On the mechanism of proton conductivity of polyantimonic acid

The study is devoted to the specification of the proton conductivity mechanism of the polyantimonic acid (PAA) H$_2$Sb$_2$O$_6\cdot n$H$_2$O, $2<n<5$, crystallized within the pyrochlore-type structure (sp. gr. Fd3m). In the introduction it is noted that the core of the structure is formed by [Sb$_2$O$_6$]$^{2-}$-octahedra, connected by apexes, and the hexagonal channels, containing H$_3$O$^+$-ions and H$_2$O molecules. H- and Cd-forms of PAA were chosen as the objects of the research. It was established that cadmium ions are located in hexagonal channels of the structure with the help of experimental methods: X-ray diffraction, scanning electron microscopy and the elemental microanalysis. The composition of the samples was determined by the thermogravimetry: it was shown that the sample of the Cd-form of PAA has a greater amount of adsorbed water compared to the H-form, but it does not contain H$_3$O$^+$-ions. Using the method of the impedance spectroscopy, it was found that the proton conductivity of the Cd-form of PAA is less by more than two orders of the H-form of PAA, which contradicts the theory of globular hydrates. A mechanism of the transport of protons in the H-form of PAA is proposed: a correlated transport of protons along the particular hydrogen bonds chain, formed by proton-containing groups, located in hexagonal channels within the structure, by oxygen anions of [Sb$_2$O$_6$]$^{2-}$-octahedra and molecules of adsorbed water.