Proton conduction in acceptor doped SnP 2O 7

Abstract

Undoped and acceptor doped tin pyrophosphate with composition D 0.1Sn 0.9P 2O 7 − 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 SnP 2O 7 was found to be significantly higher than undoped SnP 2O 7 for all the dopants. Proton transference number measurements indicated that the charge transport in acceptor doped SnP 2O 7 was mostly protonic. Within the dopants analyzed, Zn 0.1Sn 0.9P 2O 7 − 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 SnP 2O 7 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 SnP 2O 7 previously reported by other research groups.