Investigation of thermally induced Li⁺ion disorder in Li₂O using neutron diffraction

Abstract

Neutron diffraction measurements have been performed on a single crystal of 7Li2O to investigate the time-averaged positions of the nuclei at seven temperatures between 293 and 1603 K. At temperatures up to 873 K the observed Bragg peak intensities can be well fitted using the regular antifluorite structure, including harmonic and anharmonic temperature factors to describe the thermal vibrations. At higher temperatures an improved fit to the experimental data is obtained by allowing a fraction of cations to leave their regular lattice sites. This is indicative of a transition to a 'fast-ion' phase, analogous to that observed in the fluorite-structured compounds at elevated temperatures. The model of the time-averaged disordered structure which best fits the data consists of interstitial lithium ions occupying mean positions very close to the midpoint between two nearest-neighbour Li+ ions. The fraction of Li+ ions occupying these sites increases rapidly above 1200 K, reaching approximately 16% at 1603 K. There is some evidence that the presence of the interstitial cations produces slight relaxations of the two nearest-neighbour Li+ ions in (111) directions, away from the interstitial.