NbF5 and TaF5: Assignment of 19F NMR resonances and chemical bond analysis from GIPAW calculations

Correlated Anion Disorder in Heteroanionic Cubic TiOF2

Resolving anion configurations in heteroanionic materials is crucial for understanding and controlling their properties. For anion-disordered oxyfluorides, conventional Bragg diffraction cannot fully resolve the anionic structure, necessitating alternative structure determination methods. We have investigated the anionic structure of anion-disordered cubic (ReO3-type) TiOF2 using X-ray pair distribution function (PDF), 19F MAS NMR analysis, density functional theory (DFT), cluster expansion modeling, and genetic-algorithm structure prediction. Our computational data predict short-range anion ordering in TiOF2, characterized by predominant cis-[O2F4] titanium coordination, resulting in correlated anion disorder at longer ranges. To validate our predictions, we generated partially disordered supercells using genetic-algorithm structure prediction and computed simulated X-ray PDF data and 19F MAS NMR spectra, which we compared directly to experimental data. To construct our simulated 19F NMR spectra, we derived new transformation functions for mapping calculated magnetic shieldings to predicted magnetic chemical shifts in titanium (oxy)fluorides, obtained by fitting DFT-calculated magnetic shieldings to previously published experimental chemical shift data for TiF4. We find good agreement between our simulated and experimental data, which supports our computationally predicted structural model and demonstrates the effectiveness of complementary experimental and computational techniques in resolving anionic structure in anion-disordered oxyfluorides. From additional DFT calculations, we predict that increasing anion disorder makes lithium intercalation more favorable by, on average, up to 2 eV, highlighting the significant effect of variations in short-range order on the intercalation properties of anion-disordered materials.

Structural Modeling of O/F Correlated Disorder in TaOF3 and NbOF3-x(OH)x by Coupling Solid-State NMR and DFT Calculations

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., NbOF2.85(OH)0.15.

Dramatic Enhancement of Rare-Earth Metal-Organic Framework Stability Via Metal Cluster Fluorination

Rare-earth polynuclear metal-organic frameworks (RE-MOFs) have demonstrated high durability for caustic acid gas adsorption and separation based on gas adsorption to the metal clusters. The metal clusters in the RE-MOFs traditionally contain RE metals bound by μ₃-OH groups connected via organic linkers. Recent studies have suggested that these hydroxyl groups could be replaced by fluorine atoms during synthesis that includes a fluorine-containing modulator. Here, a combined modeling and experimental study was undertaken to elucidate the role of metal cluster fluorination on the thermodynamic stability, structure, and gas adsorption properties of RE-MOFs. Through systematic density-functional theory calculations, fluorinated clusters were found to be thermodynamically more stable than hydroxylated clusters by up to 8-16 kJ/mol per atom for 100% fluorination. The extent of fluorination in the metal clusters was validated through a ¹⁹F NMR characterization of 2,5-dihydroxyterepthalic acid (Y-DOBDC) MOF synthesized with a fluorine-containing modulator. ¹⁹F magic-angle spinning NMR identified two primary peaks in the isotropic chemical shift (δ_iso) spectra located at -64.2 and -69.6 ppm, matching calculated ¹⁹F NMR δ_iso peaks at -63.0 and -70.0 ppm for fluorinated systems. Calculations also indicate that fluorination of the Y-DOBDC MOF had negligible effects on the acid gas (SO₂, NO₂, H₂O) binding energies, which decreased by only ∼4 kJ/mol for the 100% fluorinated structure relative to the hydroxylated structure. Additionally, fluorination did not change the relative gas binding strengths (SO₂ > H₂O > NO₂). Therefore, for the first time the presence of fluorine in the metal clusters was found to significantly stabilize RE-MOFs without changing their acid-gas adsorption properties.

Enhanced Fluoride Uptake by Layered Double Hydroxides under Alkaline Conditions: Solid-State NMR Evidence of the Role of Surface >MgOH Sites

Layered double hydroxides (LDHs) are potential low-cost filter materials for use in fluoride removal from drinking water, but molecular-scale defluoridation mechanisms are lacking. In this research, we employed ¹⁹F solid-state NMR spectroscopy to identify fluoride sorption products on 2:1 MgAl LDH and to reveal the relationship between fluoride sorption and the LDH structure. A set of six ¹⁹F NMR peaks centered at -140, -148, -156, -163, -176, and -183 ppm was resolved. Combining quantum chemical calculations based on density function theory (DFT) and ¹⁹F{²⁷Al} transfer of populations in double resonance (TRAPDOR) analysis, we could assign the peaks at -140, -148, -156, and -163 ppm to Al-F (F coordinated to surface Al) and those at -176 and -183 ppm to Mg-F (F coordinated to surface Mg only). Interestingly, the spectroscopic data reveal that the formation of Al-F is the predominant mode of F^- sorption at low pH, whereas the formation of Mg-F is predominant at high pH (or a higher Mg/Al ratio). This finding supports the fact that the F^- uptake of 2:1 MgAl LDH was nearly six times that of activated alumina at pH 9. Overall, we explicitly revealed the different roles of the surface >MgOH and >AlOH sites of LDHs in defluoridation, which explained why the use of classic activated alumina for defluoridation is limited at high pH. The findings from this research may also provide new insights into material screening for potential filters for F^- removal under alkaline conditions.

