Mechanistic insight into the Young's modulus threshold for fracto-mechanoluminescence in manganese halides.

Fracto-mechanoluminescence (FML), a subtype of mechanoluminescence, is the phenomenon of light emission triggered by the fracturing of solids under mechanical stimuli. Although many materials have been reported with FML, the underlying mechanism remains unclear, leveraging the fundamental prerequisites and design principles to achieve FML remain elusive. In this study, we systematically investigate a series of Mn halides and find that 12 out of 18 compounds exhibit bright and clearly detectable FML. Here we show that the occurrence of FML arises from the synergistic interplay among the crystal's elastic stiffness, local electromechanical coupling, and trap states, which collectively activate Mn2+ d-d transitions upon fracture. The enhancement of FML intensity is primarily governed by the enlargement of the effective fracture area, whereas the Young's modulus determines the fracture threshold and the tolerable stress range of crystals. Additionally, as-explored Mn halides exhibit improved X-ray imaging capabilities, which are further integrated into radiation warning and damage detection devices.