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

Abstract

The structure of MOF 3 (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 3 ) layers along the c⃗ direction formed by heteroleptic cornerconnected MX 6 (X = O, F) octahedra. 19 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 cis-MO 2 F 4 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 19 F isotropic chemical shift values were determined. The agreement between the experimental and calculated 19 F spectra is excellent for TaOF 3 . The 1 H and 19 F experimental NMR spectra revealed that some of the bridging F atoms are substituted by OH groups, especially in NbOF 3 . 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 19 F NMR spectra, i.e., NbOF 2.85 (OH) 0.15 .

Correlated Anion Disorder in Heteroanionic Cubic TiOF2

“…The preference for polar cis -Ti-[O 2 F 4 ] over nonpolar trans -Ti[O 2 F 4 ] coordination in ReO 3 -type TiOF 2 , as predicted here, echoes the preference for polar cation coordination reported in other transition-metal oxyfluorides and oxynitrides. ,,,− In these materials, the preference for cis - versus trans -MX 2 Y 4 or fac - versus mer -MX 3 Y 3 coordination has been attributed to the property of a polar configuration of anions: the central cation can move off-center, resulting in shorter M–O distances and stronger net cation–anion bonding. Our results for ReO 3 -type TiOF 2 are consistent with this modelthe lowest energy structures from our calculations, which are composed entirely of cis -Ti-[O 2 F 4 ] units, have short Ti–O bonds (∼1.82 Å) and longer Ti–F bonds (∼1.99 Å), in agreement with our assignment of the split first peak in our experimental short-range PDF data (Figure ).…”

“…One approach for the quantitative prediction of 19 F δ iso values for a disordered system containing one metal cation is to derive an appropriate linear transformation function from data for a reference ordered (oxy)fluoride containing the same cation as the system of interest and with fluoride ions occupying multiple crystallographic sites. This approach has previously been used to simulate the 19 F NMR spectra of the disordered oxyfluorides MO 2 F 26 and MOF 3 95 (M = Nb, Ta), with transformation functions derived by fitting to data for the corresponding MF 5 fluorides. 74 For obtaining a transformation function σ iso →δ iso for titanium (oxy)fluorides, a reasonable reference system is TiF 4 .…”