Mechanoluminescence of Halides and Other Inorganic Crystals

Mechanoluminescence is studied in halides and other inorganic crystals. All the non-centrosymmetric crystals are mechanoluminescent and all the crystals which do not exhibit mechanoluminescence are centrosymmetric. Certain centrosymmetric crystals also exhibit ML which is comparable in intensity to that of the non-centrosymmetric crystals. The mechanoluminescence spectra of the crystals are reported. The mechanoluminescence spectra of Cs2[Pt(CN)4] · H2O crystals shift towards higher wavelengths as compared to the photoluminescence spectra. The ML activity per mole of the crystals spans four orders of magnitude. Among the alkali halides generally the harder crystals exhibit ML. The correlation of mechanoluminescence with the hardness of the crystals and the earlier disappearance of mechanoluminescence with increasing temperature suggest that the separation of the charged surfaces during the fracture of the crystals should be responsible for the mechanoluminescence excitation. The appearance or non-appearance of mechanoluminescence in centrosymmetric alkali halide crystals may be attributed to the respective ability or inability of cracks to pass through the charged directions of the misoriented regions.