Mechanisms of Enhanced Ionic Conduction at Interfaces in Ceramics

Abstract

AbstractA large enhancement in the ionic conductivity of certain compounds occurs when the compound is produced as a composite material containing a finely-dispersed non-conductor such as SiO2 or Al2O3 This effect has been reported on for more than 20 years, and it is well established that the enhancement is associated with the presence of interfaces. The popular explanation has been based on a model which contends that the enhancement is due to a space-charge layer which forms to compensate a net charge layer at an interface. A different model proposes that extended defects such as dislocations and grain boundaries, either resulting from or stabilized by the interface, are responsible for the enhancement. This paper describes recent experiments which strongly support the latter model. The ionic conductivities of LiI and CaF2 thin films grown on sapphire(0001) substrates were monitored in-situ during deposition as a function of film thickness and deposition conditions. LiI films grown at 27°C exhibited a region of enhanced conduction within 100 nm of the substrate and a lesser enhancement as the film thickness was increased further. This conduction enhancement was not stable but annealed out with a characteristic log(time) dependence. The observed annealing behavior was fit with a model based on dislocation motion which implies that the increase in conduction near the interface is due to extended defects generated during the growth process. LiI films grown at higher temperatures (100°C) in order to reduce the grown-in defects showed no interfacial conduction enhancement. X-ray diffraction measurements suggest that these high-temperature LiI films nucleate as faceted epitaxial islands with a stable misfit dislocation density defined by the epitaxial relationship between the substrate and film. CaF2 films grown at 200°C showed a behavior similar to the 27°C LiI films, with a region of thermally unstable enhanced conduction that occurs within 10 nm of the substrate. Amorphous Al2O3 films deposited over the CaF2 layers created no additional enhancement but did increase the stability of the conduction, consistent with an extended defect model. Simultaneous deposition of CaF2 and Al2O3 produced films consisting of very-fine-grained CaF2 and particles of amorphous Al2O23 (5-10 nm grain and particle size) and a high defect density which was stable even well above the growth temperature. Measured conduction in the composite at 200°C was approximately 360 times that of bulk CaF2.