Hardening of Potassium Chloride by Color Centers

Abstract

The flow stress of KCl crystals containing color centers was studied in a systematic way to determine which color centers produced significant hardening. The color centers were introduced both by additive coloring and by gamma irradiation. Annealing and optical bleaching techniques were used to alter the relative concentrations of defects. For a given concentration of electron excess centers (F, M, R, N, etc.), the irradiated crystals were about ten times harder than the additively colored ones. Possible models for the halogen defect responsible for this high hardness were considered and it was concluded that it must be an interstitial ion or atom. Annealing of additively colored crystals indicated that the ``colloid band was actually two overlapping bands centered at about 735 and 760 mμ. Prior to formation of these bands the crystals became about 40% harder than in the as-colored state indicating the presence of a coherent cluster of elementary defects. The flow stress decreased sharply upon appearance of the colloid bands. A similarly located though broader band was formed after prolonged optical bleaching.