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

## TL;DR

This study explores how adding La3+ to KNN ceramics improves transparency and electrical properties, making them suitable for advanced multifunctional devices.

## Contribution

The novel contribution is the discovery of La3+ inducing a structural transition that enhances transparency and relaxor behavior in lead-free ceramics.

## Key findings

- La3+ substitution leads to a structural transition from orthorhombic to tetragonal phase, increasing transparency.
- The composition x = 0.015 shows a transition from normal ferroelectric to relaxor state with high polarization and strain.
- PFM reveals domain evolution from macro-domains to polar nanoregions, explaining the trade-off between transparency and piezoelectricity.

## Abstract

Lead-free transparent ferroelectric ceramics with integrated opto-electro-mechanical functionalities are pivotal for next-generation multifunctional devices. In this study, K0.48Na0.52NbO3-xLa2O3 (KNN-xLa, x = 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 (Amm2) to a high-symmetry pseudocubic-like tetragonal phase (P4mm) for x ≥ 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 (T780 ≈ 53–58%, T1700 ≈ 70–72%) is achieved for 0.025 ≤ x ≤ 0.035. Simultaneously, the system undergoes a crossover from normal ferroelectric (FE) to relaxor (RF) state, governed by an FE–RF boundary at x = 0.015. While x = 0.005 exhibits robust piezoelectricity (d33 ≈ 92 pC/N), the x = 0.015 composition facilitates a transitional polar state with large strain (0.179%) and high polarization (Pm ≈ 33.3 μC/cm2, Pr ≈ 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.