Polarizable complexes in barium fluoride doped with trivalent ions

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Abstract
Ionic thermal current experiments have been carried out on BaF2 crystals doped with different trivalent rare earth and yttrium ions. The impurities studied here are La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Ho3+, Tm3+ and Y3+ ; all the samples have an impurity concentration of 10-3 expressed in molar fraction. Three principal relaxation peaks have been observed in all the crystals. The highest temperature relaxation is attributed, due to its behaviour after thermal treatments, to the dislocations present in the crystal. The low temperature peaks are identified as relaxations due to nearest neighbours and next nearest neighbours RE3+-F-i complexes that coexist in BaF2 and not very often in other fluorite structures. Two sets of values for the relaxation parameters E0 and τ0 obtained for the various impurity ions are presented. First a unique value is assumed for the reorientation energy of dipoles. Second a distribution of the reorientation energy, with a width σ around a mean value E0(σ) was assumed. The values obtained with this model for E0(σ), τ0(σ) and σ are given and discussed

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