A combination of experiments and calculations allows grasping more information on the capacity fading upon cycling of the Na2Ti3O7 electrode material in Na batteries.
state conversion. Ordering of the spin states is manifested in the corresponding superstructure reflections; these can be traced in a diffraction experiment as a function of external stimuli. By mapping reciprocal space with an area detector and synchrotron light, we have studied the temperature dependence of the superstructure reflections for NdBaCo2O5.5 and TbBaCo2O5.48. We have found that above the metal-insulator transition there are two different Co ions in the asymmetric unit, one sitting in a pyramidal and one in an octahedral environment. Below the transition temperature there are four structurally different Co ions. This observation agrees with the "spin blockade" mechanism suggested for the metal-insulator transition in cobaltites. We also present results of structural analyzes illustrating how the corresponding powder diffraction measurements could easily overlook the correct structure. A symmetry analysis bracketing the observed phase transitions within the context of Landau theory is also given. BiMnO3 has been considered as a multiferroic material due to the ferroelectric and ferromagnetic properties. The crystal symmetry is, however, controversial today. We investigated the crystal symmetry of BiMnO3 by Convergent-Beam and Selected-Area Electron Diffraction (CBED and SAED, respectively). CBED, which was used in order to discriminate the crystal axes of BiMnO3, showed that BiMnO3 belongs to space group C2/c. In the [010] SAED pattern, however, the very weak but sharp h0l (l=2n+1) reflections were observed indicating the noncentrosymmetric long-range ordered structure (C2) )O12. Singlephase powder samples of both phases were recently synthesized under high pressures at the IMEM-CNR in Parma. In both systems, the A' cation is trivalent, thus all of the Mn B-cations are expected to be trivalent. Both systems crystallize in the A'A3B4O12 complex perovskite structure consisting of a pseudo-cubic network of cornershared BO6 octahedra. This system may display a rich manifold of charge, spin and orbital orderings characteristic of mixed-valence systems. In both systems, we have determined precisely the nuclear and magnetic structures as a function of temperature, between room temperature and 2K. We have observed two magnetic transitions at low temperature in the La compound, involving crystallographically different Mn ions. Between 65K and 20K, the propagation vector k is (0,0,0), and only the Mn ions in the B-site are ordered. Below 20K, additional magnetic reflections appear, the lattice is no longer body centered, and magnetic moments of manganese atoms belonging to the A site are ordered. The magnetic structure has been solved by simulated annealing techniques, with help of symmetry analysis. It will be discussed in comparison with the Na analog which had been studied before. For the Bi compound, the effect of the lone pair of Bi on the structure will be discussed, as well as the magnetic structures observed at low temperature. While ferroelectricity and magnetism are chemically incompatible, it has r...
Since the discovery of the promising electrode material LiFePO4, recent research has been focusing on the development of new iron-based polyanionic materials for next generations Li- and Na-ion batteries displaying better performances while still preserving cost and sustainability benefits.1,2 Towards this quest, our group explored the wide family of sulfate-based compounds, with the most prominent members being monoclinic Li2Fe(SO4)2 and triplite LiFeSO4F having redox potentials of 3.83 V and 3.9 V vs. Li+/Li0, respectively.3,4 In order to further investigate the rich crystal chemistry offered by 3d-metal-based fluorosulfates, we explored the feasibility of using other alkali metals such as Na and K instead of Li. Through this approach we discovered a KFeSO4F phase, which adopts a KTiOPO4 structure and releases K+ ions via a complex electrochemical process.5 Knowing that sulfate-based compounds are prone to polymorphism, we recently unveiled a new low-temperature KFeSO4F polymorph.6 Using combined synchrotron and neutron powder diffraction as well as electron diffraction, it was shown that the compound adopts a complex layered-like structure that crystallizes in a large monoclinic unit cell. Impedance measurements together with the Bond Valence Energy Landscape approach show that the K+ ions, which are located between the layers, are mobile within the structure and can be electrochemically removed at an average