Liyf <sub> 4 </ Sub> Crystal Intrinsic Defects: F Center, V <sub> K </ Sub> Heart And Ste

The intrinsic defects in LiYF₄, such as F centers , V_k centers andself-trapped excitons (STES), are studied by the extended-ion method on the basis of one-electron Hartree-Fock approximation. The energies and the corresponding equilibrium configurations of F centers , V_k centers andself-trapped excitons(STES) are determined by variational method.The results show that there are four possible F^- holes. From the point of view of energy the probability is almost equal when an electron is captured by any anion vacancies. The excitation electron of the F centers has shown polarisation properties characteristic of dipoles oriented inthe direction of Y³⁺-Y³⁺ pairs. Among the four possible configurations of V_k centers and self-trapped excitons (STES) C configuration is the most favourable one to be occupied by the V_k center and STE. It is found that the V_k center moves toward the nearby interstitial site, still keeping C_2v, symmetry. The absorption spectrum of V_k centers comes from the opticaltransition ²∏_g�'²∑_μ + of C configuration. And the STEs undergo arelaxation consisting of an molecular axial translation superimposed with a rotation. Such a translation plus rotation relaxation of the STEs 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 spectrums of F centers ,V_k centers and the luminescence spectrum of self-trapped exciton(STE) are in a good agreement with experiments.