Self‐Recoverable Ultraviolet Mechanoluminescence in Doping‐Free MgF ₂ for Stealth Anti‐Counterfeiting

Abstract Ultraviolet (UV) mechanoluminescent (ML) materials, capable of converting mechanical stimuli into UV light without external excitation, hold significant promise for applications in pressure sensing, photodynamic therapy, and optical encryption. Realizing these applications requires UV ML materials with reliable, self‐recoverable emission. Here, self‐recoverable ML is reported from undoped magnesium fluoride (MgF 2 ), with emission from UV to the visible blue range. This ML originates from fluorine vacancies induced by oxygen incorporation during solid‐state sintering and the crystal field of the sintered MgF 2 . Density functional theory calculations reveal that these vacancies create intermediate energy levels that facilitate ML emission. Notably, X‐ray irradiation enhances ML intensity by nearly threefold, owing to the increased population of charge carriers trapped at defect states. The mechanism underlying the self‐recoverable UV ML emission is elucidated through an analysis of the piezoelectric properties and trap‐level distribution of MgF 2 . Leveraging its high transparency and invisibility under UV illumination, a stealth anti‐counterfeiting device retrievable only by mechanical stimulation—overcoming the vulnerability of conventional ML‐based systems to unauthorized retrieval is demonstrated. The findings provide new insight into the defect‐level engineering of UV ML materials and offer a strategy to advance ML‐enabled security technologies.