Intragranular Phase Proton Conduction in Crystalline Sn1–xInxP2O7 (x = 0 and 0.1)

Materials that exhibit fast ionic conduction in the intermediate temperature range are highly sought after for applications in fuel cells, electrolyzers, and sensors. Initial reports on tin pyrophosphate indicted that this material exhibited excellent protonic conductivity at 100–300 °C and that transport occurred via proton hopping through the bulk crystalline lattice. In this work, we conclusively show that the high conductivity reported by other research groups is not attributable to the bulk crystalline phase. The proton conduction mechanism of well-characterized Sn1–xInxP2O7 (x = 0 and 0.1) was investigated using ac impedance spectroscopy and inelastic neutron scattering. The crystalline MP2O7 phase possesses negligible bulk conductivity below 600 °C. Above 600 °C, the total conductivity exhibits Arrhenius behavior with an activation energy of ∼1 eV, with an increase in conductivity observed for the In-doped sample. Inelastic neutron scattering data indicates that no appreciable changes in proton con...