Key trends in the proton conductivity of Ln(6-x)MoO(12-delta) (Ln = La, Nd, Sm, Gd -Yb; x=0, 0.5, 0.6, 0.7, 1) rare-earth molybdates

We have studied the structure and proton conductivity of rhombohedral La6-xMoO12-delta (x = 0.5, 0.6, 0.7, 1) lanthanum molybdates prepared via mechanical activation of lanthanum and molybdenum oxides, followed by thermal annealing at 1650 degrees C. The La6-xMoO12-delta (x = 0.5, 0.6) materials were phase-pure and had a complex rhombohedral structure (R1). An increase in the molybdenum concentration leads to a decrease in the degree of rhombohedral distortion and proton conductivity in the La6-xMoO12-delta (x = 0.5, 0.6, 0.7, 1) series. The proton conductivity at the optimal composition La6-xMoO12-delta (x = 0.5) is similar to 4.0 x 10(-5) S/cm at 500 degrees C in wet air. A comparative analysis shows that, in the Ln(6-x)MoO(12-delta) (Ln = La, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb; x = 0-1) series, proton conductivity decreases with the Ln ionic radii decreasing regardless of the structural type. Because of this, the high proton conductivity is demostrated by the stable La6-xMoO12-delta (x = 0.5, 0.6) materials, with an inherently deficient oxygen sublattice, which crystallize in a large-volume, complex rhombohedral cell (R1). (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.