Raman spectroscopy of superionic solid solutions of alkaline-earth and rare-earth halides

We present experimental results of Raman scattering of several fluorite-type materials doped with trivalent rare-earth ions in the temperature range 300-1400 K. In all of the solid solutions we observe a Raman peak, which is associated with the excitation of a T2g phonon. In heavily doped Ba_1-xLaₓF₂₊ₓ samples we observe an additional Raman peak, which we tentatively ascribe to the excitation of local-mode vibrations in the vicinity of an interstitial fluoride ion. These defects are present in considerable concentrations in these materials as a result of charge compensation. In agreement with theoretical predictions, we do not observe this local-mode excitation peak in concentrated solid solutions Sr_1-xLaₓF₂₊ₓ. Clustering turns out to influence the Raman scattering spectra considerably. This can be seen from the results obtained for the samples Sr_1-xYbₓF₂₊ₓ. Here, apart from the T2g-phonon peak we observe for heavily doped samples an extra peak, which is ascribed to the excitation of a local-mode vibration in the neighborhood of an anion vacancy; these vacancies are present in clusters containing more than one Yb³⁺ impurity. The width of the Raman peak is broadened appreciably by jumping defects, which are present in large numbers, especially in heavily doped samples. The behavior of this contribution to the width of the Raman peak has been analyzed for a large number of solid solutions M_1-xRₓF₂₊ₓ or M_1-xRₓCl₂₊ₓ. From this analysis we have found values for the activation energies associated with the jump frequencies of the defects involved.