Na6Li4MO4(CO3)4 (M = W and Mo): An Alternative Electrolyte for High-Temperature Electrochemical Cells.

Two new acentric oxycarbonates Na6Li4MO4(CO3)4 (M = W and Mo) were synthesized via a conventional solid-state route. Their structure was determined from X-ray diffraction data on single crystals. Na6Li4MO4(CO3)4 (M = W and Mo) crystallizes in the acentric cubic P-43m space group (a ≈ 7.15 Å). It is composed of MLi4O16 units built from MO4 and LiO4 tetrahedra and linked by CO32- groups to form a three-dimensional framework in which Na+ ions are inserted. We showed from differential scanning calorimetry and powder X-ray diffraction experiments that the melting is congruent (T ∼525 °C). In the solid and molten forms, conductivity was measured for both oxycarbonates by electrochemical impedance spectroscopy with three various gas compositions (CO2 100 vol %, CO2-air 70-30 vol %, and CO2-air 20-80 vol %). Each time, the stability of the electrical behavior was checked via heating and cooling cycles. The conductivity of both solid and molten phases is purely ionic and in the same order of magnitude as for the classical molten alkali electrolyte made of Li-Na or Li-K carbonates. As activation energies are also comparable, those new oxycarbonates appear to be promising electrolytes for electrochemical devices.

Structural Characterization and Influence of Defects on the Optical Properties of the Oxygen-Deficient Perovskite Ba3LiNb2O8.5□0.5.

A new oxygen-deficient perovskite Ba3LiNb2O8.5□0.5 was synthesized via a conventional solid-state route and compared to the already known perovskite Ba3Li0.75Nb2.25O9. The structure of Ba3LiNb2O8.5□0.5 was investigated by means of X-ray and neutron diffraction, TEM, NMR, and XPS. The study of its thermal behavior revealed an unexpected color change when heated to 1400 °C in a sealed platinum tube, with conservation of the initial X-ray structure. First-principles calculations have been performed in order to better understand these observations. The geometry optimizations and the optical spectra simulations highlight the role of both Nb/Li distribution and oxygen-vacancy location.

CO2 Capture by Na2TeO4: Structure of Na2- xH xTeO4 and Kinetic Aspects.

ß-Na2TeO4 is able to trap CO2 in a humid atmosphere due to a partial Na+/H+ exchange and the formation of NaHCO3. The RT powder X-ray diffraction pattern of the resulting Na2- xH xTeO4 shows broad and narrow hkl lines preventing the structural study. We show by the DIFFaX program that Na+/H+ exchange is topotactic since the structure, as in the mother form, consists of [TeO4] n2 n- chains of TeO6 octahedra. We also show that the broadening of some hkl lines is due to stacking faults which result from the weakness of H-O···H bonds connecting the [TeO4] n2 n- chains. Upon heating, a progressive structural organization takes place which has been followed by powder X-ray diffraction, Raman, and NMR spectroscopies. Around 300 °C, a well organized structure can be described from powder X-ray diffraction refinements in the monoclinic P21/ n space group while ab initio computations allowed location of the hydrogen atoms with satisfactory H-O bonds. In addition, we present the CO2 sorption/desorption by Na2TeO4 and compare its performance to that of Na2SiO3. Finally, the existence of a Na2- xLi xTeO4 solid solution (0 ≤ x ≤ 0.9) is evidenced, and we show that the presence of lithium in the structure leads to the disappearance of the structural transition observed for ß-Na2TeO4 and to a progressive decrease of the CO2 capture ability.

ß-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 ß-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.

Instability of the Ionic Conductor Li6BaLa2B2O12 (B = Nb, Ta): Barium Exsolution from the Garnet Network Leading to CO2 Capture.

The instability of the two garnets Li6BaLa2B2O12 (B = Nb, Ta) has been studied on samples prepared in powder form by solid-state reaction. For this study, we coupled different techniques: powder X-ray diffraction, IR spectrometry, thermal analysis, transmission electron microscopy, and complex impedance spectroscopy. We showed that in ambient air and at low temperature (<150 °C), a spontaneous Li+/H+ exchange occurs. At higher temperature (500-700 °C), a progressive exsolution of the barium from the garnet framework is observed, leading to the formation of a second garnet, BaCO3, and a 3D cubic perovskite. To conclude this work, we studied the impact of barium exsolution on the ionic conductivity measured by complex impedance spectroscopy. We observed a significant decrease in the starting bulk conductivity (60%) when the pellet is heated at 500 °C for 5 h.

