Realizing Color-Tunable Photochromism in Apatite-Structured Compounds via Trapping Energy Management for Wide Gamut Encryption

Photochromic materials have shown significant potential in a plethora of possible applications. However, in practical applications, they remain limited by intricate multi-stage synthesis processes and a restricted range of photochromic colors. Modulating the chromatic properties involves an intricate manipulation of their chemical composition, crystal structure, and trapping energy levels, rendering it extremely challenging to achieve a desired color gamut. In this study, apatite-type materials are rapidly synthesized using a microwave-assisted approach. The photochromic behavior is systematically examined by substituting F⁻ ions with Cl⁻ ions, facilitating the transformation of color centers from pink to green and ultimately to blue. Notably, the prominent reflectance absorption peak ≈554 nm exhibited a gradual red shift to 625 nm. A thorough characterization reveals that this large shift in wavelength of F-center arises from the effective modulation in the trapping energy levels with differing trap distributions linked to photochromism observed across the various Sr₅(PO₄)₃F₁₋_xCl_x:1%Eu. Finally, the potential applications of these multicolor compounds are demonstrated in routine chromatic decorations and sophisticated multi-hued cryptographic technologies. These findings highlight the efficacy of trapping energy management in optimizing photochromic behavior and the considerable capability of the photochromic materials for on-demand practical applications.