Morphotropic Relaxor Boundary in a Relaxor System Showing Enhancement of Electrostrain and Dielectric Permittivity

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 ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_{3}\text{\ensuremath{-}}x{\mathrm{BaTiO}}_{3}$ 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 $\ensuremath{\sim}3$ times and the permittivity increases by $\ensuremath{\sim}1.5$ 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.