Multiscale Structural Modulation and Synergistic Enhancement of Transparency and Relaxor Behavior in La3+-Doped KNN Lead-Free Ceramics

Lead-free transparent ferroelectric ceramics with integrated opto-electro-mechanical functionalities are pivotal for next-generation multifunctional devices. In this study, K_0.48Na_0.52NbO₃-<i>x</i>La₂O₃ (KNN-<i>x</i>La, <i>x</i> = 0.005 - 0.04) ceramics were fabricated via a conventional solid-state route to investigate the La³⁺-induced multiscale structural evolution and its modulation of optical and electrical properties. La³⁺ substitution drives a critical structural transition from an anisotropic orthorhombic phase (<i>Amm</i>2) to a high-symmetry pseudocubic-like tetragonal phase (<i>P</i>4<i>mm</i>) for <i>x</i> ≥ 0.025, characterized by minimal lattice distortion (c/a = 1.0052). This enhanced structural isotropy, coupled with submicron grain refinement (<1 μm) driven by VA'-mediated solute drag, effectively suppresses light scattering. Consequently, a high-transparency plateau (<i>T</i>₇₈₀ ≈ 53-58%, <i>T</i>₁₇₀₀ ≈ 70-72%) is achieved for 0.025 ≤ <i>x</i> ≤ 0.035. Simultaneously, the system undergoes a crossover from normal ferroelectric (FE) to relaxor (RF) state, governed by an FE-RF boundary at <i>x</i> = 0.015. While <i>x</i> = 0.005 exhibits robust piezoelectricity (<i>d</i>₃₃ ≈ 92 pC/N), the <i>x</i> = 0.015 composition facilitates a transitional polar state with large strain (0.179%) and high polarization (<i>P</i>_m ≈ 33.3 μC/cm², <i>P</i>_r ≈ 15.8 μC/cm²). Piezoresponse force microscopy (PFM) confirms the domain evolution from lamellar macro-domains to speckle-like polar nanoregions (PNRs), elucidating the intrinsic trade-off between optical transparency and piezoelectricity. This work underscores La³⁺ as a potent structural modifier for tailoring phase boundaries and defect chemistry, providing a cost-effective framework for developing high-performance transparent electromechanical materials.