Bimodal polymorphic nanodomains in ferroelectric films for giant energy storage

Enhanced functional responses of ferroelectrics have often been achieved by complex compositional design of a morphotropic or polymorphic phase boundary. Here, we observe a pseudo-linear, ultra-slim polarization-electric field (P-E) hysteresis loop in simple composition, lead-free Ba(Zr,Ti)O3 of up to μm-thick. It features a low remnant polarization and a high maximum polarization after a delayed saturation process with a field-insensitive, thickness-scalable high energy efficiency of ∼90%. This giant energy storage performance is attributed to the self-assembled, bimodal polymorphic nanodomains consisting of two sets of coherent polymorphic nanodomains. The first set of domains has the best-matched, low index {110} interface. They manifest themselves as "entangled nanophases" and dominate in thinner or annealed films. The 2nd set of domains with a high-index {114} interface and an in-plane anisotropy leads to the largest reduction in bulk elastic energy. These domains show as "segregated nanophases" and prevail in as-grown thick films. The presence of multi-polar states in both sets of nanodomains helps reduce the remnant polarization and delay the polarization saturation. Furthermore, the segregated tetragonal phase in-plane nanodomains lead to a larger maximum polarization under a high poling field. Strain engineering of such nanodomain structures provides a promising alternative to chemical compositional design, for the optimization of dielectric thin films used in capacitive energy storage applications.