There is a general perception that a large piezoelectric response in ferroelectric solid solutions requires a morphotropic/polymorphic phase boundary (MPB/PPB), i.e., a composition driven inter-ferroelectric instability. This correlation has received theoretical support from models which emphasize field driven polarization rotation and/or inter-ferroelectric transformations.Here, we show that the ferroelectric system (1-x)PbTiO3-(x)Bi(Zr1/2Ni1/2)O3 (PT-BNZ), which shows d33 (~ 400 pC/N) comparable to the conventional MPB/PPB systems, does not belong to this category. In the unpoled state the compositions of PT-BNZ showing large d33 exhibits a coexistence of tetragonal and cubic-like (CL) phases on the global length scale. A careful examination of the domain strucures and global structures (both in the unpoled and poled states) revealed that the CL phase has no symptom of average rhombohedral distortion even on the local scale. The CL phase is rather a manifestation of tetragonal regions of short coherence length. Poling increases the coherence length irreversibly which manifests as poling induced CL P4mm transformation on the global scale. PT-BNZ is therefore qualitatively different from the conventional MPB piezoelectrics. In the absence of the composition and temperature driven inter-ferroelectric instability in this system, polarization rotation and inter-ferroelectric transformation are no longer plausible mechanisms to explain the large electromechanical response. The large piezoelectricity is rather associated with the increased structural-polar heterogeneity due to domain miniaturization without the system undergoing a symmetry change. Our study proves that attainment of large piezoelectricity does not necessarily require inter-ferroelectric instability (and hence morphotropic/polymorphic phase boundary) as a criterion.

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The structural, phonon, magnetic, dielectric, and magneto dielectric responses of the pure bulk Brownmillerite compound Ca2FeCoO5 are reported. This compound showed giant magneto dielectric response (10%-24%) induced by strong spin-lattice coupling across its spin reorientation transition (150-250 K). The role of two Debye temperatures pertaining to differently coordinated sites in the dielectric relaxations is established. The positive giant magneto-dielectricity is shown to be a direct consequence of the modulations in the lattice degrees of freedom through applied external field across the spin reorientation transition. Our study illustrates novel control of magneto-dielectricity by tuning the spin reorientation transition in a material that possess strong spin lattice coupling.

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