Foreign ions with d1-states in alkaline earth fluoride crystals

Starting from the model of an F-centre disturbed by an impurity ion and another centre in which the impurity ion plays an active part it is tried to explain photochromic processes in alkaline earth fluorides doped with Y, La, Ce, Gd, and Tb. Experiments and model concepts of Kiss et al. [1 to 3] are discussed and existing models investigated. Other absorption bands are associated with a so-called disturbed F-centre. These theories are supported experimentally by dichroism and bleaching experiments and theoretically by calculations on the basis of a Coulomb model of the disturbed F-centre. It can further be shown that the symmetry of the afore-mentioned activator centre is probably lower than cubic. An electron transfer between two locally separated types of centres across the conductivity band has so far been assumed by all authors [1 to 4]. Also Heyman [4] interpreted the results of his photoconductivity investigations of CaF2:Gd crystals in this way. In contrast to this it is shown that important, experimentally observable properties can be better described using a model which assumes the association of the disturbed F-centre and the activator centre [5].