Étude de l'organisation des ions sodium dans l'alumine β par résonance magnétique nucléaire

Abstract

Nuclear magnetic resonance study of sodium ion organization in beta-alumina An extensive study of the high-field (36 k-Oe) N.M.R. spectrum of ²³Na (I = 3/2) has been performed in single crystals of sodium beta-alumina at 300, 100 and 4·2 K. Only the central transition (½, −½) has been observed. Rotation spectra around c, a and a∗ axes have been performed and reveal a tendency for sodium ions to cluster into equilateral triangles at low temperature (T ∼ 100 K). The hopping frequency corresponding to the exchange of atoms within a cluster is fast compared to the second-order quadrupolar frequency. The quadrupolar spectrum at 100 K corresponds to the motional narrowing of the spectrum of three sites which are equivalent by 2π/3 rotations around the c axis. At higher temperature (T ∼ 300 K) the frequency for the hopping of ions between all vacant sites available in the structure is three orders of magnitude larger than the second-order quadrupolar frequency. Therefore the time-averaged environment of ²³Na exhibits an axial symmetry. These experiments have shown the existence of a strong triangular correlation in the diffusion of Na cations in beta-alumina.