Christophe Legein

The local field distribution of in transition metal fluoride glasses investigated by electron paramagnetic resonance

The EPR spectra of in two transition metal fluoride glasses (PZG and PBI) have been extensively studied as functions of temperature, concentration and microwave frequency (S, X, K and Q bands). The marked changes observed in the EPR spectra with temperature and concentration imply the existence of pairs of ions. The spectra of isolated ions were obtained at low temperature for samples with low doping concentration. Isolated ions are characterized by a fine-structure -parameter distribution within the range . The simulations of these spectra are computed with a Czjzek fine-structure parameter distribution (this solution prohibits the existence of axial symmetry sites). It indicates a decrease of the number of geometrical constraints compared to those for and (octahedral coordination in a fluoride medium), in agreement with the higher coordination numbers. -ion sites are found to be more distorted in PZG than in PBI, suggesting that the glass network has an influence on the polyhedron distortion amplitude.

Structure determination of β-Pb2ZnF6 by coupling multinuclear solid state NMR, powder XRD and ab initio calculations

The results from one-dimensional multinuclear (19F, 207Pb and 67Zn) magic-angle spinning nuclear magnetic resonance experiments combined with the use of the ISODISPLACE program allow for the space group determination of beta-Pb2ZnF6 (no. 138 P4(2)/ncm). The structure was refined from X-ray powder diffraction data (a = 5.633 (1) A and c = 16.247 (1) A, Z = 4). beta-Pb2ZnF6 has one six-fold coordinated Zn, one eleven-fold coordinated Pb and five F non-equivalent crystallographic sites and is built from alternated layers parallel to the (a, b) plane; tilted ZnF4(2-) layers of corner sharing ZnF6(4-) octahedra and FPb+ layers of edge sharing FPb4(7+) tetrahedra. The structure of beta-Pb2ZnF6 was then optimized using the ab initio code WIEN2k and the calculated 67Zn EFG is in agreement with the NMR results. 19F-19F proximities and 19F-207Pb connectivities were evidenced using through-space and through-bond NMR correlation experiments, respectively, and support the proposed structure. 19F-207Pb J-coupling was also used to select fluorine resonances depending on the number of neighbouring lead ions, leading to an unambiguous assignment of the different 19F resonances.

From crystalline to glassy gallium fluoride materials: an NMR study of 69Ga and 71Ga quadrupolar nuclei

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of 69Ga and 71Ga are obtained in crystallised (PbGaF5, Pb3Ga2F12, Pb9Ga2F24 and CsZnGaF6) and glassy (PbF2-ZnF2-GaF3) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters (nu(Q) up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzeks distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.

Investigation of fluorine octahedron connectivities in transition metal fluoride glasses by solid-state magic-angle-spinning nuclear magnetic resonance spectroscopy

High-resolution magic-angle-spinning (MAS) NMR spectroscopy is used to investigate the structural properties of some transition metal fluoride glasses ( - - ) related to the fluorine network. Several glass compositions are investigated in order to vary the ratios. MAS NMR experiments are carried out on certain crystalline compounds selected as being the initial constituents or recrystallization compounds of glassy phases, or because they have some specific particular connectivities of the fluorine octahedron network (e.g. ). It is shown that three types of fluorine are involved in the glass network: free fluorines which are not connected to transition metal ions, and shared and unshared fluorines belonging to and octahedra. Quantitative information on these three different fluorine sites, their relative ratios in the glassy networks, and the degree of cross-linking of the fluorine octahedra is obtained. Our results prove the validity of the previously adopted assumption according to which the glass network is built up from corner-sharing fluorine octahedra centred on transition metal ions.

Double-quantum 19F–19F dipolar recoupling at ultra-fast magic angle spinning NMR: application to the assignment of 19F NMR spectra of inorganic fluorides

A broadband dipolar recoupling method robust to chemical shift is introduced to observe (19)F-(19)F proximities in fluoroaluminates in high magnetic field and at ultra-fast magic angle spinning (>60 kHz).

Short-range order quantification in transition metal fluoride glasses (TMFG) through EPR spectra simulation

Cr3+ and Fe3+ EPR spectra in transition metal fluoride glasses (TMFG), pyrochlore CsZnGaF6 and amorphous GaF3 are found to be very similar. Then, it is inferred that the constituent octahedra of all these disordered fluoride compounds are characterized by closely related distortions. Two short-range structural models, based on CsZnGaF6 and GaF3, are worked out with the aim of characterizing octahedra in TMFG. The CsZnGaF6 model is developed using structural information on this compound. Amorphous GaF3 structure is simulated by using molecular dynamics calculations starting from rhombohedral GaF3. From the M-F distance and bonding angle distributions obtained from these two models, the Cr3+ and Fe3+ fine-structure parameter distributions are calculated with the help of the superposition model and finally the corresponding EPR spectra are computed. A good agreement is obtained simultaneously for the two local paramagnetic probes, which leads to a quantification of the short-range order in TMFG: constituent octahedra are only slightly distorted. It is interesting to note that the distributions of fine-structure parameters are very similar to the Czjzek ones previously used to simulate Cr3+ and Fe3+ spectra in TMFG.

Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO2

In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg2+ and Al3+ into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg2+ and Al3+ insertion in anatase TiO2, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO2. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.

Neutron powder diffraction, multinuclear, and multidimensional NMR structural investigation of Pb5Ga3F19.

The room temperature structure of Pb5Ga3F19 is investigated by combining neutron diffraction and multinuclear 19F, 71Ga, and 207Pb one-dimensional and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments. Two models built in space group I4cm are reported for the description of the crystalline structure of Pb5Ga3F19. The structure is built from a network of both opposite corner-sharing Ga2F6(3-) octahedra forming infinite chains along the c-axis and isolated Ga1F6(3-) octahedra. The two models present two slightly different views of the strong static disorder of the fluorine ions belonging to the Ga2F6(3-) octahedra. 71Ga NMR results show that the local environment of all Ga2 ions is identical, which indicates a tilt of the Ga2F6(3-) octahedra within the chains. 207Pb NMR experiments confirm that the environment of only one of the two lead sites, Pb1, is strongly affected by the disorder, which gives rise to three broad distinct 207Pb NMR lines for this site. All 19F NMR lines are assigned using the 19F DQ-SQ MAS experiment. 19F-207Pb through-bond and through-space heteronuclear correlation experiments are carried out for the first time, supporting assignment of both the 19F and 207Pb NMR spectra. These correlation experiments also show that both models correctly describe the medium-range order of the structure of Pb5Ga3F19.

Ca2+/vacancies and O2−/F− ordering in new oxyfluoride pyrochlores Li2xCa1.5−x□0.5−xM2O6F (M = Nb,Ta) for 0 ≤x≤ 0.5

New oxyfluorides Li(2x)Ca(1.5-x) square (0.5-x)M2O6F (M = Nb, Ta), belonging to the cubic pyrochlore structural type (Z = 8, a approximately 10.5 angstroms), were synthesized by solid state reaction for 0 < or = x < or = 0.5. XRD data allowed us to determine their structures from single crystals for the two alpha and beta-Ca(1.5) square (0.5)Nb2O6F forms and from powder samples for the others. This characterisation was completed by TEM and solid state 19F NMR experiments. For the Ca(1.5) square (0.5)M2O6F (x = 0) pyrochlore phases, the presence of a double ordering phenomenon is demonstrated, involving on one hand the Ca(2+) ions and the vacancies and on the other hand the oxide and the fluoride anions which are strictly located in the 8b sites of the Fd3m aristotype space group. The Ca(2+) ions/vacancies ordering leads to a reversible phase transition, a (P4(3)32) <--> beta (Fd3m). The 19F NMR study strongly suggests that, in the beta-phases, the fluoride ions are only on average at the centre of the Ca3 square tetrahedron. It shows that slightly different Ca-F distances occuring in alpha-Ca(1.5) square (0.5)Nb2O6F may be related to a more difficult thermal ionic and vacancies diffusion process than in the tantalate compound. This may explain the hysteresis phenomenon presented by the phase transition. A solid solution Li(2x)Ca(1.5-x) square (0.5-x) Ta2O6F (0 < or = x < or = 0.5) was prepared and the order-disorder phase transition observed for Ca(1.5) square (0.5)M2MO6F compounds disappears for all the other compositions where less or no more vacancies exist in the 16d sites. In the LiCaM2O6F compounds, the 19F NMR study allows us to determine the Ca(2+) and Li+ ions distributions around the fluoride ions and shows that the [FLi2Ca2] environment is clearly favoured.

Nuclear magnetic resonance quadrupolar parameters and short range order in disordered ionic fluorides

Information on both radial and angular atomic coordinations is provided by the quadrupole interaction between the quadrupole moment of the atomic nuclei and the electric field gradients (EFGs) originating from the distribution of electric charges around the nuclei. 69Ga and 71Ga are quadrupolar nuclei active in NMR. Their spectra are recorded in amorphous GaF3 which may be considered as a model compound for disordered fluorides with fluorine corner sharing octahedra. An unique and continuous quadrupolar parameter Czjzek distribution allows us to simulate the experimental spectra. The measured chemical shifts indicate that the Ga3+ ions are at the centre of (GaF6)3- octahedra in the amorphous GaF3 phase. A polarizable point charge model is used to explain NMR quadrupolar parameters which are related to the electric field gradients at the Ga site. Lattice summations are performed in the direct space over spherical volumes. Atomic position sets generated by molecular dynamics are shown to give quadrupolar parameter distributions which look like Czjzek ones whatever the EFG calculation approximations. Provided the polarizabilities are adjusted, they allow us to reconstruct the experimental NMR spectra which correspond in any case to slightly distorted (GaF6)3- octahedra. The present approach may be applied to any ionic disordered compound which contains quadrupolar nuclei and used to quantify short range order around such nuclei.