A study of the intrinsic defects in LiY F4 with extended-ion method

The intrinsic defects in LiY F4, such as F centers, V k centers and self-trapped excitons (STEs), are studied within the extended-ion approximation. It is found that the V k center moves toward the nearby interstitial site, still keeping C 2 v symmetry. The absorption spectrum of V k comes from the optical transition ( Σ u → Π g ) of C configuration. The STE is in fact a nearest-neighbour F–H pair with C configuration, and the STE undergoes a relaxation consisting of a molecular axial transition superimposed with a rotation. Such a transition plus rotation relaxation of the STE is quite different from those in alkali halides, but there are some similarities with those in alkali-earth halides. The calculated results for the absorption of F centers, V k centers and the luminescence spectrum of self-trapped exciton (STE) are in a good agreement with experiments.