DYNAMICAL EFFECTS ON THE ELECTRONIC SPIN RELAXATION OF AN OFF-CENTER INTERSTITIAL HYDROGEN IN SRF2 AND BAF2

A study of the spin relaxation times of interstitial hydrogen centres created in hydrolyzed SrF2 and BaF2 by X-irradiation at room temperatu re, H-i(0)(I), and at liquid nitrogen temperature, H-i(0)(II), is pres ented. The thermal evolution of T-l and T-M have been obtained by meas uring the spin echo decays at different temperatures. Marked differenc es have been found for H-i(0)(I), and H-i(30)(II). In the case of H-i( 0)(II) the relaxation times are too short and can only be measured bel ow 25 K. The echo decays for H-i(0)(I), which can be measured below 15 0 K, show an anomalous behaviour around 60 K and below 20 K. In order to explain this anomaly, a model is proposed consisting of a hydrogen atom in a [100] off-centre position which reorients among the six equi valent minima by [100] and [110] jumps for which the associated freque ncies are omega(110) < omega(100) at any temperature. This model harmo nizes our results with the previous CW-EPR measurements which show a c ubic spectrum due to an atomic hydrogen in the centre of a cube intera cting with eight equivalent fluoride ions.