Mechanoluminescence of Coloured Alkali Halide Crystals

Abstract

The present paper reports both the experimental and mathematical aspects of elastico-mechanoluminescence (EML), plastico-mechanoluminescence (PML) and fracto-mechanoluminescence (FML) of coloured alkali halide crystals in detail, and thereby provides a deep understanding of the related phenomena. The additively coloured alkali halide crystals do not show ML during their elastic and plastic deformation. The ML emission during the elastic deformation takes place due to the mechanical interaction between bending dislocation segments and F-centres, and the ML emission during plastic deformation takes place due to the mechanical interaction between the moving dislocations and F-centres. The ML emission during fracture is also caused by the mechanical interaction between the moving dislocations and F-centres; however, in certain hard crystals like LiF, NaCl, NaF, etc., fracto ML also occurs due to the gas discharge caused by the creation of oppositely charged walls of cracks. The EML, PML, and solid state FML spectra of coloured alkali halide crystals are similar to their thermoluminescence spectra and afterglow spectra. However, the fracto ML spectra of certain hard crystals like LiF, NaCl, NaF, etc., also contain gas discharge spectra. The solid state ML spectra of coloured alkali halide crystals can be assigned to deformation-induced excitation of halide ions inV2-centres or in other hole-centres. Whereas, the intensity of EML and FML increases linearly with the applied pressure and the impact velocity, the intensity of PML increases quardratically with the applied pressure and the impact velocity because of the plastic flow of the crystals. Both Imand ITincrease with the density of F-centres in the crystals and strain rate of the crystals; however, they are optimum for a particular temperature of the crystals. The ML of diminished intensity also appears during the release of applied pressure. Expressions are derived for the elastico ML, plastico ML and fracto ML of coloured alkali halide crystals, in which a good agreement is found between the experimental and theoretical results. Many parameters of crystals such as band gap between the dislocation band and interacting F-centre energy level, radius of interaction between dislocations and F-centres, pinning time of dislocations, work hardening exponent, velocity of cracks, rise time of applied pressure, lifetime of electrons in the dislocation band, lifetime of electrons in shallow traps, diffusion time of holes, critical velocity of impact, etc., can be determined from the ML measurements. The ML of coloured alkali halide crystals has potential for self-indicating method of monitoring the microscopic and macroscopic processes; mechanoluminescence dosimetry; understanding dislocation bands in crystals; interaction between the dislocations and F-centres; dynamics of dislocations; deformation bleaching of coloration, etc. The ML of coloured alkali halide crystals has also the potential for photography, ML memory, and it gives information about slip planes, compression of crystals, fragmentation of crystals, etc.Contents of Paper