Vibrational study of and conduction mechanism in β alumina. I. Stoichiometric β alumina

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Abstract
Single crystals of the stoichiometric β alumina containing H3O+, Na+, K+, Ag+, Tl+ ions have been studied by Raman spectroscopy between 2 and 4000 cm−1 in the 10–1400 K temperature range. Single crystals of isomorphous gallates have also been studied. Far infrared measurements have been performed between 10 and 250 cm−1. The Raman bands assigned to the spinel block vibrational modes of the stoichiometric compound are very narrow, Δν1/2?2 cm−1 as expected for an ordered crystal, while those of the nonstoichiometric compound are much broader Δν1/2?10 cm−1, this confirms the mechanism of the Al3+ Frenkel defects. The in plane cation vibrations have been identified below 100 cm−1 in infrared and in Raman. The most prominent Raman feature corresponds to the in plane E2g species and its halfwidth was measured as a function of temperature. The results indicate that there is a transition from an ordered state at low temperature to a state where the conducting cations are dynamically disordered. The stoichiometric β alumina can thus be considered as a superionic conductor above a characteristic temperature (Tc?350 K for Ag+, Tc?100 K for Na+) which is consistent with the x-ray study. The conductivity mechanism and activation energy are also discussed.