Characteristic Volume Expansion of Point Defects in KBr

Abstract

The change in volume of crystals of KBr, x irradiated at low temperature, has been measured using a capacitive dilatometer. Simultaneous measurement of volume expansion and optical density permitted a determination of the relaxations around individual defect pairs. By assuming that the $F$-center Frenkel pair relaxation volume was constant between 78 and 5 K, and by using previously measured values for the $\alpha$-center and $F$-center relaxations, the relaxations around the interstitial defect could be evaluated. It was found that at 78 K the $F$-center associated interstitial had a relaxation $(0.7\mathrm{}\pm 0.3)\frac{1}{2}{a}^3$. Over the temperature range 5-30 K, an apparent change in the relaxation due to the $\alpha$-center Frenkel pair is observed. Optical bleaching of the $F$ band between 5 and 20 K produced no change in the crystal volume. A possible reason for this is presented.