(Oxo)(Fluoro)–Aluminates in KF–Al₂O₃ System: Thermal Stability and Structural Correlation

Precise investigation of part of the phase diagram of KF–Al₂O₃ system was performed in an experiment combining different techniques. Solidified mixtures of KF–Al₂O₃ were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO₂, K₂Al₂₂O₃₄, α-K₃AlF₆, KAlF₄, and K₂Al₂O₃F₂. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF–Al₂O₃ with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K₂Al₂O₃F₂

Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X‑ray Diffraction, and Density Functional Theory Calculations

The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K₅Sc₃F₁₄ was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF₄, Li₃ScF₆, KSc₂F₇, and Na₃ScF₆ compounds were studied in detail from solid-state ¹⁹F and ⁴⁵Sc NMR experiments. The ⁴⁵Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The ¹⁹F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the ¹⁹F and ⁴⁵Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (<i>CASTEP</i>), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state ⁴⁵Sc NMR spectroscopy

Solid-State NMR of the Family of Positive Electrode Materials Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ for Lithium-Ion Batteries

The possibilities offered by ex situ and in situ operando ⁷Li solid-state nuclear magnetic resonance (NMR) are explored for the Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ family (0 < <i>y</i> < 1), shown previously to display cationic and anionic redox activity when used as a positive electrode for Li ion batteries. Ex situ NMR spectroscopic studies indicate a nonrandom Sn/Ru substitution in the family. In the first charge, an increased metallicity at 4 V is deduced from the NMR spectra. Surprisingly, no striking difference is observed at 4.6 V compared to the pristine electrode, although the electronic structure is expected to be very different and the local cation environment to be distorted. For in situ operando measurements, we designed a new electrochemical cell that is compatible with NMR spectroscopy and one-dimensional magnetic resonance imaging (MRI). These operando measurements validate the ex situ observations and indicate that the environment formed at 4 V is specific of the initial charge and that there is little, if no, electrolyte decomposition, even at 4.6 V. This is another attractive feature of these compounds

Solid-State NMR of the Family of Positive Electrode Materials Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ for Lithium-Ion Batteries

The possibilities offered by ex situ and in situ operando ⁷Li solid-state nuclear magnetic resonance (NMR) are explored for the Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ family (0 < <i>y</i> < 1), shown previously to display cationic and anionic redox activity when used as a positive electrode for Li ion batteries. Ex situ NMR spectroscopic studies indicate a nonrandom Sn/Ru substitution in the family. In the first charge, an increased metallicity at 4 V is deduced from the NMR spectra. Surprisingly, no striking difference is observed at 4.6 V compared to the pristine electrode, although the electronic structure is expected to be very different and the local cation environment to be distorted. For in situ operando measurements, we designed a new electrochemical cell that is compatible with NMR spectroscopy and one-dimensional magnetic resonance imaging (MRI). These operando measurements validate the ex situ observations and indicate that the environment formed at 4 V is specific of the initial charge and that there is little, if no, electrolyte decomposition, even at 4.6 V. This is another attractive feature of these compounds

Synthesis and Structure Resolution of RbLaF₄

The synthesis and structure resolution of RbLaF₄ are described. RbLaF₄ is synthesized by solid-state reaction between RbF and LaF₃ at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (<i>Pnma,</i> <i>a</i> = 6.46281(2) Å, <i>b</i> = 3.86498(1) Å, <i>c</i> = 16.17629(4) Å, <i>Z</i> = 4). One-dimensional ⁸⁷Rb, ¹³⁹La, and ¹⁹F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the ¹⁹F resonances is performed on the basis of both ¹⁹F–¹³⁹La <i>J</i>-coupling multiplet patterns observed in a heteronuclear DQ-filtered <i>J</i>-resolved spectrum and ¹⁹F–⁸⁷Rb HMQC MAS experiments. DFT calculations of both the ¹⁹F isotropic chemical shieldings and the ⁸⁷Rb, ¹³⁹La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF₄ and luminescence properties of Eu-doped LaRbF₄ are investigated

Synthesis and Structure Resolution of RbLaF₄

The synthesis and structure resolution of RbLaF₄ are described. RbLaF₄ is synthesized by solid-state reaction between RbF and LaF₃ at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (<i>Pnma,</i> <i>a</i> = 6.46281(2) Å, <i>b</i> = 3.86498(1) Å, <i>c</i> = 16.17629(4) Å, <i>Z</i> = 4). One-dimensional ⁸⁷Rb, ¹³⁹La, and ¹⁹F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the ¹⁹F resonances is performed on the basis of both ¹⁹F–¹³⁹La <i>J</i>-coupling multiplet patterns observed in a heteronuclear DQ-filtered <i>J</i>-resolved spectrum and ¹⁹F–⁸⁷Rb HMQC MAS experiments. DFT calculations of both the ¹⁹F isotropic chemical shieldings and the ⁸⁷Rb, ¹³⁹La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF₄ and luminescence properties of Eu-doped LaRbF₄ are investigated