Protonic conduction and defect structures in Sr-doped LaPO4

Protonic conduction and defect structures in Sr-doped LaPO4 (Sr=0–7 mol%) were investigated by using conductivity, MAS-NMR spectroscopy and FT-Raman spectroscopy measurements. The results of MAS-NMR and FT-Raman spectroscopy measurements revealed that hydrogen phosphate groups, possibly hydrogen phosphate ions HPO42−, and pyrophosphate ions P2O74− existed in Sr-doped LaPO4 under wet and dry conditions, respectively, in addition to orthophosphate ions PO43−. Based on these results, we proposed a defect structure model for Sr-doped LaPO4. According to this model, La substitution with Sr leads to formation of P2O74− as intrinsic positive defects, and in wet atmosphere, protons are introduced into the phosphate forming hydrogen phosphate groups through the equilibrium between P2O74− and water vapor. The conductivity behavior of Sr-doped LaPO4 versus water vapor and oxygen partial pressures supported this defect structure model.