Self-Polarization and Zero-Bias Piezoelectricity Driven by Giant Interfacial Strain Gradient in Lead-Free...

Artificially engineered oxide superlattices offer a powerful platform for manipulating strain, interfacial coupling, and ferroelectric/piezoelectric properties, thereby enabling functionalities that cannot be achieved in single-phase materials. In this work, we investigate the structural and functional properties of 250-nm-thick (BiFeO₃)_0.65Λ/(BaTiO₃)_0.35Λ (BFO_0.65Λ/BTO_0.35Λ) superlattices with a modulation period Λ ranging from 24 to 195 Å, grown by pulsed laser deposition on (001) SrTiO₃ substrates buffered with a conductive SrRuO₃ electrode. Comprehensive structural analyses combining high-resolution X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy (STEM) reveal phase-pure films with good epitaxial quality and a defined nanoscale chemical modulation of the layers. The analysis of the average lattice parameters shows that the BTO layers are subjected to compressive strain, resulting in an enhanced tetragonality, whereas the BFO layers experience tensile strain accompanied by a slight reduction in tetragonality relative to the bulk. However, STEM-GPA mapping reveals large interfacial strain gradients of about 10⁵ m^-1 within the first ∼30 nm near the SRO interface for Λ ≥ 95 Å, which are associated with a strong tetragonal distortion in the BTO layers and a c/a ratio greater than unity in the BFO layers, together with the coexistence of rhombohedral-like and tetragonal-like BFO domains. Ferroelectric and dielectric measurements show enhanced polarization and permittivity for short-period superlattices, while larger-period structures (Λ ≥ 95 Å) exhibit a giant internal bias field (Es ≈ -200 kV/cm) that stabilizes a robust self-polarized state. PFM measurements confirm the ferroelectric nature of the films and reveal nonvolatile piezoelectric shape memory, as well as self-piezoelectric responses driven by the strong built-in field induced by the interfacial strain gradient. The stabilization of the self-piezoelectric state, combined with the low dielectric permittivity at zero field, provides a promising route to maximizing the figure of merit of lead-free nanostructures for next-generation piezoelectric devices.