Impedance spectroscopy and proton transport number measurements on Sr-substituted LaPO4 prepared by combustion synthesis

La1−xSrxPO4 samples of nominal composition x=0.025 and x=0.050 were prepared by powder combustion synthesis, cold-pressing, and sintering at 1300 °C. Samples with Pt paste electrodes were characterized by conductivity measurements, impedance spectroscopy, and transport number measurements. The samples are predominantly protonic conductors below 700 °C in wet oxygen, as proven for the first time with the EMF/water vapour concentration cell method. The modelling of the conductivity data over a large temperature range suggests that the entropy and enthalpy changes of the incorporation of water vapour to hydrolyse the P2O74− group into two HPO42− groups are −90 J/mol K and −83 kJ/mol, respectively. Impedance spectroscopy showed that grain boundary resistance is dominant only below 400 °C and thus not a severe problem for applications. The synthesis procedure is convenient and upscaleable, but at this stage, the proton conductivity appears to be somewhat lower than reported elsewhere, assigned in part to insufficient dopant homogenisation.