Abstract
Lanthanum tungstate (LWO) and LWO with 20 at.% and 35 at.% molybdenum substituting tungsten were prepared by the Pechini method. Phase purity and successful Mo substitution inside these dense LWO membrane materials were confirmed by conventional and high resolution transmission electron microscopy techniques. The split of La2/W2 site by around 0.3 Å was proven. Extra reflections show up in the diffraction patterns from Mo-substituted LWO, and together with simulations, these reflections were recognized as forbidden reflections in a non-substituted LWO system, while the extinction rules are broken by Mo substitution due to the different scattering factors of W and Mo. Energy-dispersive X-ray chemical mapping allowed direct visualization of individual atomic columns, and revealed that all Mo is located at the W1 sites in the Mo-substituted LWO. Moreover, the diffuse scattering in diffraction patterns provides direct evidence of short range clustering of oxygen vacancies and could be further related to the oxygen conduction of the LWO membranes.
Highlights
Mixed proton-electron conductors are promising gas separation membranes and have attracted great research interests[1,2,3,4,5]
In spite of extensive studies on the conducting properties of LWO or Mo/Re-substituted LWO and several reports determining their structural models by neutron and X-ray diffraction (XRD)[14,15,19,25,26], only a few attempts were made to characterize the mixed conductors using transmission electron microscopy (TEM) with atomic resolution
The as-sintered pellets of the three types were first analyzed by XRD, in which all of them were proven to be phase pure within the experimental detection limit (Supplementary Fig. S1)
Summary
Mixed proton-electron conductors are promising gas separation membranes and have attracted great research interests[1,2,3,4,5]. The LWO holds appreciable mixed proton and electron conductivity, thermal and hydrothermal stability as well as chemical stability in reducing and water-containing atmospheres at elevated temperatures. It has been pointed out that single-phase LWO can be prepared at 1500 °C with a La/W ratio in the window between 5.3 and 5.716,17 Outside this compositional range, segregation of either La2O3 or La6W2O15 was found. In spite of extensive studies on the conducting properties of LWO or Mo/Re-substituted LWO and several reports determining their structural models by neutron and X-ray diffraction (XRD)[14,15,19,25,26], only a few attempts were made to characterize the mixed conductors using transmission electron microscopy (TEM) with atomic resolution. Combining electron diffraction and high resolution TEM, nanodomains with an incipient superstructure were reported in La28−y(W0.6Mo0.4)4+yO54+δ, and when the Mo content goes higher, a new phase with rhombohedral structure was proposed[15,20]
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