Clustering in rare-earth-doped alkaline earth fluorides (dielectric relaxation)

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Abstract
Audio-frequency capacitance and conductance measurements have been carried out for calcium fluoride doped with 1.0 mol.% of thirteen rare earths, and 10 and 20 mol.% of erbium and lanthanum; strontium fluoride doped with 0.1 and 1.0 mol.% of twelve rare earths, yttrium and lanthanum; and barium fluoride doped with 0.1 mol.% of twelve rare earths, yttrium and lanthanum. For the moderately doped calcium fluoride samples, it is found that the low-temperature (RIII) relaxation dominates the spectrum for small rare earths. For large rare earths, the dimer-associated RIV ('gettered' 2:2:2) relaxation dominates the spectrum. The primary result for strontium fluoride is the observation of a new relaxation in the heavily doped samples with an activation energy which depends strongly upon the size of the rare earth and thus is attributed to RIV dimers. Other results may be summarised as follows. The RIV dimer becomes less stable as the host size increases but also as the rare earth size decreases. For smaller rare earth dopants, RIII is more stable in the smaller hosts and appears at lower concentrations in smaller hosts. Correspondingly, the complexity of the dielectric spectrum increases for smaller hosts and smaller rare earth dopants.