Microscopic origin of giant piezoelectricity in ferroelectric xBi(Ni0.5Hf0.5)O3−(1−x)PbTiO3 beyond the morphotropic phase boundary

Local structural correlations in a novel ferroelectric solid solution [$x\mathrm{Bi}({\mathrm{Ni}}_0.5{\mathrm{Hf}}_0.5){\mathrm{O}}₃\text{\ensuremath{-}}(1\ensuremath{-}x){\mathrm{PbTiO}}₃$ at $x=0.39$] have been drawn out through atomistic modeling against neutron pair distribution functions and complementary Raman scattering measurements. By examining polar displacements of different cations from the refined structural models, we reveal the distributions of cation-specific dipolar fluctuations at the key composition where the maximum piezoelectric response was recorded. It becomes clear that the unusual structural modifications observed after the poling are simply a manifestation driven by the varying extent of the dipolar fluctuations and not a structural transformation caused by adaptive cationic displacements. Moreover, in addition to strong dipolar flexibility under an electric field, both poled and unpoled ceramics exhibit orientation-independent polarized Raman scattering, which is more typical of a dipolar glass with a responsive and polarizable matrix than a polycrystalline material composed of ferroelectric domains. Therefore, compared to other analogous $x\mathrm{Bi}(\mathrm{Me}){\mathrm{O}}₃\text{\ensuremath{-}}(1\ensuremath{-}x){\mathrm{PbTiO}}₃$ systems, the current material is a unique case where chemical substitution has led to a highly polarizable ionic-covalent matrix that seems to be the key ingredient to generate giant piezoelectricity at a composition beyond the morphotropic phase boundary.