We study ionic transport in nano- and microcrystalline $(1-x){\mathrm{Li}}_2\mathrm{O}:x{\mathrm{B}}_2{\mathrm{O}}_3$ composites using standard impedance spectroscopy. In the nanocrystalline samples (average grain size of about 20 nm), the ionic conductivity ${\sigma }_{\mathrm{dc}}$ increases with increasing content $x$ of ${\mathrm{B}}_2{\mathrm{O}}_3$ up to a maximum at $x\approx 0.5$. Above $x\approx 0.92$, ${\sigma }_{\mathrm{dc}}$ vanishes. By contrast, in the microcrystalline samples (grain size about $10\mu \mathrm{m}$), ${\sigma }_{\mathrm{dc}}$ decreases monotonically with $x$ and vanishes above $x\approx 0.55$. We can explain this strikingly different behavior by a percolation model that assumes an enhanced conductivity at the interfaces between insulating and conducting phases in both materials and explicitly takes into account the different grain sizes.

• Received 16 August 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.2889