Atomistic origin of huge response functions at the morphotropic phase boundary of(1−x)Na0.5Bi0.5TiO3−xBaTiO3

Abstract

The ferroelectric solid solution $(1\ensuremath{-}x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}\text{\ensuremath{-}}{\mathrm{BaTiO}}_{3} (\mathrm{NBT}\text{\ensuremath{-}}x\mathrm{BT})$ is a promising material to substitute for the environmentally undesired Pb-based ferroelectrics. The strong enhancement of the dielectric permittivity and piezoelectric coefficients near the morphotropic phase boundary (MPB) in ferroelectric $AB{\mathrm{O}}_{3}$-type solid solutions is commonly related to the existence of a single or several long-range-ordered phases resulting in a complex nanodomain pattern. Here, $\mathrm{NBT}\text{\ensuremath{-}}x\mathrm{BT}$ is studied by Raman scattering and complementary synchrotron x-ray total elastic scattering to follow the temperature evolution of the mesoscopic-scale structural transformations for $x$ below, at, and slightly above ${x}_{\mathrm{MPB}}$. The most remarkable result is that at $x\ensuremath{\sim}{x}_{\mathrm{MPB}}$ the phonon mode involving vibrations of both off-centered A-site Bi and B-site Ti experiences strong softening and damping near the triple-point temperature in the phase diagram, in strong contrast to the compounds with $x<{x}_{\mathrm{MPB}}$ or $x>{x}_{\mathrm{MPB}}$. The chemically enhanced coupling between the Bi and Ti subsystems at ${x}_{\mathrm{MPB}}$ is facilitated by the subtle disturbance of the coupling processes within the Bi subsystem induced by Ba at ${x}_{\mathrm{MPB}}$. These mesoscopic-scale structural and dynamic phenomena seem to be significant for the enhanced response functions at the MPB in $\mathrm{NBT}\text{\ensuremath{-}}x\mathrm{BT}$.