Phase diagram and structure evolution mechanism in ultrahigh energy storage ${\mathrm{NaNbO}}_3$-based superparaelectric relaxor ferroelectric ceramics

The construction of superparaelectric (SPE) systems has been demonstrated to be an essential means of enhancing energy storage properties, while the underlying physical behavior is still unclear. Here, the structure evolution of SPE was investigated on (1-$x$)($0.85{\mathrm{NaNbO}}_3-0.15{\mathrm{Sr}}_{0.7}{\mathrm{Bi}}_{0.2}{\mathrm{TiO}}_3$)-$x\mathrm{Bi}\left({\mathrm{Mg}}_{0.5}{\mathrm{Zr}}_{0.5}\right)O_3$ (NN-SBT-BMZ) ceramics by analyzing the lattice structure and electronic transitions behavior under the regulation of chemical content and temperature. The cell volume and optical band gap has been enhanced significantly, with increasing the doping contents. Moreover, the detailed phase diagram of NN-SBT-BMZ with a temperature content was derived by combining the evolution of dielectric, optical transitions, lattice structure, and phonon behavior under thermodynamic field, in which the evolution of the lattice structure is closely related to the domain structure. Noteworthy, the oscillatory processes of the main phonons near the $T_B$ temperature clearly reflect the relaxation state of the lattice structure, which is caused by the fact that the static displacements of the atoms in the crystal with respect to their equilibrium positions do not occur simultaneously. This work describes the comprehensive study of structural properties on ${\mathrm{NaNbO}}_3$-based SPE ceramics, which display positive implications for the development of energy storage capacitors.