Study of Colour Centres in BaCl 2 and BaBr 2

After additive or electrolytic coloration, BaCl 2 , and BaBr 2 , single crystals exhibit F‐centres, whose proper symmetry is C 3 . The hyperfine spectrum in BaCl 2 can be accounted for with the assumption that there exist two types, F I and F II , of F‐centres, and that each hyperfine constant referring to a Ba 2+ ion ( k ) is given by an empirical expression a k = g N A / d   k 3 , where d k is the distance between the ion k and the F‐centre. The two types of F‐centres have the same g‐tensor with principal values, in the case of BaCl 2 , gXX = 1.966; gYY = 1.976; gZZ = 1.986. The F‐centres can be optically bleached at room temperature, and then regenerated by heating above 100 °C.