A coherent scattering generated by nanodomains mimics diffraction patterns that could be confused with a pattern generated by a homogeneous monoclinic phase.