Multimodal Structure Solution with 19F NMR Crystallography of Spin Singlet Molybdenum Oxyfluorides

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.

Polymorph Identification for Flexible Molecules: Linear Regression Analysis of Experimental and Calculated Solution- and Solid-State NMR Data

The Δδ regression approach of Blade et al. [ J. Phys. Chem. A 2020, 124(43), 8959-8977] for accurately discriminating between solid forms using a combination of experimental solution- and solid-state NMR data with density functional theory (DFT) calculation is here extended to molecules with multiple conformational degrees of freedom, using furosemide polymorphs as an exemplar. As before, the differences in measured 1H and 13C chemical shifts between solution-state NMR and solid-state magic-angle spinning (MAS) NMR (Δδexperimental) are compared to those determined by gauge-including projector augmented wave (GIPAW) calculations (Δδcalculated) by regression analysis and a t-test, allowing the correct furosemide polymorph to be precisely identified. Monte Carlo random sampling is used to calculate solution-state NMR chemical shifts, reducing computation times by avoiding the need to systematically sample the multidimensional conformational landscape that furosemide occupies in solution. The solvent conditions should be chosen to match the molecule's charge state between the solution and solid states. The Δδ regression approach indicates whether or not correlations between Δδexperimental and Δδcalculated are statistically significant; the approach is differently sensitive to the popular root mean squared error (RMSE) method, being shown to exhibit a much greater dynamic range. An alternative method for estimating solution-state NMR chemical shifts by approximating the measured solution-state dynamic 3D behavior with an ensemble of 54 furosemide crystal structures (polymorphs and cocrystals) from the Cambridge Structural Database (CSD) was also successful in this case, suggesting new avenues for this method that may overcome its current dependency on the prior determination of solution dynamic 3D structures.

A new high-pressure polymorph of K2MoO2F4

In this paper, a new high-pressure (HP) polymorph of the otherwise known oxyfluoride K2MoO2F4 is presented. The crystal structure was determined by use of single-crystal X-ray diffractometry and its features are described in detail herein. HP-K2MoO2F4 crystallizes in the monoclinic space group C2/m (no. 12) with the cell parameters a = 13.8579(5), b = 5.8109(2), c = 6.9442(3) Å, β = 90.36(1)°, V = 559.18(4) Å3, and Z = 4 at T = 301(2) K. Bond valence (BV) and charge distribution (CHARDI) calculations were carried out to support the assignment of oxygen and fluorine to the various anion positions and Madelung part of lattice energy (MAPLE) calculations were used to validate the structure model. Infrared spectroscopy provided further information on the structure and water content of the inseparable side phase.

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.

Wide Bandgaps and Strong SHG Responses of Hetero-Oxyfluorides by Dual-Fluorination-Directed Bandgap Engineering

A wide bandgap is an essential requirement for a nonlinear optical (NLO) material. However, it is very challenging to simultaneously engineer a wide bandgap and a strong second-harmonic generation (SHG) response, particularly in NLO materials containing second-order Jahn–Teller (SOJT) distorted units. Herein, we employ a bandgap engineering strategy that involves the dual fluorination of two different types of SOJT distorted units to realize remarkably wide bandgaps in the first examples of 5d⁰-transition metal (TM) fluoroiodates. Crystalline A₂WO₂F₃(IO₂F₂) (A = Rb (RWOFI) and Cs (CWOFI)) exhibit the largest bandgaps yet observed in d⁰-TM iodates (4.42 (RWOFI) and 4.29 eV (CWOFI)), strong phase-matching SHG responses of 3.8 (RWOFI) and 3.5 (CWOFI) × KH₂PO₄, and wide optical transparency windows. Computational studies have shown that the excellent optical responses result from synergism involving the two fluorinated SOJT distorted units ([WO₃F₃]³⁻ and [IO₂F₂]⁻). This work provides not only an efficient strategy for bandgap modulation of NLO materials, but also affords insight into the relationship between the electronic structure of the various fluorinated SOJT distorted units and the optical properties of crystalline materials.

Wide bandgaps, strong SHG responses, and sufficient birefringence are observed in the first examples of 5d⁰-transition metal fluoroiodates, A₂WO₂F₃(IO₂F₂) (A = Rb, and Cs), which were constructed by dual-fluorination-directed bandgap engineering.

Perovskite-like K3TiOF5 Exhibits (3 + 1)-Dimensional Commensurate Structure Induced by Octahedrally Coordinated Potassium Ions

Elpasolite- and cryolite-type oxyfluorides can be regarded as superstructures of perovskite and exhibit structural diversity. While maintaining a similar structural topology with the prototype structures, changes in the size, electronegativity, and charge of cation and/or anion inevitably lead to structural evolution. Therefore, the nominal one-to-one relation suggested by a doubled formula of perovskite does not guarantee a simple 2-fold superstructure for many cases. Herein, the commensurately modulated perovskite-like K₃TiOF₅ was refined at 100 K from single-crystal X-ray diffraction data by using a pseudotetragonal subcell with lattice parameters of a = b = 6.066(2) Å and c = 8.628(2) Å. The length of the modulation vector was refined to 0.3a* + 0.1b* + 0.25c*. In the commensurate supercell of K₃TiOF₅, the B-site Ti⁴⁺ and K⁺ cations in [TiOF₅]^3- and [KOF₅]^6- octahedral units were found to be significantly displaced from the average atomic positions refined in the subcell. The displacements of the K⁺ cations are ±0.76 Å, and those for the Ti⁴⁺ cations are approximately ±0.13 Å. One- and two-dimensional solid-state ¹⁹F NMR measurements revealed two tightly clustered groups of resonances in a ratio of ca. 4:1, assigned to equatorial and axial fluorine, respectively, consistent with local [TiOF₅]^3- units. S/TEM results confirmed the average structure. Electronic structure calculations of the idealized I4mm subcell indicate the instability to a modulated structure arises from soft optical modes that is controlled by the octahedrally coordinated B-site potassium ions in the cryolite-type structure.

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.

Topotactic desolvation and condensation reactions of 3D Zn3TiF7(H2O)2(taz)3·S (S = 3H2O or C2H5OH)

Thermodiffraction, IR, DFT calculations, and ¹H and ¹⁹F NMR characterizations of the desolvatation and reversible condensation reactions of Zn₃TiF₇(taz)₃ family.