The morphotropic phase boundary (MPB) is the cornerstone of designing high-performance piezoelectric ceramics, enabling materials such as $\mathrm{Pb}\left(\mathrm{Zr},\mathrm{Ti}\right){\mathrm{O}}_3, \mathrm{Pb}\left(\mathrm{Mg},\mathrm{Nb}\right){\mathrm{O}}_3–\mathrm{PbTi}{\mathrm{O}}_3, \left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3$-based and $\mathrm{BaTi}{\mathrm{O}}_3$-based (BT) ceramics to have excellent piezoelectric coefficients (exceeding 600 pC/N). In contrast, MPB in $\left(\mathrm{Bi},\mathrm{Na}\right)\mathrm{Ti}{\mathrm{O}}_3$-based (BNT) materials exhibits a notable anomaly: despite their high spontaneous polarization, their piezoelectric coefficients are limited to ∼300 pC/N. This work systematically compares the properties and in situ microstructures of MPB in BT-based (no oxygen octahedral tilting) and BNT-based (with oxygen octahedral tilting) systems, and reveals that oxygen octahedral tilting leads to a unique MPB type in the BNT-based system. In BT-based systems, the MPB composition (no oxygen octahedral tilting) maintains a two-phase coexistence structure before and after the application of an electric field and exhibits a low polarization rotation energy barrier, thereby generating a high piezoelectric coefficient. In contrast, in BNT-based systems, when an electric field is applied, the MPB composition (with oxygen octahedral tilting) undergoes a field-induced transition, transforming from a two-phase coexistence structure to a single-phase structure. This single-phase structure and oxygen octahedral tilting result in a high energy barrier for polarization rotation, leading to a low piezoelectric coefficient and a high coercive field. These results provide insights into the microstructural origin of the abnormal properties of BNT-based MPB piezoelectrics and the design of high-performance lead-free materials.

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• Crystal structure
• Dielectric properties
• Electric polarization
• Ferroelectricity
• Piezoelectricity