Colour Centres in Alkaline Earth Fluorides

Abstract

Abstract The effect of trace elements on the optical and UV absorption of X‐irradiated calcium fluoride crystals was investigated. Crystals displaying the oxygen cut‐off at about 220 nm in the unirradiated state yielded the α and β absorption bands at about 375 and 550 nm, respectively. In the absence of the cut‐off the well known four‐band spectrum (225, 335, 400, and 580 nm) appeared. This was not significantly influenced by the level of cation impurities, including yttrium in amounts of 0.5 to 1000 ppm. A possible relation between the two spectra is suggested. An attempt was made to locate the F‐band by evaluating the 1‐2p transition energy of electrons trapped at vacant fluorine sites, and the calculations were extended to the fluorides of strontium and barium. The calculated absorption wavelengths for CaF 2 (400 nm) and BaF 2 (610 nm) are reasonably close to those of 375 and 660 nm ascribed by Arends to F‐centers on the basis of his ESR data. The value derived for SrF 2 (495 nm) suggests that the strongest peak, occurring close to 450 nm in additively coloured crystals, corresponds to the F‐centre. The origin of a ‘Mollwo’ relation satisfied by the bands of these crystals, is discussed.