Sm 6-x MoO 12-δ (x = 0, 0.5) and Sm 6 WO 12 – Mixed electron-proton conducting materials

Samarium molybdates Sm 6-x MoO 12-δ (x = 0, 0.5) and samarium tungstate Sm 6 WO 12 – potential mixed electron-proton conductors have been studied by X-ray diffraction, Raman spectroscopy, SEM and impedance spectroscopy (in ambient air and in dry and wet air). Solid solutions differing in structure have been obtained in the Sm 2 O 3 -MoO 3 system at 1600 °C. The samarium molybdate Sm 6 MoO 12 has the fluorite structure ( Fm¯3m ). The less samarium rich solid solution Sm 5.5 MoO 11.25 crystallizes in a rhombohedral ( R 3 ¯ ) structure. The morphotropic transformation is due to the change in the chemical composition of the solid solution with decreasing Sm 3 + concentration. The total conductivity of the cubic fluorite phase Sm 6 MoO 12 at 750 °C in air (1.48 × 10 − 3 S/cm, E a = 1.22 eV) is an order of magnitude higher than that of rhombohedral Sm 5.5 MoO 11.25 (2.34 × 10 − 4 S/cm, E a = 1.11 eV). At low temperatures (T 6 MoO 12 and Sm 5.5 MoO 11.25 in air deviates from linearity, suggesting that there is a proton contribution to its conductivity at these temperatures, like in the case of the Sm 5.4 Zr 0.6 MoO 12.3 zirconium-doped molybdate. Below ~ 500 °C, Sm 6 MoO 12 fluorite and fluorite-like Sm 6 WO 12 have identical Arrhenius plots of conductivity in ambient air. The region of dominant proton conductivity is wider for Sm 6 WO 12 than Sm 6 MoO 12 , reaching temperatures as high as 750 °С for the former. The absolute values of total conductivity obtained for samarium tungstate and molybdate at 400 °С in wet air are virtually identical and close to 3 × 10 − 6 S/cm, which suggests the conductivity of both compounds is dominated by protons at low temperatures and the proton transport numbers are similar.