Preparation-Dependent Composition and O/F Ordering in NbO 2 F and TaO 2 F.

Through an analysis combining powder XRD, TGA, and19 F and1 H solid-state NMR, it is confirmed for NbO2 F and shown for TaO2 F that both contain hydroxyl defects and metal vacancies when prepared by aqueous solution synthesis. The formulations M1-x □x O2-5x (OH,F)1+5x of both the samples are determined. The effects of the usually applied thermal treatments are examined. Obtaining pure NbO2 F and TaO2 F from these samples, that is, fully removing metal vacancies and hydroxide, while avoiding the formation of M2 O5 , 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 NbO2 F and then apply it to the synthesis of TaO2 F. The local structure of NbO2 F and TaO2 F is poorly described by an ideal cubic ReO3 -type model with O and F randomly distributed over the available anion sites. Since O/F ordering was previously highlighted, NbO2 F and TaO2 F 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 MX6 octahedra as expected in disordered compounds. Moreover, the structural modeling of NbO2 F and TaO2 F by these geometry-optimized supercells is supported by the computed19 F and93 Nb NMR parameters, which give very good agreement with the experimental ones.