F‐Centres of Two Types in BaFCl Crystals

F‐centres in X‐irradiated and additively coloured tetragonal BaFCl single crystals have been studied. An F‐centre at a fluorine site absorbs light polarized parallel to the crystallographic c ‐axis in a band at 438 nm and light polarized perpendicular to c in a band at 550 nm. A chlorine F‐centre has a band at 532 nm ( E ‖ c ) and a double band at 440 nm ( E ‖ c ). This dichroism arises from the tetragonal surroundings of the F‐centres. The splitting of the double bands is probably due to a dynamic Jahn‐Teller effect. In an X‐irradiated crystal there is a partial transformation of fluorine into chlorine F‐centres. In additively coloured BaFCl predominantly fluorine type F‐centres are formed above 300 °C in thermal equilibrium.