Vibrational study of and conduction mechanism in β alumina. II. Nonstoichiometric β alumina

Single crystals of nonstoichiometric β alumina 11 Al2O3, (1+x)M2O3(x = 0.25 to 0.6) with M = Na, K, Tl, Ag have been studied by Raman spectroscopy between 2 and 1000 cm−1 in the 20–1400 K temperature range. Far infrared measurements have been performed between 10 and 250 cm−1. Ag(Ag)3 and (Tl)3 clusters of coupled cations have been evidenced in the low temperature spectra of Ag+ and Tl+ compounds. Ag–Ag and Tl–Tl ’’bond stretching’’ force constants were found equal, respectively, to 0.10 and 0.23 mdyn Å−1. In the low temperature state, potential barriers separating sites available for cations have been found to increase at a small extent from x = 0 to x = 0.6 compounds. The temperature dependence of the low frequency Raman spectra is reported and shows that vibrational correlations between silver ions are partly responsible for the decrease of the energy necessary for the cation hopping mechanism.