Preparation-Dependent Composition and O/F Ordering in NbOF and TaOF.

Through an analysis combining powder XRD, TGA, and F and H solid-state NMR, it is confirmed for NbOF and shown for TaOF that both contain hydroxyl defects and metal vacancies when prepared by aqueous solution synthesis. The formulations M□O(OH,F) of both the samples are determined. The effects of the usually applied thermal treatments are examined. Obtaining pure NbOF and TaOF from these samples, that is, fully removing metal vacancies and hydroxide, while avoiding the formation of MO, 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 NbOF and then apply it to the synthesis of TaOF. The local structure of NbOF and TaOF is poorly described by an ideal cubic ReO-type model with O and F randomly distributed over the available anion sites. Since O/F ordering was previously highlighted, NbOF and TaOF cubic 3 × 3 × 3 supercells featuring -M-O-M-O-M-F- chains along ⟨100⟩ have been built and geometry optimized. These optimized supercells lead to more realistic structures than the previously proposed models, that is, really disordered structures with angularly and radially distorted MX octahedra as expected in disordered compounds. Moreover, the structural modeling of NbOF and TaOF by these geometry-optimized supercells is supported by the computed F and Nb NMR parameters, which give very good agreement with the experimental ones.