Structure Resolution of Ba5Al3F19 and Investigation of Fluorine Ion Dynamics by Synchrotron Powder Diffraction, Variable-Temperature Solid-State NMR, and Quantum Computations

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.

Fluoride solid electrolytes: investigation of the tysonite-type solid solutions La1−xBaxF3−x (x < 0.15)

La_1−xBa_xF_3−x solid solutions for x < 0.15 were prepared by solid state synthesis in a platinum tube under an nitrogen atmosphere with subsequent quenching for 0.07 ≤ x < 0.15.

NMR crystallography: Applications to inorganic materials

Current developments of NMR crystallography as well as some recent applications to diamagnetic inorganic solids are presented. First, we illustrate how solid-state NMR data can be used in combination with diffraction data for the determination of the periodic part of the crystal structures, from the space group selection, to the structure determination over the refinement and validation processes. As ss-NMR, contrary to diffraction (powder and single-crystal), is not restricted to periodic boundary conditions, ss-NMR data can be used to further complete the structural description of materials, including studies of local order/disorder, etc. This illustrated through examples, which are shown and discussed in the second part of this review.

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.

A general protocol for determining the structures of molecularly ordered but noncrystalline silicate frameworks

A general protocol is demonstrated for determining the structures of molecularly ordered but noncrystalline solids, which combines constraints provided by X-ray diffraction (XRD), one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles quantum chemical calculations. The approach is used to determine the structure(s) of a surfactant-directed layered silicate with short-range order in two dimensions but without long-range periodicity in three-dimensions (3D). The absence of long-range 3D molecular order and corresponding indexable XRD reflections precludes determination of a space group for this layered silicate. Nevertheless, by combining structural constraints obtained from solid-state (29)Si NMR analyses, including the types and relative populations of distinct (29)Si sites, their respective (29)Si-O-(29)Si connectivities and separation distances, with unit cell parameters (though not space group symmetry) provided by XRD, a comprehensive search of candidate framework structures leads to the identification of a small number of candidate structures that are each compatible with all of the experimental data. Subsequent refinement of the candidate structures using density functional theory calculations allows their evaluation and identification of "best" framework representations, based on their respective lattice energies and quantitative comparisons between experimental and calculated (29)Si isotropic chemical shifts and (2)J((29)Si-O-(29)Si) scalar couplings. The comprehensive analysis identifies three closely related and topologically equivalent framework configurations that are in close agreement with all experimental and theoretical structural constraints. The subtle differences among such similar structural models embody the complexity of the actual framework(s), which likely contain coexisting or subtle distributions of structural order that are intrinsic to the material.

Multiple resonance heteronuclear decoupling under MAS: dramatic increase of spectral resolution at moderate magnetic field and MAS frequencies

The effects of multiple-resonance heteronuclear decoupling under magic angle spinning (MAS) on the resolution of one-dimensional (19)F and (31)P and various two-dimensional MAS NMR spectra and on the residual non-refocusable coherence lifetimes in fluorinated aluminophosphate AlPO(4)-CJ2, i.e. a compound that contains numerous highly abundant nuclei but no homonuclear spin bath, has been investigated. The design of the four-channel ((1)H, (19)F, (27)Al, (31)P) MAS probe used for this study is first described. (1)H and (1)H-(27)Al double-resonance decouplings allows lengthening the optimized transverse relaxation T(2)(opt) and increasing the resolution in the (19)F and (31)P dimensions. Under the application of multi-nuclear decoupling, a two-dimensional (19)F-(31)P CP-HETCOR correlation spectrum for AlPO(4)-CJ2 is recorded with unprecedented high-resolution in the two dimensions. Moreover, because (1)H-decoupling increases the (19)F T(2)(opt), it has been applied during the entire duration of the 2D NMR experiments, allowing the direct use of residual small interactions to generate (19)F-(19)F and (19)F-(27)Al 2D NMR correlation spectra in AlPO(4)-CJ2.

Observation of "hidden" magnesium: first-principles calculations and 25Mg solid-state NMR of enstatite

(25)Mg NMR parameters have been determined for two polymorphs of enstatite (MgSiO(3)), an important magnesium silicate phase present as a major component of the Earth's upper mantle. The crystal structures of both polymorphs contain two crystallographically distinct magnesium sites; however, only a single resonance is observed in (25)Mg MAS NMR spectra recorded at 14.1 and 20.0 T. First-principles calculations performed on geometry-optimised crystal structures reveal that the quadrupolar interaction for the second site is expected to be very large, resulting in extensive broadening of the spectral resonance, explaining its apparent absence in the NMR spectrum. (25)Mg QCPMG NMR experiments employing variable offset cumulative spectroscopy (VOCS) are used to observe the broadened site and enable measurement of NMR parameters. The large difference in quadrupolar interaction between the two crystallographic magnesium sites is rationalised qualitatively in terms of the distortion of the local coordination environment as well as longer-range effects using a simple point charge model.