title: Preparation-Dependent Composition and O/F Ordering in NbO2F and TaO2F.
abstract: Through an analysis combining powder XRD, TGA, and19F and1H 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+5xof 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 anion sites. Since O/F ordering was previously highlighted, NbO2F and TaO2F 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 MX6octahedra as expected in disordered compounds. Moreover, the structural modeling of NbO2F and TaO2F by these geometry-optimized supercells is supported by the computed19F and93Nb NMR parameters, which give very good agreement with the experimental ones.
authors.affiliation: Université Bretagne Loire, Université du Maine , UMR CNRS 6283, Institut des Molécules et des Matériaux du Mans (IMMM), Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Jamal
authors.initials: J
authors.lastname: Dabachi
authors.affiliation: Université Bretagne Loire, Université du Maine , UMR CNRS 6283, Institut des Molécules et des Matériaux du Mans (IMMM), Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Monique
authors.identifier: http://orcid.org/0000-0002-5895-3731
authors.initials: M
authors.lastname: Body
authors.affiliation: Université Bretagne Loire, Université du Maine , UMR CNRS 6283, Institut des Molécules et des Matériaux du Mans (IMMM), Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Cyrille
authors.initials: C
authors.lastname: Galven
authors.affiliation: Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS , 2 Rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France.
authors.forename: Florent
authors.initials: F
authors.lastname: Boucher
authors.affiliation: Université Bretagne Loire, Université du Maine , UMR CNRS 6283, Institut des Molécules et des Matériaux du Mans (IMMM), Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Christophe
authors.identifier: http://orcid.org/0000-0001-7426-8817
authors.initials: C
authors.lastname: Legein