Structural Modeling of O/F Correlated Disorder in TaOF₃ and NbOF_3-x(OH)_x by Coupling Solid-State NMR and DFT...

The structure of MOF₃ (M = Nb, Ta) compounds was precisely modeled by combining powder X-ray diffraction, solid-state NMR spectroscopy, and semiempirical dispersion-corrected DFT calculations. It consists of stacked _∞(MOF₃) layers along the c⃗ direction formed by heteroleptic corner-connected MX₆ (X = O, F) octahedra. ¹⁹F NMR resonance assignments and occupancy rates of the anionic crystallographic sites have been revised. The bridging site is shared equally by the anions, and the terminal site is occupied by F only. An O/F correlated disorder is expected since cis-MO₂F₄ octahedra are favored, resulting in one-dimensional -F-M-O-M- strings along the <100> and <010> directions. Ten different 2 × 2 × 1 supercells per compound, fulfilling these characteristics, were built. Using DFT calculations and the GIPAW approach, the supercells were relaxed and the ¹⁹F isotropic chemical shift values were determined. The agreement between the experimental and calculated ¹⁹F spectra is excellent for TaOF₃. The ¹H and ¹⁹F experimental NMR spectra revealed that some of the bridging F atoms are substituted by OH groups, especially in NbOF₃. New supercells involving OH groups were generated. Remarkably, the best agreement is obtained for the supercells with the composition closest to that estimated from the ¹⁹F NMR spectra, i.e., NbOF_2.85(OH)_0.15.