Location of H+ sites in the fast proton-conductor (H3O)SbTeO6 pyrochlore

Abstract

The defect pyrochlore (H₃O)SbTeO₆ oxide is an excellent proton conductor, showing a conductivity value of 10⁻¹ S cm⁻¹ at 30 °C under saturated water vapor partial pressure. It can be prepared by ion exchange from KTeSbO₆ pyrochlore in sulfuric acid at 453 K for 12 h. The full characterization of the structure of the (H₃O)SbTeO₆ pyrochlore, including the location of the H₃O⁺ units within the three-dimensional framework, has been carried out by neutron powder diffraction. A first Rietveld refinement of the [SbTeO₆]⁻ framework was performed in the Fdm space group (a = 10.1510(1) Å); a difference Fourier map enabled the unambiguous location of the O2 atoms from the H₃O⁺ ions at 32e (x,x,x) positions, and subsequently the H atoms at 96g (x,x,z). The (H₃O)SbTeO₆ crystal structure is constituted by a network of randomly distributed Sb^VO₆ and Te^VIO₆ octahedra linked by their corners with (Sb,Te)–O1–(Sb,Te) angles of 136.2°. Hydronium ions are located off-center around the large 8a cages of the pyrochlore. The geometry of the (O2)–H₃ units is that of an almost regular tetrahedron, with O2 atoms at the center and the three H atoms in three of the vertices; the fourth vertex is supposed to be occupied by the O2 lone pair. The three O2–H bonds have equal distances of 1.020(8) Å. The H₃O⁺ units are linked to the O1 framework oxygens by weaker hydrogen bonds, with O1–H bond lengths of 1.649(7) Å. The relatively large thermal factors of O2 and H, of 2.5 and 3.7 Ų, respectively, suggest that both kinds of atoms are not static at fixed positions but could be dynamically fluctuating between crystallographically equivalent sites.