Effect of a tetragonal phase on heterophase states in perovskite-type ferroelectric solid solutions

Results on the role of the ferroelectric tetragonal (P4mm) phase in an elastic matching and phase coexistence are discussed for perovskite-type solid solutions (1−t)BiFeO3–tPbTiO3, (1−z)Pb(ZrwTi1−w)O3–zPb(Zn1/3Nb2/3)O3 and (1−r)(Na1/2Bi1/2)TiO3–rPbTiO3 near morphotropic phase boundaries. An important feature is a combination of single- and polydomain regions of the tetragonal phase that coexists with the adjacent ferroelectric phase, either monoclinic or rhombohedral, at the complete stress relief in a heterophase sample. A dependence of the volume fraction of the tetragonal phase on the composition and domain types is found for various heterophase states in the aforementioned systems. The remarkable role of the tetragonal phase and its unit-cell parameters behaviour is discussed. Some correlations between changes in unit-cell parameters of the tetragonal phase and its volume fraction are revealed. Agreement between predicted and known experimental data on the phase contents in the studied ferroelectric solid-solution systems is stated.