Processing-induced secondary phase formation in Mo-substituted lanthanum tungstate membranes

The compositional homogeneity of a technically relevant hydrogen separation membrane, La_5.4W_0.8Mo_0.2O_12-δ (LWO-Mo₂₀), was studied using comprehensive transmission electron microscopy (TEM) techniques. The membrane is predominantly composed of dense LWO-Mo₂₀ grains with a defect fluorite structure. In addition to the primary phase, the observed secondary phase (SP) grains were identified as La_2/3(Mg_1/2W_1/2)O₃, with the W sites partially occupied by Mo, Fe and Al. Part of the SP grains were incorporated into single LWO-Mo₂₀ grains through smart orientations, in which massive structural defects at the interface of the LWO-Mo₂₀ and SP grains are efficiently avoided. Slight elemental disorder is limited within a few atomic layers. In contrast, the LWO-Mo₂₀ grains share barely common features with neighboring SP grains, and are unstable under electron beam irradiation. The formation of the SP was tracked back to the traces of impurities in the precursors. Excluding such impurities is technically challenging and unacceptable in terms of cost. Hence, our results here show an opportunity to remedy these impurities through engineering the SP into individual primary grains, in which even a significant cost reduction could thus be realized.