Ionic conductivity of LixLa10-x(SiO4)6O3-x sinters

A series of Li2O-La2O3-SiO2 sinters were prepared with the various ratios of the associated elements, Li: La: Si=x: (10-x): 6, and their ionic conductivities were examined. All the sinters were mainly composed of the apatite-type LixLa10-x(SiO4)6O3-x crystalline phases but sub-phases were produced in some cases. The lattice parameters of the main phase decreased with an increase in the Li-content, x, up to x=2.0 and became constant above x=2.0, indicating that La 4f-site was partially replaced by Li up to the maximum of x=2.0. The conductivity of the sinters gradually increased with an increase in x to 1.5, but it suddenly dropped down at x=2.0 and increased again by a further increase in x. The XRD and 29Si-NMR results suggested that the first conductivity increase was due to the decrease in the content of the La2SiO5 sub-phase with the Li replacement. Furthermore, the 7Li-NMR measurements gave evidence that lithium silicate sub-phase was formed at above x=2 in the grain-boundary of the crystalline phase as a high Li-ionic conducting phase. The Nernst's EMF response of the O2 and CO2 concentration cells suggested that the main ionic carriers were oxide ion at x < 2.0 and lithium ion at x ≥ 2. (author)