Triboluminescence

Abstract

Triboluminescence (TL)—defined here as the light emitted when a material is stressed to the point of fracture—has been known since the sixteenth century, although serious investigations of the phenomenon were only begun in this century. As a rough guide, some 50% of all crystals exhibit TL. This review, partly historical in approach, seeks to correlate the TL properties of a material, as characterized by the TL spectrum, with other known luminescent properties of the material (for example, its photoluminescent spectrum) and to elucidate the mechanism whereby the TL is excited.

In many materials (for example, sugar) the TL originates from dielectric breakdown of the surrounding air. If the material can be excited to photoluminescence by the ultra-violet content of the gas discharge (as, for example, uranyl nitrate can be) the TL spectrum will also contain the material's photoluminescent spectrum. In several cases (for example, sugar and uranyl nitrate) piezoelectrically generated fields are able to account for the gas discharge observed at fracture. Electroluminescent materials, such as the doped zinc sulphides, which are also piezoelectric have a TL spectrum which is the same as the electroluminescent spectrum. Less well understood are the materials whose TL spectrum approximates to the photoluminescent spectrum but where no nitrogen discharge spectrum is evident (as in hexaphenylcarbodiphosphorane). Even more puzzling are coumarin (and other large organic molecules) whose TL spectra contain features not seen in their photoluminescent spectra. It is suggested that these features may arise through changes in the Franck-Condon factors brought about by the high stresses existing at the tips of growing cracks.

Besides exhibiting TL at fracture, γ or X-ray irradiated alkali halide crystals luminesce when deformed elastically or plastically. Two processes are involved: the release of electrons when moving dislocations interact with F-centres and the recombination of these free electrons with luminescent centres. Unirradiated, or merely additively coloured, alkali halides only emit light at fracture. Although this light emission is largely attributable to a gas discharge, resonance radiation from the alkali metal atoms can be produced under certain conditions (for example, when NaCl crystals are fractured in an argon atmosphere).

The review also discusses the light sometimes emitted when materials crystallize from aqueous solutions and when phase transitions occur in solids. In some cases this light has a TL origin. Other topics discussed include the phenomenon of temporary triboluminescence and the relationship between TL and thermoluminescence. The main experimental techniques used in TL studies—notably fracturing techniques and the measurement of TL spectra—are described in some detail.