LOCAL ENVIRONMENT AND LITHIUM ION MOBILITY IN LINBO2 - INFERENCES FROM STRUCTURE, PHYSICAL-PROPERTIES, AND NMR

The structure and properties of LiNbO2, including the mobility of lith ium ions, have been determined by Rietveld refinement of the structura l model from powder neutron diffraction data and by analysis of Nb-93 and Li-7 NMR spectra, magnetic susceptibility data, and the optical sp ectrum. The material prepared for this study was found to be slightly deficient in lithium (relative to niobium), Li0.96NbO2 from chemical a nalysis and Li0.98NbO2 from the analysis of neutron diffraction data, but fully stoichiometric in oxygen. Close agreement between the lithiu m content and the lithium site occupancy suggests that there are few a ntisite defects, if any. The structural model for LiNbO2 first propose d by Meyer and Hoppe was confirmed by the refinement of the structural model from the powder neutron diffraction data; the static quadrupola r and dipolar interactions obtained from the NMR spectra are also cons istent with this model, While impure samples are paramagnetic and exhi bit field-dependent magnetic behavior, the purest samples are diamagne tic, and there is no field dependence. An absorption onset at approxim ately 2 eV was observed, providing further evidence that LiNbO2 is ind eed a diamagnetic semiconductor, as predicted by the band structure ca lculation of Burdett and Hughbanks. Finally, variable temperature and frequency Li-7 NMR studies indicate two different types of lithium mot ion in the system, occurring at significantly different rates: a relat ively high-frequency rattling process of the Li ion within a given oct ahedral site, accompanied by occasional hops to adjacent octahedral si tes.