CHAPTER X 



TRIBOLUMINESCENCE, PIEZOLUMINESCENCE, 



CRYSTALLOLUMINESCENCE, AND 



LYOLUMINESCENCE 



Introduction 



LITERALLY interpreted triboluminescence should refer to a low- 

 -i temperature light which results from rubbing a material ( Greek 

 tribo, to rub) , whereas piezoluminescence should infer that pressing 

 (Greek, piezo, to press) has excited the light. Rubbing and scratch- 

 ing from local pressure often involve fracture of the material and 

 in many cases breaking or separation appears to be necessary for 

 such luminescences. Hence the name " Trennungsleuchten," first 

 applied by Heinrich (1820) , and often used in Germany. Light 

 resulting from collision, friction, fracture, or attrition is also con- 

 sidered in this chapter. Triboluminescence and piezoluminescence 

 are synonymous and the former word will be used as the preferred 

 term. 



Actually radiation described under these headings may be due to 

 three different activities.^ It may result (1) from frictional elec- 

 tricity, from the building up of a potential which discharges through 

 the air, an electroluminescence. It may be (2) a light emission from 

 unstable molecules (centers, electron traps) which have previously 

 been excited by exposure to radiation of some kind, a delayed phos- 

 phorescence like thermoluminescence, or what might be called tribo- 

 phosphorescence. Certainly not all triboluminescences are due to 

 previous radiation, and the word " tribostimulation " cannot be 

 applied. Finally (3) there may be emission from molecules excited 

 by the unbalanced charges on the two faces of the separated crystal 

 planes, what has been called true triboluminescence. Without at- 

 tempting to distinguish which process or combination of processes 

 is involved in any one case, the experience of a long list of investi- 

 gators has indicated that many minerals (amorphous or crystalline) 

 and many organic crystals exhibit the phenomenon of tribolumi- 

 nescence. 



Allied to triboluminescence is crystalloluminescence, the light 

 which appears when solutions crystallize, whereas lyoluminescence 



^ See F. G. Wick, Uber Triboluminescence, Sitzungsber. Akad. Wiss. Wien, Math-Nat. 

 Klasse Ila, 145: 689-705, 1936, and Jour. Opt. Soc. Amer. 27: 275-285, 1937. 



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