Phase contents and piezoelectric activity in lead-free ferroelectric solid solutions containing (Ba, Ca)(Me, Ti)O3 (Me = Zr or Sn)

The coexistence of ferroelectric P4mm and Amm2 phases is studied in advanced perovskite-type (Ba0.95Ca0.05)(Zr0.05Ti0.95–x(Zn1/3Ta2/3)x)O3 and Ba0.92Ca0.08Sn0.09Ti0.91O3 solid solutions (0 ≤ x ≤ 0.02) near the morphotropic phase boundaries. Heterophase structures are analyzed within the framework of a model where large two-phase regions (up to four domain types) and interlayers (one of six domain types) coexist. The role of specific non-180° domain types in stress relief is discussed. Complete stress relief is achieved in the two-phase regions “almost single-domain—polydomain” or “single-domain—single-domain” and in the heterophase structures containing such regions and single-domain interlayers. In the (Ba0.95Ca0.05)(Zr0.05Ti0.95−x(Zn1/3Ta2/3)x)O3 system, the largest number of versions of changes in the phase content is four, which corresponds to x = 0.005 at the largest tetragonality cT/aT, and small cO/bO and aO/bO ratios of unit-cell parameters. Near x = 0.005, an average optimum volume fraction ⟨nT,opt⟩ of the 90° domains in the tetragonal phase is correlated with changes of the longitudinal piezoelectric coefficient d33(x). In Ba0.92Ca0.08Sn0.09Ti0.91O3, the polydomain P4 mm phase coexists with the almost single-domain Amm2 phase. The conditions for the unit-cell parameters aO=aT, bO=aT at the relatively small cT/aT, cO/bO, and aO/bO ratios favor complete stress relief in Ba0.92Ca0.08Sn0.09Ti0.91O3 at almost equal volume fractions of the coexisting phases. A link between parameters of the heterophase structure and the piezoelectric activity is discussed. The evaluated results on the phase content are consistent with experimental data.