ENDOR analysis of the microscopic structures of the thermally driven bistable configurations of FH(OH-) centres in KBr

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Abstract
The aggregation of F centres and OH- molecules to FH(OH-) centres in KBr leads to a very interesting optical and thermal bistability. The structures of the bistable FH(OH-) centres were investigated by using optical and conventional detection of electron nuclear double resonance (ENDOR). The analysis of the ENDOR spectra of both FH(OH-) configurations is presented. It is shown that in both bistable configurations the OH- molecule resides on a next-nearest anion site (fourth shell). The bistable configurations differ in the orientation of the OH- dipole with respect to the F centre. The thermal bistability is tentatively explained as being due to entropy changes with temperature.