Nuclear magnetic resonance study of diffusion of bound and free fluorine interstitials in alkaline earth fluorides doped with trivalent impurities

When alkaline-earth fluorides are doped with trivalent impurities, interstitial fluorines are created to maintain charge neutrality. We have performed NMR dipolar-energy relaxation measurements over a wide temperature range and have observed the diffusion of both free and locally bound fluorine interstitiais F(7} in the extrinsic region. We have observed that these motions depend strongly on the host-crystal lattice parameter. In particular, we have observed that the motion of which is at the nearest neighbor (nn) site to the trivalent impurity dominates the relaxation above room temperature in CaF2:Y3 + but is unobserved in BaF2:Y3 + . In addition, a second type of bound F(7) motion, characterized by a much smaller activation energy, appears over a very narrow temperature range (150-185 °C) in CaF2:Y3 + , but over a large temperature range (below room temperature to 130°C) in BaF2:Y3 + . SrF2:Y3 + shows similar behavior over a temperature range (54-160°C), which is intermediate between that of CaF2:Y3 + and BaF2:Y3 + . Possible explanations in terms of the motion of a more remotely bound F^ [e.g., at a next-nearest-neighbor (nnn) site] and the motion of F@ near clusters of dipoles are discussed. We measured activation energies for all these F(7) motions. A comparison of our results with those by other techniques (specifically, EPR, ENDOR, optical spectroscopy, ionic conductivity, ionic thermocurrent, dielectric and anelastic loss) is also given.