Françoise Le Berre

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.

Li+ ionic conduction in the layered perovskite Li2La2/3Ta2O7

The Li+ ionic conduction properties of the Li2La2/3Ta2O7 layered perovskite compound have been investigated by complementary techniques: impedance spectroscopy, 7Li NMR and thermal neutron powder diffraction. Up to 770 K, the results are consistent with an electrical conductivity dominated by Li+ ions jumping between Li1 and Li2 sites, the adjacent centers of the two kinds of LiO4 tetrahedra constituting the interlayer region of the structure. The sudden event, observed near 770 K on the curve log(σT) = f(1000/T), is associated with significant changes to the 7Li NMR signal and to a structural modification followed by neutron powder diffraction up to 973 K. All these facts are consistent with the displacement at 770 K of the lithium ions residing in the Li1 sites. Neutron powder diffraction showed that, in the interlayer region, the location of the lithium ions in the Li2 sites remains unchanged. However, 17% of the Li1 population leaves its tetrahedral position to occupy a new Li3 site inside the perovskite cages, very close to their four O2− bottlenecks. The other Li+ ions (83%) remain in the interlayer but their coordination changes from tetrahedral to a five fold one.

Reinvestigation of the total Li(+)/H(+) ion exchange on the garnet-type Li5La3Nb2O12.

Li(+)/H(+) exchange was performed on Li5La3Nb2O12 using CH3COOH. After X-ray powder diffraction experiments to check the quality of Li5-xHxLa3Nb2O12, the chemical formulation was determined by thermogravimetric analysis coupled with mass spectrometry and flame photometry. The results showed unambiguously that the Li(+)/H(+) exchange was not total and that some CH3COOH remained in the sample. Raman experiments revealed in addition that the organic contribution on the spectrum was due either to metal acetate or to ionic bond to the crystal.

β-Na2TeO4: Phase Transition from an Orthorhombic to a Monoclinic Form. Reversible CO2 Capture

The present work concerns the tellurate Na2TeO4 which has a 1D structure and could then present a CO2 capture ability. It has been synthesized in a powder form via a solid-state reaction and structurally characterized by thermal X-ray diffraction experiments, Raman spectroscopy, and differential scanning calorimetry. The room temperature structure corresponds to the β-Na2TeO4 orthorhombic form, and we show that it undergoes a reversible structural transition near 420 °C toward a monoclinic system. Ab initio computations were also performed on the room temperature structure, the Raman vibration modes calculated, and a normal mode attribution proposed. In agreement with our expectations, this sodium oxide is able to trap CO2 by a two-step mechanism: Na+/H+ exchange and carbonation of the released sodium as NaHCO3. This capture is reversible since CO2 can be released upon heating by recombination of the mother phase.