Proton Conduction and Microstructure of Lanthanum Phosphates

Lanthanum phosphate (LaPO4) has been recently studied as a potential proton conducting ceramic electrolyte for fuel cells. The complexity of the La2O3 - P2O5 system, particularly in the 1000 - 1600K range, leads to the difficulty in forming phase pure LaPO4 by traditional synthesis methods. Complex microstructures involving amorphous and crystalline phases at grain boundaries have been detected by high resolution transmission electron microscopy, and have been shown to affect proton conductivity by several orders of magnitude. A simple model is used to predict conductivity of the material based on grain size and grain-boundary phases, and experimental results match up well with the model. Conductivities as high as 3.6E-5 S/cm at 773 K were found for undoped LaPO4.