Sm6-xMoO12-δ (x=0, 0.5) and Sm6WO12 - Mixed electron-proton conducting materials

Samarium molybdates Sm6-xMoO12-delta (x = 0, 0.5) and samarium tungstate Sm6WO12 - 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 Sm2O3-MoO3 system at 1600 degrees C. The samarium molybdate Sm6MoO12 has the fluorite structure (Fm 3m). The less samarium rich solid solution Sm5.5MoO11.25 crystallizes in a rhombohedral structure. The morphotropic transformation is due to the change in the chemical composition of the solid solution with decreasing Sm3+ concentration. The total conductivity of the cubic fluorite phase Sm6MoO12 at 750 degrees C in air (1.48 x 10(-3) S/cm, E-a = 1.22 eV) is an order of magnitude higher than that of rhombohedral Sm5.5MoO11.25 (234 x 10(-4) S/cm, E-a = 1.11 eV). At low temperatures (T < 500 degrees C), the Arrhenius plot of total conductivity for Sm6MoO12 and Sm5.5MoO11.25 in air deviates from linearity, suggesting that there is a proton contribution to its conductivity at these temperatures, like in the case of the Sm5.4Zr0.6MoO12.3 zirconium-doped molybdate. Below similar to 500 degrees C, Sm6MoO12 fluorite and fluorite-like Sm6WO12 have identical Arrhenius plots of conductivity in ambient air. The region of dominant proton conductivity is wider for Sm6WO12 than Sm6MoO12, reaching temperatures as high as 750 degrees C for the former. The absolute values of total conductivity obtained for samarium tungstate and molybdate at 400 degrees C in wet air are virtually identical and close to 3 x 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. (C) 2017 Elsevier B.V. All rights reserved.