Significantly Photo‐Thermochromic KNN‐Based “Smart Window” for Sustainable Optical Data Storage and Anti‐Counterfeiting

Abstract Ferroelectric photo‐thermochromic (PTC) materials with both high coloration contrast and high transparency have attracted great attention for their applications in “smart window” for non‐contact optical information storage. Nevertheless, it has been challenging to develop a ferroelectric PTC material that simultaneously meets the distinct transparency and coloration contrast requirements. Herein, dual rare‐earth ions doping of 1%Er‐(K_0.5Na_0.5)_0.985La_0.015NbO₆ (Er‐KNLN) ferroelectric ceramics is aimed at increasing the optical transmittance and obtaining an ideal coloration contrast. The traditional solid‐state reaction is exploited for ceramics fabrication, and the optimal optical transmittance (≈70% at 900 nm) can be obtained due to finer grains, more symmetrical structure, and compact structure in samples sintered at a suitable temperature (such as ≤1165 °C). Contributed from the trapping and de‐trapping of charge carriers in vacancy‐related defects, the Er‐KNLN ceramics exhibit obvious PTC behavior and the novel impedance modulation is developed. The coupling between PTC behavior and optical transparency, lanthanide up‐conversion emission, patterned display, and room temperature impedance enables nondestructive rewritable and readout characteristics of multi‐mode “0” and “1” states. This work solves the main problems hampering the promising applications of ferroelectric materials and paves a way for developing multifunctional optical memory smart windows.