Proton conduction in acceptor doped SnP2O7

Undoped and acceptor doped tin pyrophosphate with composition D0.1Sn0.9P2O7 − δ (D = In, Ga, Zn, Sc, and Mg) were synthesized by the conventional solid state reaction method and sintered to obtain well densified pellets (> 90% relative density). The conductivity of all the synthesized materials was measured in unhumidified atmosphere in the temperature range 300 °C–600 °C using electrochemical impedance spectroscopy (EIS). The conductivity of acceptor doped SnP2O7 was found to be significantly higher than undoped SnP2O7 for all the dopants. Proton transference number measurements indicated that the charge transport in acceptor doped SnP2O7 was mostly protonic. Within the dopants analyzed, Zn0.1Sn0.9P2O7 − δ showed the highest conductivity of 2.84 × 10−6 S/cm at 600 °C. This value is several orders of magnitude lower than previously reported for doped tin pyrophosphate compounds but it is consistent with a recent publication. The reason for the discrepancy was investigated and a possible explanation has been proposed based on results obtained from 31P MAS–NMR spectroscopy as follows. It was observed that the as-calcined powder of doped SnP2O7 contained a significant fraction of leftover phosphoric acid from synthesis procedure. This phosphoric acid is likely to be responsible for the high conductivity values for acceptor doped SnP2O7 previously reported by other research groups.

Research Highlights
► Synthesized materials with composition D0.1Sn0.9P2O7 − δ (D = In, Ga, Zn, Sc, and Mg).
► Performed 31P MAS–NMR to compare as-calcined and as-sintered samples.
► Measured conductivity and performed transference number measurements.
► Explained large differences in previously reported conductivities of doped SnP2O7.