Infrared Absorption Edges in Inorganic Optical Crystals: Understanding and Prediction

The optical transparent region is a crucial parameter for optoelectrical functional crystals. Herein, the relationship between the infrared (IR) absorption edge and phonon frequencies in inorganic optical crystals is investigated. It is demonstrated that the two-phonon absorption mechanism plays a significant role in determining the IR absorption edge in millimeter- and centimeter-level crystals, which are commonly employed for practical applications, and their quantitative correspondence is built based on first-principles calculations. Moreover, the phonon forbidden gap is highlighted to expand the transparent range. Accordingly, we predict two optical crystals with an ultrawide transparent region, i.e., Ag₂HgI₄ (with IR absorption edge at more than 30 μm) and Li₃VO₄ (with a transparent window from 6 to 9 μm). This work paves a new roadmap to evaluate the applied wavelength of optical crystals before large crystal growth that is often very resource-consuming and is beneficial to the exploration of new optoelectrical functional materials with wide transparent windows.