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Abstract:

Apatites are a structurally flexible group of materials, which are promising candidates for advanced optical applications due to their capacity for extensive cationic and anionic substitution. Recent attention has been drawn to their defect-related spectroscopic properties, particularly photochromism – an optically induced, reversible color change. A typical mechanism of the photochromic effect in inorganic materials involves the photoinduced generation of charge carriers, which are trapped at point defects, forming color centers. Currently, the photochromism of satisfactory efficiency in apatites requires rare-earth doping, most notably with Eu2+; however, the photochromic efficiency of such materials still remains modest. The main goal of this project is to prepare rare-earth-free photochromic haloapatites in the form of powders and ceramics. The molten salt method will be employed for the preparation of single-phase M5(PO4)3X (where M=Ca-Ba; X=F-Br) materials with a significantly enhanced photochromic response. The influence of synthesis conditions on the phase formation and photochromic properties will be investigated in detail, including the effects of flux origin and composition, temperature, annealing time, cooling rate, precursor-to-flux ratio, etc. Systematic and comprehensive characterization of dominant point defects responsible for the photochromic effect will be performed, combining the methods of optical spectroscopy with electron paramagnetic resonance and optically detected magnetic resonance spectroscopy analysis. As a result, high-contrast photochromic apatites will be prepared, and the mechanism of the photochromic effect will be proposed. This work will open new possibilities for the development of robust, rare-earth-free photochromic materials for applications such as optical data storage, smart coatings, anti-counterfeiting technologies, and others.