Self-recoverable elastico mechanoluminescence of a hybrid metal halide crystal

Materials exhibiting self-recoverable elastico mechanoluminescence (EML) are highly sought after due to their utility in sensing and information encryption. However, the current pool of identified EML materials remains limited, exclusively comprising purely inorganic substances. This study delves into the investigation of the EML properties of a zero-dimensional (0D) organic-inorganic metal halide denoted as [C₁₉H₁₈P]₂MnBr₄ (where C₁₉H₁₈P⁺ signifies methyl triphenyl phosphonium). Notably, [C₁₉H₁₈P]₂MnBr₄ manifests two distinct polymorphs, with the piezoelectric and non-piezoelectric polymorphs exhibiting thermodynamic and kinetic stability, respectively. Despite both compounds showing bright greenish luminescence, only the piezoelectric phase exhibits desirable EML behavior. The EML in this context is distinguished by its high intensity, strong fatigue resistance and prompt self-recovery. The underlying mechanism of EML in the piezoelectric polymorph can be explained by the piezoelectric effect and the stress-induced energy band tilting. Calorimetric and piezoelectric experiments reveal the self-recoverable EML arises from the relaxation of the stress-induced kinetically stable non-piezoelectric to the thermodynamically favored piezoelectric phase. This work paves a new pathway in the exploration of self-recoverable EML materials in the realm of hybrid organic-inorganic crystals.