States of H+-containing species and proton migration forms in hydrated niobates and tantalates of alkaline-earth metals with a perovskite-related structure
Abstract Hydrated niobates and tantalates of alkaline-earth metals Sr 6 Ta 2 O 11 , Sr 6 Nb 2 O 11 , and Ba 4 Ca 2 Nb 2 O 11 with a perovskite-related structure have been investigated by using wide-line 1 H nuclear magnetic resonance (NMR), thermogravimetry (TG), conductivity measurements, and IR spectroscopy. TG investigations show that these phases can incorporate up to ∼1 mol of water, which corresponds to the theoretical limit of hydration. A wide-line 1 H-NMR study indicates that the proton signal consists of three components, because protons were present in the form of (i) relatively isolated OH groups, (ii) paired OH groups bound with the same metal atom, and (iii) closely spaced OH groups belonging to different octahedra and localized within a defect cluster. The niobates and the tantalates have different ratios of those groups that may be explained by different coordination tendencies of niobium and tantalum and as a consequence a different arrangement of oxygen vacancies. A dehydration scheme may be proposed from a comparison analysis of the TG and 1 H-NMR data for partially hydrated samples.