Epitaxial films of the proton-conducting Ca-doped LaNbO4 material and a study of their charge transport properties

Ca-doped LaNbO4 (LCNO) material has shown outstanding stability in harsh environments in combination with moderate proton-conductivity in polycrystalline samples at intermediate temperatures below 800 °C under a wet atmosphere. However, in polycrystalline materials the role of grain boundaries is believed to play a predominant role in protonic conductivity, which may make more difficult the study of the intrinsic properties of the material. For that reason the present study focuses on the preparation of epitaxial films by pulsed laser deposition on NdGaO3 (110) single crystal substrates. Under certain deposition conditions the films grow epitaxially with a monoclinic structure (fergusonite) and b-axis orientation. A polymorph change between the monoclinic and tetragonal structure has been observed at 250 °C. The film charge transport properties are analyzed under different atmospheres, dry and wet O2 and H2 as well as under deuterated conditions by using D2 and D2O in order to check for the characteristic isotopic effect of the proton transport. Although a certain proton conductivity was observed under reducing wet atmospheres unexpected large DC conductivity values (of about 5.5 S/cm for the 80 nm thin LCNO film at 800 °C in dry O2) have been observed for all films and ambient conditions.