Phase Transitions in the Ruddlesden-Popper Phase Li2CaTa2O7: X-ray and Neutron Powder Thermodiffraction, TEM, Raman, and SHG Experiments.

The structure of the Ruddlesden-Popper layered perovskite Li2CaTa2O7, known for its high photocatalytic water activity since its discovery in 2008, is reinvestigated. This oxide has been characterized by powder X-ray and neutron thermodiffraction, TEM, second harmonic generation (SHG), and Raman experiments on powders and single crystals. It is shown that it undergoes two structural phase transitions (i) around 220 °C, mainly characterized by the progressive emergence of SHG signal at low temperatures, and (ii) at 660 °C, mainly characterized by changes of the temperature behavior of lattice parameters and by the emergence of Raman signals that linearly increase on decreasing temperature. It is shown by powder neutron diffraction profile refinements at RT, 400, and 800 °C that the space groups of the successive phases of Li2CaTa2O7 are the acentric Pna21 (RT ≤ T ≤ 220 °C), Pnma (220 °C ≤ T ≤ 660 °C), and Cmcm (T ≥ 660 °C). A soft mode associated with the transition to the highest symmetry for this structural arrangement (I4/mmm) is also found in the Raman spectra. All these transitions appear continuous: the high temperature ones can be attributed to progressive vanishings of the octahedra tiltings (displacives) while the transition in the vicinity of 220 °C from Pna21 to Pnma exhibits order-disorder character.

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

Pure tysonite La1-xBaxF3-x solid solutions for x < 0.15 were prepared by solid state synthesis in a platinum tube under an azote atmosphere with subsequent quenching for 0.07 ≤x < 0.15. The solid solutions were studied by X-ray, electron and neutron diffractions and by (19)F NMR and impedance spectroscopy. The evolution of the cell parameters obeying Vegard's rule was determined for 0 < x≤ 0.15 and atomic position parameters were accurately refined for x = 0.03, 0.07 and 0.10. The chemical pressure induced by large Ba(2+) cations leads to an increase of the unit cell parameters. Fluorine environment and mobilities are discussed on the basis of the results of neutron diffraction and (19)F solid state NMR. The F1 subnetwork is lacunar; fluorine exchange occurs according to the order: F1-F1 and F1-F2,3. 2D EXSY NMR spectra of La0.97Ba0.03F2.97 reveal, for the first time, a chemical exchange between F2 and F3 sites that requires two successive jumps. The ionic conductivity was evaluated from sintered pellets and different shaping methods were compared. The only structural features which could explain the conductivity maximum are a crossover together with a smaller dispersion of F1-F1,2,3 distances at x = 0.05-0.07.

Thermal structural characterization of the acentric layered perovskite LiHSrTa2O7: X-ray and neutron diffraction, SHG and Raman experiments.

The present work concerns the thermal structural characterization of the acentric Ruddlesden-Popper LiHSrTa2O7. A previous study, performed with powder neutron diffraction data, has revealed that at room temperature, LiHSrTa2O7 crystallizes in the Ama2 space group and that the acentric character is mainly due to the unequal distribution of the Li(+) and H(+) cations on their sites. In this new paper, the thermal behaviour has been studied by several techniques: powder X-ray and neutron diffraction, SHG experiments and Raman spectroscopy. All of them have revealed that LiHSrTa2O7 undergoes a reversible structural transition from an orthorhombic to a tetragonal symmetry around 200 °C. This transition is associated with the progressive vanishing of the TaO6 octahedra tilting, becoming completely straight in the high temperature form (S.G. I4/mmm), and with a variation of the Li(+) and H(+) distribution in the interlayer spacing.

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.

Structural characterization of a new acentric Ruddlesden-Popper layered perovskite compound: LiHSrTa2O7.

A new n = 2 member acentric Ruddlesden-Popper layered perovskite LiHSrTa(2)O(7) (LiDSrTa(2)O(7)) has been synthesized and structurally characterized from Rietveld treatment of its powder X-ray and high-resolution neutron diffraction data. It can be synthesized by a partial Li(+)/H(+) exchange from the mother phase Li(2)SrTa(2)O(7) either in solid state by NH(4)Cl or in dilute HNO(3) by controlling the amount of H(+). This compound crystallizes in the orthorhombic acentric space group Ama2 (no. 40) with lattice constants a = radical2a(p) approximately 5.5522(1) A, b = radical2a(p) approximately 5.5248(1) A and c approximately 18.7745(4) A. Classically, Ta(5+) ions occupy the octahedral sites of the kinked perovskite blocks and Sr(2+) ions completely fill the perovskite cages while Li(+) and D(+) ions are found in the interlayer spacing. Efficient positive second harmonic generation response, performed at room temperature on a polycrystalline sample, shows unambiguously the acentric character of this new phase. Interestingly, the choice of the acentric Ama2 space group to describe the structure is revealed only by high-resolution neutron diffraction data: in the interlayer spacing, Li(+) and D(+) cations are unequally distributed on different sites (two 4a sites for Li(+) and two 4b sites for D(+) ions).

