Composition-driven structural and dielectric transitions in epitaxial (1-x)PbMg_1/3Nb_2/3O₃-(x)PbZrO₃ relaxor-antiferroelectric thin films

Antiferroelectrics (AFE) are antipolar at zero field and undergo a field-induced reversible transition to polar ferroelectric (FE) phase at a threshold field, resulting in double polarization-electric field hysteresis loop. This transition makes AFEs attractive for applications like capacitive-energy storage, electromechanical actuation, and electrothermal-energy conversion. Challenges with conventional AFEs, however, are their low dielectric constants near zero field and large hysteretic losses at high fields. In contrast, relaxor ferroelectrics have complex nanoscale polar order at low fields but exhibit robust saturation polarization at high fields as well as high dielectric tunability and low hysteretic loss. Combining these two systems into relaxor antiferroelectrics (RAFE) can offer potential advantages like higher dielectric constants at low fields and less hysteresis at high fields enhancing energy storage capacities and improved electromechanical properties. Previous research in RAFE bulk ceramics focused on improved energy storage due to breaking of long-range antiferroelectric order. Despite their promising features, RAFEs are relatively underexplored, especially in thin-films, and there is not a consensus view of their local structural/polar order, phase evolution with chemistry and fields, and mechanisms underlying their enhanced properties. This work addresses this gap by investigating fundamental physics of RAFEs in thin films, that could unlock new functionalities in antiferroelectrics.

Here, a canonical solid solution of a relaxor and an AFE based on (1-x)PbMg_1/3Nb_2/3O₃-(x)PbZrO₃ [PMN-xPZ, with x = 1, 0.96, 0.94, 0.92, 0.86] system is studied to understand structural and physical behavior including dielectric, ferroelectric, energy-storage capacity and electromechanical response of RAFEs. Prior work in bulk ceramics showed existence of AFE-to-FE boundary near x = 0.9; however, evolution of crystal and polar structure and properties across this boundary have not been well understood. Here, solid solutions of PMN-(x)PZ (0.86 ≤ x ≤ 1) were grown as epitaxial thin films on SrTiO₃ (001) substrates. X-ray diffraction (XRD) studies show that, as PMN content increases, orthorhombic PbZrO₃ transitions through orthorhombic-rhombohedral structural coexistence at intermediate compositions (x = 0.94-0.92), transforming to pure rhombohedral structure at x = 0.86. This structural evolution is corroborated by atomic-resolution scanning transmission electron microscopy (STEM) mapping and cross-sectional dark-field imaging studies which show that, while PbZrO₃ possesses antiparallel dipole arrangement, intermediate compositions such as x = 0.94 exhibit periodic defect planes with randomly arranged dipoles breaking the antiferroelectric regions, ultimately transitioning into relaxor-like dipole arrangement at x = 0.86. In line with these changes, dielectric studies show increasing dielectric constant and tunability response as system transitions from AFE (x = 1) phase to mixed relaxor-AFE behavior at x = 0.92-0.94, to relaxor phase at x = 0.86. Polarization-electric field hysteresis loops show similar transition from AFE to RAFE to relaxor behavior corresponding to threshold field reducing from ~400 kV/cm to ~220 kV/cm to zero with increasing PMN content. Notably, these solid solution films had twice the breakdown field as compared to PbZrO₃, with leakage current densities two orders of magnitude lower due to changes in the transport mechanism. This increases maximum possible electromechanical strain in solid-solution films to 1.5-1.8% (compared to ~1% in PbZrO₃) and energy-storage efficiency to ~80% (compared to ~60% in PbZrO₃), extending the storage capacity as larger external fields can be applied. On the whole, this study provides insights into the crystal and polar structure evolution and corresponding electrical properties of RAFE systems and underlying mechanisms that govern these transitions and enhanced functionalities.