Interstitial Stabilization and Color-Center Production in KBr

This paper deals with the color-center production in KBr containing alkali impurities (Li, Na, and Rb). It turns out that alkali impurities enhance the color-center production at 80^∘{}K only if their size is smaller than that of the substituted ion. Typically, in sodium- or lithiumdoped samples (1% in the melt) the average α- and F-center production efficiencies are increased by more than an order of magnitude and by a factor of 2-3, respectively. In KBr:Na, the enhancement of the α- and F-center production is related to the growth of the well-known V₁ and I (interstitial negative ion) bands (not observed at 80^∘{}K in sufficiently pure samples). Similarly, in KBr:Li, the increase of the α- and F-center production is related to the growth of an L-type band (peaking at 223 m{μ}) and of two V₁-type bands (peaking at 390 and 438 m{μ}). The analysis of the growth- and thermal-decay properties of the I- and V₁-type bands, with respect to those of the α and F bands, leads to the conclusion that the enhancement of the α- and F-center production is due to the trapping and the stabilization of interstitial halogens by sodium or lithium impurities. Finally, the fact that lithium-trapped interstitials are more stable than those trapped by sodium, along with the fact that rubidium impurities (1% in the melt) do not affect the color-center production, supports an interstitial-stabilization mechanism based on the reduction of the repulsive forces acting on the interstitials when they are trapped near suitable impurities.