potential of 3.7 V vs. Li+/Li0. Lastly, neutron diffraction experiments coupled with SQUID measurements reveal a long range antiferromagnetic ordering of the Fe2+ magnetic moments. These results confirm once again the richness of polymorphisms in sulfate-based materials, which, besides unusual electrochemical properties, show interesting physical properties. (1) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Goodenough, J. B. Mapping of Transition Metal Redox Energies in Phosphates with NASICON Structure by Lithium Intercalation. J. Electrochem. Soc. 1997, 144 (8), 2581–2586. (2) Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113 (8), 6552–6591. (3) Reynaud, M.; Ati, M.; Melot, B. C.; Sougrati, M. T.; Rousse, G.; Chotard, J.-N.; Tarascon, J.-M. Li2Fe(SO4)2 as a 3.83 V Positive Electrode Material. Electrochem. Commun. 2012, 21, 77–80. (4) Ati, M.; Melot, B. C.; Chotard, J.-N.; Rousse, G.; Reynaud, M.; Tarascon, J.-M. Synthesis and Electrochemical Properties of Pure LiFeSO4F in the Triplite Structure. Electrochem. Commun. 2011, 13 (11), 1280–1283. (5) Recham, N.; Rousse, G.; Sougrati, M. T.; Chotard, J.-N.; Frayret, C.; Mariyappan, S.; Melot, B. C.; Jumas, J.-C.; Tarascon, J.-M. Preparation and Characterization of a Stable FeSO 4 F-Based Framework for Alkali Ion Insertion Electrodes. Chem. Mater. 2012, 24 (22), 4363–4370. (6) Lander, L.; Rousse, G.; Abakumov, A. M.; Sougrati, M.; Tendeloo, G. van; Tarascon, J.-M. Structural, Electrochemical and Magnetic Properties of a Novel KFeSO4F Polymorph. J. Mater. Chem. A 2015, 3 (39), 19754–19764.
Over the last two decades, energy storage devices and in particular lithium-ion batteries, have been the object of a steady growing demand due to their feasible implementation for automotive electric transportation (PHEV and HEV)[ 1 ]. However, this market requires the research of new electrode materials with higher energy density and greater power rate. When one looks back on history, the early nineties were solely devoted to oxide-based 3d-metal electrodes[ 2 ] and in the last 15 years three-dimensional frameworks built on transition metals and polyanions (XO4)n- have become subject of very intensive research[ 3 ]. LiFePO4 polyaionic compound offers low cost, environmental compatibility, high theoretical specific capacity (170 mAh/g) and has become the most praised material for the next generation of Li-ion batteries for EV’s [ 3-4 ]. Aside from the inorganics polyanionic compounds, we have recently shown the cost-wise attractiveness of some Fe-based phases having organic polyanions such as carbonates, oxalates, malonates, etc. These materials could be attractive electrodes due to their cost, low molecular weight and electronegativity. Herein, we report a new synthesis route to prepare Fe2(C2O4)3·4H2O and determine its crystal structure through X-ray powder diffraction coupled with neutron powder diffraction (Figure 1). We also show for the first time that iron(III) oxalate compound is electrochemical active versus lithium as it can reversibly insert 1.6 Li atom per formula unit at 3.35 V versus Li+/Li0 (Figure 2). Beside reporting the synthesis and the crystal structure of Fe2(C2O4)3·4H2O phase, we will determine the structural changes driven by Li insertion using both in situ XRD and Operando Mössbauer measurements as will be discussed. References [1] M. Ati, M. T. Sougrati, G. Rousse, N. Recham, M. L. Doublet, J. C. Jumas and J. M. Tarascon, Chemistry of Materials 2012, 24, 1472-1485. [2] J. M. Tarascon, W. R. McKinnon, F. Coowar, T. N. Bowmer, G. Amatucci and D. Guyomard, Journal Electrochemical Society 1994, 141, 1421-1431. [3] C. Masquelier and L. Croguennec, Chemical Reviews 2013, 113, 6552-6591. [4] N. Recham, J. Oró-Solé, K. Djellab, M. R. Palacín, C. Masquelier and J. M. Tarascon, Solid State Ionics 2012, 220, 47-52. Figure 1
Recent results have demonstrated an exceptionally high dielectric constant in the range 200 K-330 K in a crystalline tianium oxide : Rb 2 Ti 2 O 5 . In this article, the possibility of a structural transition giving rise to ferroelectricity is carefully inspected. In particular X-Ray diffraction, high resolution transmission electron microscopy and Raman spectroscopy are performed. The crystal structure is shown to remain invariant and centrosymmetric at all temperatures between 90 K and 450 K. The stability of the C 2/m structure is confirmed by DFT calculations. These important findings allow to discard the existence of a conventional ferroelectric phase transition as a possible mechanism for the unusual dielectric constant observed in this material.
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