Defect structure of NH4+ β-alumina studied by NMR

Abstract

The behavior of proton NMR absorption spectra in nonstoichiometric NH+4 β-alumina single crystal has been studied between 90 and 230 K, below the onset of translational motion narrowing. Below 140 K the signal is the superposition of a broad line due to motionless NH+4 ions in mid-oxygen sites and of a narrow line due to rapidly rotating ions in Beevers–Ross sites. Above 160 K all the ions rotate and both lines merge into a single line. A detailed quantitative analysis of the second moments of these lines enables us to show that the defect related to the nonstoichiometry is made of two pairs of NH+4 ions in displaced mid-oxygen sites. Perfect agreement between theory and experiment is found if one takes into account small displacements of the Beevers–Ross ions from their high symmetry sites, the size and the shape of the NH+4 ions, and the specific orientations of the displaced Beevers–Ross and mid-oxygen ions. This study shows in particular that the population of anti-Beevers–Ross sites is vanishingly small at the temperatures where the translational motion is slow, showing that there is a close link between ionic conductivity and anti-Beevers–Ross occupancy.