Vacancy Production and Volume Expansion in KBr and KBr-KCl Mixed Crystals

Abstract

The generation of Frenkel defects in KBr and KBr-KCl mixed crystals after x irradiation at 66\ifmmode^\circ\else\textdegree\fi{}K has been investigated. The growth of the optical absorption bands induced by the defects was studied, and the volume expansion was measured with the photoelastic technique. The relative defect production rate at 66 and 87\ifmmode^\circ\else\textdegree\fi{}K can be explained simply in terms of the thermal stability of the vacancy-interstitial pairs. The energy required to produce a Frenkel defect pair at 66\ifmmode^\circ\else\textdegree\fi{}K does not depend upon the quality of the x rays used to irradiate the crystals. The variation in the $\frac{\ensuremath{\alpha}}{F}$ ratio indicates that recombination luminescence is not an important factor in determining this ratio at 66\ifmmode^\circ\else\textdegree\fi{}K. The volume expansion associated with the $F$-center Frenkel pair in pure KBr agrees with the results due to L\"uty et al., for KBr-KH, indicating that the interstitial ${\mathrm{Br}}^{0}$ atom causes little expansion. No volume expansion could be attributed to the ${V}_{4}$ center; thus, it cannot be the exact antimorph of the $F$ center, but must involve a localized hole as in Kanzig's ${V}_{F}$ center of Itoh's model of the ${V}_{4}$ center. Analysis of the $\ensuremath{\alpha}$-center expansion data yielded an expansion at 66\ifmmode^\circ\else\textdegree\fi{}K caused by the interstitial bromine ions which was smaller than that produced at 5\ifmmode^\circ\else\textdegree\fi{}K. The expansion caused by $F$-center Frenkel pairs was larger in KBr-KCl crystals than in pure KBr. This indicates that the $F$-center production process involves the displacement of ${\mathrm{Cl}}^{\ensuremath{-}}$ ions from their substitutional sites. The expansion induced by $\ensuremath{\alpha}$-center Frenkel pairs in the mixed crystals was half that observed in KBr. Possible explanations for this anomalous result would appear to be aggregates of $\ensuremath{\alpha}$ centers which are not additive in their effect and overlap to change the bulk compressibility of the crystal in their vicinity, or a similar change produced by the vacancy-interstitial pairs.