Eur. Phys. J. B 28, 91-101 (2002)
https://doi.org/10.1140/epjb/e2002-00204-0

The role of OH⁻ ions in the stabilization of F₂⁺ colour centres in LiF

¹ Irkutsk State University, Institute of Applied Physics, 20 Gagarin Boulevard, 664003 Irkutsk, Russia
² Irkutsk State Institute of Transport Engineers, 15 Tchernyshevsky Street, 664074 Irkutsk, Russia
³ University of Athens, Department of Physics, Solid State Physics Section, Panepistimiopolis, 157 84 Zografos, Greece

Corresponding author: ^a ikaloger@cc.uoa.gr

Received: 31 August 2001
Revised: 30 March 2002
Published online: 9 July 2002

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 F₂⁺ – 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 O^2-–V_a⁺ dipole and which possibly is the “nucleus” for the F₂⁺ centre. The proposed F₂⁺(ND, O^-) model seems to better explain the dielectric results, compared to the older F₂⁺(O^2-) and F₂⁺(O^-) models. The estimate for the electric dipole moment derived from the experimental TSDC bands, gives a value for the F₂⁺ – like centre in LiF:OH^- between those of the F₂⁺(O^-) and F₂⁺(O^2-) defects, in good agreement with the proposed F₂⁺(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 Mg₂⁺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.