Mechanoluminescence in crystalline inorganic materials: local disorder and the elastic distortion hypothesis

In this exploratory work, we aim to understand the mechanoluminescence (ML) phenomenon manifested by various inorganic compounds by focusing on the influence of mechanical loading on local distortion and loss of symmetry at active sites. To this end, we have analyzed the elastic deformation of several relevant crystalline phases and shown, through a kinematic analysis based on elastic constants, that the loading-induced distortion is smaller than, but remains comparable in magnitude to, the intrinsic structural distortion as quantified by the Baur descriptor. Although the structural distortion has previously been proposed as a structural fingerprint of the ML potential of a compound, it is by nature a static parameter, and must be supplemented by a dynamic distortion contribution that develops under mechanical load. By approximating the crystal deformation by that of simple polygonal motifs, we have been able to propose a rationale for otherwise puzzling observations: in particular, the marked differences in sensitivity to hydrostatic pressure and to shear, the appearance of an intense ML peak upon unloading in certain compounds, and the contrast in behavior depending on whether the preliminary UV irradiation is performed under load or at rest.