A new type of F₃‐centre in additively coloured CaF₂

Two new absorption bands at 450 and 320 nm appear in additively coloured CaF 2 crystals by bleaching in the α‐band (375 nm) at temperatures ≈ 160 K. These two bands have dichroic behaviour, from which is found, that they are due to two different optical transitions in the same centre, which have their electric dipoles perpendicular, and parallel to the 〈110〉 directions, for the 450 nm band; and along the 〈100〉 directions, for the 320 nm band. Thermal reorientation measurements show that the dichroism induced by light with E ∥ [110], disappears at 210 K, while the dichroism induced by light with E ∥ [100] does not disappear until 270 K. This 270 K disorientation is accompanied by a transformation to the linear F 3 ‐centre, which appears with some dichroism. A model for the centre responsible for these two bands, consisting of three F‐centres forming a right‐angled triangle in a (100) plane, gives good agreement with the experimental results. This is a new type of F 3 ‐centre in the fluorite structure, in contrast with the linear F 3 ‐centre found by bleaching in the α‐band at RT.