Evolution of local heterogeneity towards flat free energy landscape: Optimization of piezoelectric response in PNN-PZ-PT at morphotropic phase boundary

A precisely specified compositional landscape of two distinct ferroelectric systems – morphotropic phase boundary (MPB) – possesses ultrahigh piezoelectricity, where generically a flat energy profile is favoured under thermodynamic consideration. A more exotic and technologically appealing phase is unlocked when ternary-based morphotropic phase boundary compositions are formulated via revisiting the thermal and compositional stability. Local structure heterogeneity is another generic route towards optimization of the piezoelectric performance via rare earth doping, as rare-earth doping introduces the local structural distortions. To enhance piezoelectric response, we adopt rare earth Sm³⁺ doping into ternary based morphotropic phase boundary 0.55Pb(Ni_1/3Nb_2/3)O₃–0.135PbZrO₃–0.315PbTiO₃ samples. The effect of Sm³⁺doping on the structure, microstructure, dielectric, ferroelectric and piezoelectric properties of 0.55Pb(Ni_1/3Nb_2/3)O₃–0.135PbZrO₃–0.315PbTiO₃ were investigated. Dielectric spectroscopy and order parameter analysis collectively reveal that the free energy landscape of morphotropic phase boundary is further softened via local structural heterogeneity, enabled via rare earth doping. As a result of free energy flattening, dielectric and piezoelectric responses of Sm³⁺ doped system are significantly enhanced. Piezoelectric coefficient increases from 545pC/N (x = 0%) to 810pC/N (x = 1%) with Sm³⁺ doping. Observed results suggest that the piezoelectric and ferroic performances of morphotropic phase boundary based 0.55Pb(Ni_1/3Nb_2/3)O₃–0.135PbZrO₃–0.315PbTiO₃ can further be improved by hetero-structural tuning via optimized rare earth doping.