In ferroelectric and relaxor-ferroelectric materials, piezoelectric and dielectric properties are significantly enhanced at the morphotropic phase boundary (MPB), a boundary between different ferroelectric phases with different macroscopic symmetries. By contrast, in relaxor systems, such an MPB does not exist because relaxors of different compositions possess the same macroscopic symmetry. Here, we report the existence of a morphotropic relaxor boundary (MRB) in the single phase relaxor region of a ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_3\text{−}x{\mathrm{BaTiO}}_3$ system, which is a composition-induced boundary between two relaxors with different local polar symmetries (tetragonal versus rhombohedral) but with the same macroscopic cubic symmetry. At the MRB the electrostrain increases by $\sim 3$ times and the permittivity increases by $\sim 1.5$ times over a wide temperature range of more than 100 K, as compared with off-MRB compositions. Our Letter demonstrates that the MRB may become an effective mechanism to enhance the dielectric and electrostrictive properties of relaxors.

Transformative technologies demand polycrystalline piezoelectric ceramics with piezoelectric coefficients ( d ₃₃ ) exceeding 6000 picocoulomb per Newton (pC/N), but this goal has remained elusive because of the intrinsically weak nature of piezoelectricity and incomplete polarization alignment in polycrystals. We overcome this barrier by placing a polycrystalline lead zirconate titanate (PZT) ceramic in a temperature and electric-field control module so that it operates at a quadruple phase point (QP). This QP ceramic exhibited a d ₃₃ of ~6850 pC/N, which surpasses that of commercial PZT ceramics by 10 to 30 times and commercial lead magnesium niobate–lead titanate single crystals by ~4 times. This exceptional property arises from the tricritical nature of the QP, a thermodynamic singularity that produces an ultrasoft lattice and enables complete polarization alignment in polycrystals. The module maintained this performance for surrounding ambient temperature ranging from 25° to 350°C.