Conductivity of SnP2O7 and In-doped SnP2O7 prepared by an aqueous solution method

SnP2O7 and In-doped SnP2O7 have been prepared by an aqueous solution method using (NH4)2HPO4 as phosphorous source. It was found that the solid solution limit in Sn1 − xInx(P2O7)1 − δ was at least x = 0.12. All pyrophosphates in the Sn1 − xInx(P2O7)1 − δ (x ≤ 0.12) series exhibit 3 × 3 × 3 superlattice structures. The conductivities of Sn0.92In0.08(P2O7)1 − δ in air are 6.5 × 10− 6 and 8.0 × 10− 9 S/cm at 900 and 400 °C, respectively, when prepared by an aqueous solution method and annealed at 1000 °C. The conductivity of undoped SnP2O7 is slightly lower. However, it was also found that the low-temperature conductivities of pyrophosphates annealed only at 650 °C are several orders of magnitude higher than those annealed at 1000 °C, which could be related to a trace amount of an amorphous secondary phase. The peak conductivity was in this case observed at around 250 °C, which is the same temperature as previously observed in In-doped SnP2O7 although the conductivity is still three orders of magnitude lower in the present study. These differences can be related to large differences in particle size and morphology, and all in all, the conductivities of SnP2O7-based materials are very sensitive to the synthetic history.