Impurities alkali halide sensitivity

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

The triboluminescence of minerals has been studied visually (see the footnotes to Table I) but only a few minerals have been examined spectroscopically. There are a few clear examples of noncentric crystals, such as quartz, whose emission is lightning, sometimes with black body radiation. Most of the triboluminescent minerals appear to have activity and color which is dependent on impurities, as is the case for kunzite, fluorite, sphalerite and probably the alkali halides. Table I attempts to distinguish between fracto-luminescence and deformation luminescence, but the distinctions are not clear cut. A detailed analysis of the structural features of triboluminescent and nontriboluminescent minerals may make it possible to draw conclusions about the nature and concentration of trace impurities that are not obvious from the color or geological site of the crystals. Triboluminescence could be used as an additional method for characterizing minerals in the field, using only the standard rock hammer, with the sensitive human eye as a detector. 

While it is plausible that dislocations also act as internal surfaces which serve as centers for the initiation of decomposition, detailed studies of the role of dislocations require further examination. Recent advances in electron-microscope techniques may permit direct observation of the role of dislocations and impurities in the mechanisms of colloid or nucleus formation. Indeed, such studies on alkali halide crystals (at liquid-helium temperatures, to avoid radiation damage by the electron beam) confirmed the special role of dislocations in sensitizing the nucleation of colloids [52]. Such techniques should be possible for at least the (more stable) alkali azides. 

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