Axis 1 : Investigative Crystal Chemistry and Modeling

Axis 1: Investigative crystal chemistry and modeling

Action 1.1: Investigative Crystal Chemistry

This central action for the MI thematic feeds the other actions. In this action as in the others, our knowledge in crystal chemistry and our skills in synthesis and crystallography are mobilized to discover and describe new compounds with potentially interesting properties. In these structural studies, electron diffraction (microscopy), solid state NMR, Mossbauer (J.-M. Grenèche, PSC) and Raman (A. Bulou, PSC) spectroscopies and even frequency doubling (D. Mounier, PSC) are used in addition to X-ray diffraction (laboratory or synchrotron) or neutron diffraction.

New fluorinated coordination polymers (CPs) with variable porosity are developed by combining an inorganic metallic entity with organic azole ligands. Several works have highlighted the key synthesis parameters influencing the class, dimensionality, composition and topology of the crystal edifices of these PCs [1–5]. If the use of monocyclic azole ring molecules, mostly commercialized, leads to PCs with moderate porosities (5 to 25% by volume), some of them develop unexpected properties, especially in magnetism and optics [6–9]. The size of the cavities could be increased by the use of original ligands with multiple tetrazole rings synthesized in collaboration with G. Dujardin of the SO thematic. These new fluorinated architectures (Fig. 1) built with Fe^2,3+ and/or Zn²⁺ cations and the H₂bdt ligand show remarkable porosities (up to 60% by volume) approaching those of reference materials (MIL-53, for example).

The work was partly devoted to the investigation and characterization of new compounds likely to be O^2- conductive because of the combination of cations with high polarizability (La) and high valence (Nb, Mo or W). The careful exploration of the La₂O₃-MoO₃ binary and La₂O₃-Nb₂O₅-(Mo,W)O₃ ternary diagrams allowed the identification and structure determination of three new phases La₃₄Mo₈O₇₅¤₉ [10], La₅NbMo₂O₁₆ ([11], Fig. 2) and La₃NbWO₁₀ [12]) whose crystal structures deriving from the fluorite CaF₂. Although, these 3 phases exhibit a modest anionic conductivity, their discovery testifies to the relevance of the reasoned approach of these investigations.

Fig. 2 Structural relation of La₅NbMo₂O₁₆ with CaF₂ structural type.

The existence of cations with two oxidation states in the octahedra of the columnar perovskite-like structure of La₇W⁶⁺₄M⁵⁺₃O₃₀ (with M= Nb and Ta) led to the idea of new compositions (La,Pr)₇W⁶⁺_7-xM^m+_xO₃₀ in which the cation M would have a valence m= 4, 3 or 2. Such compositions have been prepared and their structures determined [13,14]. The M cations occupy the octahedra in the center of each hexagonal section column (Fig. 3) inducing a distortion of the octahedra [WO₆] at the column periphery. This cationic distribution and distortion are comparable to those found in the octahedra inside and at the periphery of the sheets of the A_nB_nO³⁺_2n oxide series (Fig. 3). It has been proposed to describe the 2D structures A_nB_nO³⁺_2n and 1D A₇B₇O₃₀ structures as resulting from a cutting, respectively into sheets and columns, of the 3D ABO₃ structure that an oxygen insertion accompanies (Fig. 3).

Fig. 3 Oxygen excess d in ABO_3+d.

The structure of the lamellar perovskite Li₂CaTa₂O₇, a compound that otherwise exhibits good photo-catalytic properties at room temperature, has been re-examined [15]. Diffraction techniques revealed that it is in fact non-centrosymmetric (space group Pna2₁) at room temperature. Two reversible structural transitions were also revealed (Pna2₁ -> Pnma -> Cmcm). They occur respectively around 220°C and 650°C and involve the concerted rotation and tilting of the [TaO₆] octahedra within the [Ta₂O₇]^4- sheets of the structure. These rotations/tiltings are at the origin of the disappearance of the non-centrosymmetry of the structure at 220°C, which has been confirmed by frequency doubling measurements at temperature.

Action 1.2: Modeling

Following the regional project RMN3MPL (2019-2013), we have continued to apply the GIPAW method to inorganic fluorides [16–18] but also to ordered [19] and disordered [20] oxyfluorides and hybrids [8], in the framework of a thesis [21] and of three collaborative studies. The DFT approach, i.e. Density Functional Theory, for the precise determination of NMR parameters of periodic systems, applied to inorganic fluorides since 2002 at Le Mans, has indeed become almost indispensable in solid state NMR to interpret and attribute to crystallographic sites the NMR lines of complex spectra.

The interest of the modelling is illustrated by the ³⁹K spectrum of KYF₄ which does not allow to determine the NMR parameters of the six potassium sites. On the other hand, it is satisfactorily reproduced by the calculated spectrum (Fig. 4) showing the accuracy of the DFT calculations of the NMR parameters and thus of the structure [18].

On the other hand, assigning NMR lines to crystallographic sites is necessary when one wishes to identify or study the structural factors influencing the NMR parameters of a nucleus such as the isotropic chemical shift of ⁸⁹Y, particularly sensitive to the number of potassium atoms present in the second sphere of coordination for compounds of the binary KF-YF₃ [18].

The "NMR crystallography" approach has been applied to two hybrid titanium oxyfluorides for which, the comparison of the chemical shifts of the protons calculated for the complete structure, for the organic cations alone and for an isolated cation (Fig. 5) has allowed to evaluate the hydrogen bonds, by adapting a method applied until now exclusively to organic solids [8].

References

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(17)        Martel, L.; Capelli, E.; Body, M.; Klipfel, M.; Beneš, O.; Maksoud, L.; Raison, P. E.; Suard, E.; Visscher, L.; Bessada, C.; Legein, C.; Charpentier, T.; Kovács, A. Insight into the Crystalline Structure of ThF ₄ with the Combined Use of Neutron Diffraction, ¹⁹ F Magic-Angle Spinning-NMR, and Density Functional Theory Calculations. Inorganic Chemistry 2018, 57 (24), 15350–15360. https://doi.org/10.1021/acs.inorgchem.8b02683.

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