The role of OH $ \mathsf {^-}$ ions in the stabilization of F $ \mathsf {_2^+}$ colour centres in LiF

Abstract

Thermally Stimulated Depolarisation Current (TSDC) and optical methods are applied to a range of alkali-fluoride crystals in order to establish a model for the stable F 2 + - like colour centres in LiF:OH-. The experimental results for LiF:OH- suggest that the OH- defects are partially destroyed under ionising irradiation or during crystal growth. The low-temperature dielectric relaxation signals in LiF:OH- and LiF:Mg2+,OH- are attributed to highly interacting hydroxide ions and products of their destruction located in extended lattice defects. In LiF:OH-, in contrast to other alkali halides, the results advocate for a defect-structure model, which considers a neutral defect (ND, probably O2 or H2) sited at the anion vacancy of the O2--V a + dipole and which possibly is the “nucleus” for the F 2 + centre. The proposed F 2 + (ND, O-) model seems to better explain the dielectric results, compared to the older F 2 + (O2-) and F 2 + (O-) models. The estimate for the electric dipole moment derived from the experimental TSDC bands, gives a value for the F 2 + - like centre in LiF:OH- between those of the F 2 + (O-) and F 2 + (O2-) defects, in good agreement with the proposed F 2 + (ND,O-) model. The reduction of the activation energy barrier of the (re)orientation process of the Mg2+V c - (OH-) complexes in LiF:Mg2+,OH-, and the low-temperature shift of their TSDC band, compared to the single Mg 2 + V c - peak in LiF:Mg2+, are tentatively ascribed to an increase in the crystal-lattice parameters owing to the presence of OH- and/or products of its destruction.