Fluoridation of HfO2

Fluoridation of HfO₂ was carried out with three commonly used solid-state fluoridation agents: PVDF, PTFE, and NH₄HF₂. Clear and reproducible differences are observed in the reaction products of the fluoropolymer reagents and NH₄HF₂ with the latter more readily reacting in air. Strong evidence of distinct, previously unreported hafnium oxyfluoride phases is produced by both reactions, and efforts to isolate them were successful for the air-NH₄HF₂ reaction. Synchrotron XRD, ¹⁹F NMR, and elemental analysis were employed to characterize the phase-pure material which appears to be analogous to known Zr-O-F phases with anion-deficient α-UO₃ structures such as Zr₇O₉F₁₀. Comparison with the hydrolysis of β-HfF₄ under identical conditions depicts that the NH₄HF₂ route produces the oxyfluoride with greater selectivity and at lower temperatures. Thermodynamic calculations were employed to explain this result. Potential reaction pathways for the NH₄HF₂ fluoridation of HfO₂ are discussed.

Insight into the factors influencing NMR parameters in crystalline materials from the KF-YF3 binary system

Solid state NMR signals are very sensitive to the local environment of the observed nucleus; however, their interpretation is not straightforward. On the other hand, first-principles DFT calculations of NMR parameters can now be applied to periodic compounds to predict NMR parameters. Thus, ab initio calculations can help to interpret the NMR spectra exhibited by complex materials, to assign NMR lines to structural environments, and even to enlighten the environmental factors influencing the NMR parameters for a given nucleus. Both techniques have been applied to crystalline compounds of the KF-YF3 binary system, γ-K3YF6, K2YF5, KYF4, β-KY2F7 and α-KY3F10, which present a variety of YFn and KFm polyhedra. First, the structure of K2YF5 was refined in the Pnma space group and, for all compounds, atomic positions were optimized by DFT. The 19F, 89Y and 39K NMR spectra have been recorded and the measured NMR parameters are compared to those calculated from the first-principles DFT method, allowing unambiguous assignments of NMR lines to crystallographic sites. Linear correlations between the experimental δiso and calculated σiso values for the three nuclei are used to predict the theoretical 19F spectra of KYF4 (24 F sites) and β-KY2F7 (19 F sites) as well as the 39K spectrum of KYF4 (6 K sites). For 89Y and 39K, both computational and experimental results show a decrease of the isotropic chemical shift values when the cation coordination number increases. Above all, 89Y isotropic chemical shift values correlate with the number of K atoms present in the Y second coordination sphere. For 19F, the combination of isotropic chemical shift and chemical shift anisotropy allows for distinguishing four kinds of F environments.

Preparation-Dependent Composition and O/F Ordering in NbO2F and TaO2F

Through an analysis combining powder XRD, TGA, and ¹⁹F and ¹H 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 M_1-x□_xO_2-5x(OH,F)_1+5x of 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 MX₆ octahedra as expected in disordered compounds. Moreover, the structural modeling of NbO₂F and TaO₂F by these geometry-optimized supercells is supported by the computed ¹⁹F and ⁹³Nb NMR parameters, which give very good agreement with the experimental ones.

The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce1−xSrxF3−x solid solutions

Evolution with x of RT ionic conductivity of RE_1−xAE_xF_3−x showing that Ce_1−xSr_xF_3−x is the best F⁻ tysonite conductor is discussed.

Structure determination of Ba5AlF13 by coupling electron, synchrotron and neutron powder diffraction, solid-state NMR and ab initio calculations

The structure and dynamics of Ba₅AlF₁₃ are resolved by combining complementary information from powder diffraction, ²⁷Al and ¹⁹F ultra-fast MAS NMR and DFT calculations.

Structural refinement of the RT LaOF phases by coupling powder X-Ray diffraction, (19)F and (139)La solid state NMR and DFT calculations of the NMR parameters

The structures of the β- and t-LaOF phases have been refined from XRPD patterns. For both phases, (19)F and (139)La solid-state NMR spectra recorded at high magnetic fields show the presence of a single F and a single La local environment, indicating a full anionic ordering in these oxyfluoride compounds. DFT calculations of the (19)F and (139)La chemical shielding tensors and of the (139)La EFG tensor have been performed for the proposed structural models. The observed good agreement between experimental and calculated NMR parameters for both phases highlights the accuracy of the structural data.

NMR parameters in column 13 metal fluoride compounds (AlF₃, GaF₃, InF₃ and TlF) from first principle calculations

The relationship between the experimental (19)F isotropic chemical shift and the (19)F isotropic shielding calculated using the gauge including projector augmented-wave (GIPAW) method with PBE functional is investigated in the case of GaF3, InF3, TlF and several AlF3 polymorphs. It is shown that the linear correlation between experimental and DFT-PBE calculated values previously established on alkali, alkaline earth and rare earth of column 3 basic fluorides (Sadoc et al., Phys. Chem. Chem. Phys. 13 (2011) 18539-18550) remains valid in the case of column 13 metal fluorides, indicating that it allows predicting (19)F solid state NMR spectra of a broad range of crystalline fluorides with a relatively good accuracy. For the isostructural α-AlF3, GaF3 and InF3 phases, PBE-DFT geometry optimization leads to noticeably overbended M-F-M bond angles and underestimated (27)Al, (71)Ga and (115)In calculated quadrupolar coupling constants. For the studied compounds, whose structures are built of corner shared MF6 octahedra, it is shown that the electric field gradient (EFG) tensor at the cationic sites is not related to distortions of the octahedral units, in contrast to what previously observed for isolated AlF6 octahedra in fluoroaluminates.