Mechanism of a reversible CO2 capture monitored by the layered perovskite Li2SrTa2O7.

We demonstrate for the first time, a new CO(2) capture ability monitored by a Ruddelsden-Popper compound. Under a humid CO(2) atmosphere, Li(2)SrTa(2)O(7) is transformed into LiHSrTa(2)O(7) releasing lithium hydroxide which combined with the atmospheric CO(2) leads to Li(2)CO(3). The presence of carbonate is confirmed by IR, thermal analysis coupled with mass spectroscopy and diffraction experiments (X-ray and neutron). The structural study of LiHSrTa(2)O(7) performed with X-ray and neutron diffraction data showed that the structure differs from that of LiHSrTa(2)O(7) obtained by ionic exchange from Li(2)SrTa(2)O(7) by the Li(+)/H(+) repartition in the interlayer spacing. In the case of the LiHSrTa(2)O(7) studied in this paper, the Li(+) and H(+) ions are ordered, while in the other form, each cation is unequally distributed on 2 sites. By heating, Li(2)SrTa(2)O(7) is recovered showing that the CO(2) capture is reversible and the cyclability of the CO(2) capture has been tested during six cycles.

NMR investigations of Li(+) ion dynamics in the NASICON ionic conductors [Formula: see text].

NMR studies of (7)Li and (31)P nuclei are reported in the 150-900 K temperature range for the [Formula: see text] NASICON compounds with x = 0.8, 0.7, 0.6 and 0.3. Magic angle spinning (MAS mode) experiments were performed at room temperature on the (7)Li and (31)P nuclei. The linewidth and the spin lattice relaxation times of these nuclei are studied versus temperature in the static mode. The spectra recorded in the MAS mode show that the (7)Li ions occupy three chemical sites, the occupation of which being very sensitive to the x values but not sensitive to the coexistence of the two varieties [Formula: see text] and [Formula: see text] observed at room temperature in compounds with x≤0.5. On the other hand, the (31)P nucleus MAS spectra are very sensitive to lithium content but also to the variety coexistence. T(1) measurements were performed in a static mode on the (7)Li and (31)P nuclei. In all the compounds, the (7)Li spin lattice relaxation time exhibits two branches with several minima, indicating the complex dynamics for this ion. One of these minima appears in the same temperature range as the minimum of the (31)P nucleus T(1), strongly suggesting a cross-relaxation process between these nuclei. T(1ρ) measurements on (7)Li (static mode) allow us to show a slow motion different from the one probed by the T(1). The analysis of the T(1ρ) behaviour with temperature and composition allows us to ascribe the motion probed by this time to the oxygen ion motion which monitors the opening and closing of the lithium pathways. A qualitative interpretation of the (7)Li  T(1) results is done; it takes into account the cross-relaxation phenomena between (31)P and (7)Li and quadrupolar fluctuations.

Double NASICON-type cell: ordered Nd3+ distribution in Li0.2Nd0.8/3Zr2(PO4)3.

The NASICON compound Li(0.2)Nd(0.8/3)Zr(2)(PO(4))(3), synthesized by a sol-gel process, has been structurally characterized by TEM and powder diffraction (neutron and X-ray). It crystallizes in the space group R3[combining macron] (No. 148): at room temperature, the Nd(3+) ions present an ordered distribution in the [Zr(2)(PO(4))(3)](-) network which leads to a doubling of the classical c parameter (a = 8.7160(3) A, c = 46.105(1) A). Above 600 degrees C, Nd(3+) diffusion occurs leading at 1000 degrees C to the loss of the supercell. This reversible cationic diffusion in a preserved 3D [Zr(2)(PO(4))(3)](-) network is followed through thermal X-ray diffraction. Ionic conductivity measurements have been undertaken by impedance spectroscopy, while some results concerning the sintering of the NASICON compound are given.

Ca(2+)/vacancies and O2-/F- ordering in new oxyfluoride pyrochlores Li(2x)Ca1.5-x square 0.5-xM2O6F (M = Nb,Ta) for 0 < or = x < or = 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.