Preparation-Dependent Composition and O/F Ordering in NbO2F and TaO2F

Through an analysis combining powder XRD, TGA, and 19F and 1H solid-state NMR, it is confirmed for NbO2F and shown for TaO2F that both contain hydroxyl defects and metal vacancies when prepared by aqueous solution synthesis. The formulations M1–x□xO2–5x(OH,F)1+5x of both the samples are determined. The effects of the usually applied thermal treatments are examined. Obtaining pure NbO2F and TaO2F from these samples, that is, fully removing metal vacancies and hydroxide, while avoiding the formation of M2O5, is not that easy. Since thermal treatments result in dehydroxylation and defluorination, it requires, at least, a larger amount of fluorine than metal initially, which may not be the case. We also confirm that the solid-state synthesis is an efficient method to avoid metal vacancies and hydroxyl defects in NbO2F and then apply it to the synthesis of TaO2F. The local structure of NbO2F and TaO2F is poorly described by an ideal cubic ReO3-type model with O and F randomly distributed over the available ani...