Structure and ionic conductivity of reactively sputtered apatite-type lanthanum silicate thin films

La–Si–O thin films with Si/(La + Si) atomic ratios ranging from 0.36 to 0.43 were produced by magnetron sputtering in a reactive Ar/O discharge gas. The as-deposited films have large X-ray diffraction peak characteristic of quasi-amorphous materials and oxygen contents from 29 to 35 at.%. The Apatite-type lanthanum silicate phase was formed in all the as-deposited films upon annealing at 900 °C for 1 h. The lanthanum silicate films obtained by annealing the as-deposited films with lower Si/(La + Si) atomic ratios have a preferential orientation with the c-axis perpendicular to the substrate while low intensity diffraction peaks ascribed to La2Si2O7 phase were detected in the films deposited with higher Si content. The preferentially oriented films have higher activation energy and lower ionic conductivity as the ionic conductivity measurements were performed in the direction perpendicular to the c-axis. The highest ionic conductivity was obtained for the film deposited with a Si/(La + Si) atomic ratio of 0.42, with a value of 1.2 × 10− 2 S·cm− 1 at 750 °C.