Proton NMR in NH4+ β-alumina: defect structure and low dimensionality effects

NMR spectra of protons in NH4+ β-alumina single crystal has been studied in detail between 90 K and 530 K. Below 140 K the signal is the superposition of a broad line due to motionless NH4+ ions in mO sites and of a narrow line due to rapidly rotating ions in BR sites. Above 160 K, all the ions rotate and both lines merge into a single line. A quantitative analysis of the second moment in the two regimes enables to determine the geometry of the defect related to the non-stoichiometry: two pairs of NH4+ ions in displaced mO sites. Above 250 K translational diffusion causes complete motion narrowing. Low dimensionality effects are observed in that regime. (Auth.)