Complexes of trigonal symmetry involving hydride ions paired with trivalent rare earth ions in calcium fluoride

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Abstract
When hydrogen is diffused into calcium fluoride crystals which contain both rare earth (RE) and oxygen impurities, RE3+- H- complexes of trigonal symmetry, labelled T3, are formed. These centres have been detected by measurements of the infrared absorption from the localized vibrations of the paired H- (or D-) ions, and in the case of crystals containing Yb3+ ions these measurements have been correlated with EPR spectra and with optical electronic transitions. It is shown that the RE3+ and H- ions occupy nearest neighbour substitutional sites and that T3 is a simple modification of another trigonal centre, T1, which is sometimes present in crystals grown by the Czochralski technique. It is suggested that models proposed previously for T1 and for another centre of trigonal symmetry (T2) may be incorrect, and alternative proposals are put forward.