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

The ferroelectric solid solution (1 - x)Na0.5Bi0.5TiO3-BaTiO3 (NBT-xBT) 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 ABO(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, NBT-xBT 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(MPB). The most remarkable result is that at x similar to x(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(MPB) or x > x(MPB). The chemically enhanced coupling between the Bi and Ti subsystems at x(MPB) is facilitated by the subtle disturbance of the coupling processes within the Bi subsystem induced by Ba at x(MPB). These mesoscopic-scale structural and dynamic phenomena seem to be significant for the enhanced response functions at the MPB in NBT-xBT.