Structural Modeling of O/F Correlated Disorder in TaOF and NbOF(OH) by Coupling Solid-State NMR and DFT Calculations.

The structure of MOF (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 (MOF) layers along the direction formed by heteroleptic corner-connected MX (X = O, F) octahedra. F 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 -MOF 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 F isotropic chemical shift values were determined. The agreement between the experimental and calculated F spectra is excellent for TaOF. The H and F experimental NMR spectra revealed that some of the bridging F atoms are substituted by OH groups, especially in NbOF. 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 F NMR spectra, i.e., NbOF(OH).