Photochromic apatite skeletal structure materials: recent advances and potential applications

Rare earth ion-doped inorganic photochromic (PC) materials have recently garnered increasing research interest due to their excellent reversibility, fatigue resistance, and diverse PC performances. Apatite-type materials, in particular, have emerged as a focal point, offering the potential for exchanging multiple cations and anions with stronger tolerance. In comparison to other inorganic PC materials, apatite-type PC materials exhibit more abundant color changes and absorption bands and centers, creating new and substantial opportunities for adjusting and optimizing PC properties, deepening the understanding of PC mechanisms, and enabling construction of novel PC materials and development of multifunctional applications. This review comprehensively summarizes and discusses recent progress in apatite-type PC materials. It covers various aspects, including the PC properties of reported apatite-type PC materials, the pivotal role of traps in PC reactions, the energy transfer mechanism between luminescent centers and color centers, the prediction of PC properties through first-principles calculations, and the systematic regulation of color changes and absorption bands and centers during PC reactions. Benefiting from multiple color variations and extensive absorption band regulation, apatite configuration PC materials exhibit broad prospects and significant potential for various frontier applications, such as optical modulation, photo-switching, anti-counterfeiting, erasable optical information storage, and temperature sensing. Subsequently, this review outlines some challenges and prospects in the research and design of apatite-type PC materials, aiming to guide the exploration of structural design of new inorganic PC materials, further regulating and controlling the PC effect and expanding practical application domains. This review summarizes the reported photochromic properties of apatite skeletal structure photochromic materials. Apatite-type PC materials exhibit more abundant color changes and absorption bands and centers, creating new and substantial opportunities for adjusting and optimizing PC properties.