Intragranular Phase Proton Conduction in Crystalline Sn_1-xIn_xP₂O₇ (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 Sn_1-xIn_xP₂O₇ (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 concentration occur between hydrated and dehydrated samples of SnP₂O₇ while changes in proton vibrational mode amplitudes occur with indium doping. The vibrational modes identified for the two materials are consistent with atomistic models of the bulk crystalline conductivity mechanism, where our calculations show that doping of In does not increase the mobility; instead, it helps to incorporate protons. This is also consistent with the Arrhenius behavior of the conductivity in which the activation energy is very similar between the undoped and doped material but with the doped material showing a larger pre-exponential constant. Our modeling results indicate that the interoctahedra hop between two M-O-P bridges is the most dominant transport pathway irrespective of doping. This work helps resolve the ongoing discrepancies in the literature regarding the mechanism of proton conduction in this material system. There exist two distinct conduction mechanisms between stoichiometric Sn_1-xIn_xP₂O₇ and excess polyphosphate containing Sn_0.9In_0.1P_2+xO_7±z. Intragranular proton transport through the bulk crystalline Sn_1-xIn_xP₂O₇ material occurs only at elevated temperatures. An amorphous polyphosphate phase residing at the crystalline grain boundaries (not the intercrystalline grain boundaries of the metal pyrophosphate itself) is required to obtain the high conductivity at reduced temperatures reported in the literature.