Surface diffusion, migration, and conjugated processes at heterophase interfaces between WO3 and MeWO4 (Me = Ca, Sr, Ba)

Abstract

With the aid of a complex of methods it is demonstrated that at heterophase interfaces between WO3 and MeWO4 (Me = Ca, Sr, Ba) there occurs penetration of components WO3 and MeWO4 into one another under spontaneous conditions and after the imposition of an electric field. Experimental data concerning the electrosurface migration in potentiostatic and galvanostatic regimes are compared. It is demonstrated that the amount of WO3 transported onto inner surface of MeWO4 is defined by the magnitude of the electric charges passed through the system but does not depend on the I–U parameters of experiment. It is established that the magnitude of the faradaic efficiency of the WO3 transport in an electric field at 900°C is close for all compounds of the type MeWO4 (Me = Ca, Sr, Ba) and amounts to 0.42 ± 0.02 for a galvanostatic regime of the process. Methods of x-ray diffraction analysis, x-ray-fluorescence analysis, XPS, and electron microscopy are employed to explore the properties and compositions of regions adjacent to the WO3|MeWO4 interface after experiments in spontaneous and field-induced regimes. Data are obtained that confirm the reality of formation of nonautonomous phase MeW-s and its crucial role in the origin and mechanism of processes that occur at the heterophase interface WO3|MeWO4. The real architecture of the interface may be portrayed by the scheme WO3⋮MeW-s|MeW-s⋮MeWO4, which reflects penetration of MeW-s into both initial briquettes. The reasons for the loss of weight of briquettes of MeWO4 when annealed in contact with WO3 under spontaneous conditions are analyzed. It is shown that the weight loss may be caused by congruent sublimation of the MeW-s phase, which is directly connected with its low surface energy and relatively low sublimation energy.