Synthesis and electrical conductivity of Sn_1−x(Ni_2/3Ta_1/3)_xP₂O₇ electrolytes for low-temperature solid oxide fuel cells

In this study, co-substitution of Ni and Ta was performed on SnP2O7, and their crystal structure and electrical conductivity were investigated. X-ray powder diffraction patterns showed that the compounds had a cubic structure identical to that of SnP2O7, but a secondary phase containing SnO2 and Ni(PO3)2 was observed for x = 0.20. Rietveld analysis was used to refine the crystal structures, revealing that the lattice parameters increased with an increase in x, up to 0.15, and then reached saturation for x = 0.20. The increase in lattice parameter is considered contradictory, but it is suggested that it may be related to the introduction of oxygen vacancies caused by the substitution of lower valence cations, resulting in an increase in P–O bond length. As a result, the co-substitution of Ni and Ta for Sn increases the interstitial proton concentration in the lattice and the electrical conductivity of the compounds was effectively improved when increasing the value of composition x; the highest electrical conductivity of 1.7 × 10−2 S cm−1 at 250 °C exhibited for Sn0.85(Ni2/3Ta1/3)0.15P2O7.