Photoelastic Measurements of the Volume Expansion by theU→FandU→αTransformation in KBr

Volume expansion by the introduction and transformation of point defects in alkali halide crystals was measured using the photoelastic technique. KBr crystals containing $U$ centers were used as convenient model cases, which allow the transformation of $U$ centers into $F$ or $\alpha$ centers and hydride interstitials by uv or x irradiation. All products of these reactions can be quantitatively controlled in their formation and annealing behavior. Their concentrations can be monitored by optical absorption measurements, and their microscopic structure models are well known. Volume-expansion measurements with an improved automatic photoelastimeter were performed parallel to the formation and annealing processes, and could thus be correlated to specific center transformations. From the dilatation, which is found to be strictly proportional to the $U\to F$ transformation at room temperature, the volume expansion per $F$ center can be determined. $U\to \alpha$ conversion by uv irradiation at low temperature produces volume expansion proportional to the $\alpha$-center concentration. Thermal annealing of the $\alpha$ centers occurs in correlation with the annealing of the volume expansion. This allows the accurate determination of the volume change per vacancy $H^{-}$-interstitial pair. Using Eshelby's theory and previous data on the lattice parameter of KBr: KH crystals, values for the microscopic dilatations around $F$ centers and $\alpha$ centers are obtained.