Structural Modeling of Disordered Inorganic Oxyfluorides by Coupling Solid State NMR and DFT
Résumé
Tantalum and niobium oxyfluorides are part of the heteroanionic inorganic compounds that present interesting physical proprieties. In order to interpret and enhance such properties, a detailed structural modeling of the structure of this type of compounds is required. Structural modeling of inorganic disordered oxyfluorides is more challenging, since their structure is majoritively chemically disordered, precisely, anionic disorder, that is difficult to characterize using conventional diffraction techniques, as O and F atoms are indistinguishable due to their quasi-similar scattering factors. The average structure given by such techniques stays inadequate to describe disorder in such type of solids.
On the other hand, solid-state NMR spectroscopy (ss-NMR) sensitivity to short-range environmental effects exerted on probed nuclei, coupled DFT calculations bring forth accurate structural solutions that were unfeasible before determining theoretical NMR parameters using the gauge including projector augmented wave (GIPAW) approach, is crucial to make accurate assignments of the experimental NMR lines, also to provide profound structural insights.
In this study, the structure of NbOF3 and TaOF3 was precisely modeled, combining X-ray powder diffraction, 19F and 1H MAS ss-NMR spectroscopy and DFT calculations. The results suggests unambiguously the existence of one-dimensional strings of correlated O/F disorder in the equatorial crystallographic site along <100> and <010> directions for both compounds. The bridging site is distributed with equal anionic occupancies. Whereas, the non-bridging site is totally occupied by F. Using 2x2x1 supercells, DFT calculations suggests the possibility of ten unique models for each compound.