Oxygen-participated electrochemistry of new lithium-rich layered oxides Li3MRuO5 (M = Mn, Fe).

We describe the synthesis, crystal structure and lithium deinsertion-insertion electrochemistry of two new lithium-rich layered oxides, Li3MRuO5 (M = Mn, Fe), related to rock salt based Li2MnO3 and LiCoO2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m) where Li and (Li0.2Mn0.4Ru0.4) layers alternate along the c-axis, while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R3[combining macron]m) structure where Li and (Li0.2Fe0.4Ru0.4) layers are stacked alternately. Magnetic measurements indicate for Li3MnRuO5 the presence of Mn(3+) and low spin configuration for Ru(4+) where the itinerant electrons occupy a π*-band. The onset of a net maximum in the χ vs. T plot at 9.5 K and the negative value of the Weiss constant (θ) of -31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g(-1) (2.3 Li per f.u.) and 144 mA h g(-1) (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge-charge cycling points to partial reversibility (ca. 160 mA h g(-1) and 45 mA h g(-1) for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn(3+) and Ru(4+) are partially oxidized to Mn(4+) and Ru(5+) in the sloping region at low voltage, while in the long plateau, O(2-) is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O(2-) ligand in the reversible redox processes. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O(2-) (plateau) while Fe seems to retain its 3+ state.

Ahead of the lanthanide contraction; pressure and ionic size in the synthesis of MSr2RECu2O8 (RE = Rare Earth, M = Ru, Cr, Ir): a Gaussian relation.

We have been working for sometime on the synthesis at high pressure (P < or = 12.5 Gpa) and high temperature (T < or = 1400 K) of new materials of the type MSr2RECu2O8 (RE = Rare Earth), which formally derive from YBCO (i.e., CuBa2YCu2O7) by replacing the [Cu-O4] squares in the basal plane of the structure by [M-O6] octahedra (M = Ru, Cr or Ir). The adequate formation of these cuprates, as majority phases, can only be performed in a particular and relatively narrow window of P and T, outside which they cannot be obtained pure or even obtained at all. These "optimum conditions" bear a remarkable Gaussian correlation with the rare earth ion size, the rare earth cation being at the center of the unit cell in the YBCO setting, and they do not follow the classic lanthanide contraction so often observed in the chemistry of those elements. Instead, interelectronic repulsion seems to play a major role in fixing the synthesis conditions. Moreover, the position of the Gaussian tip in the pressure-ionic radii space is also dependent on the transition metal that sits in the octahedron, in a way that seems related to the thermodynamic stability of their simpler oxides.

High pressure synthesis of polar and non-polar cation-ordered polymorphs of Mn2ScSbO6.

Two new cation-ordered polymorphs of Mn2ScSbO6 have been synthesised at high-pressure. At 5.5 GPa and 1523 K Mn2ScSbO6 crystallizes in the Ni3TeO6-type structure with the polar R3 space group and cell parameters a = 5.3419 (5) Å and c = 14.0603 (2) Å. Below TC = 42.0 K it exhibits ferrimagnetic order with a net magnetization of 0.6µB arising from unusual site-selective Mn/Sc disorder and is thus a potential multiferroic material. A double perovskite phase obtained at 12 GPa and 1473 K crystallizes in the non-polar P21/n monoclinic space group with cell parameters a = 5.2909 (3) Å, b = 5.4698 (3) Å, c = 7.7349 (5) Å and ß = 90.165 (6) °. Magnetization and neutron diffraction experiments reveal antiferromagnetic order below TN = 22.3 K with the spins lying in the ac plane.

Magnetic and crystal structure determination of Mn2FeSbO6 double perovskite.

The perovskite form of the Mn2FeSbO6 oxide has been obtained at 5.5 GPa and 1523 K. X-ray and neutron diffraction data reveal that this compound crystallizes in the monoclinic P21/n space group with a = 0.5234 nm, b = 0.5389 nm, c = 0.7642 nm and ß = 90.372° lattice parameters. AC and DC magnetic susceptibility measurements suggest the existence of a complex magnetic behavior below 200 K. The magnetic structure has been determined and can be described on the basis of an elliptical spiral with an incommensurate propagation vector, k = [0,0.426,0], where the magnetic moments of 2Mn(2+) and Fe(3+) cations are confined to the ac-plane.

Spin glass to superconducting phase transformation by oxidation of a molybdo-cuprate: Mo0.3Cu0.7Sr2TmCu2Oy.

A detailed study of the structure and properties for the as-prepared and oxygen annealed Mo0.3Cu0.7Sr2TmCu2Oy material is reported. The Cu/Mo cationic distribution is established using a combination of x-ray/neutron powder diffraction refinement. The chemical substitution of the Mo ions for the Cu ions in the CuYSr2Cu2O(7-δ) structure is found to occur in both of the copper sites for the as-prepared sample. Interestingly, no trace of Mo substitution in the copper plane site is found to occur after oxygenation. The as-prepared Mo0.3Cu0.7Sr2TmCu2Oy material is found to be a spin glass (SG) system and explained on the basis of the cluster-by-cluster freezing model. On the other hand, the oxygen annealed material is superconducting (SC) (T(SC,onset) = 31 K). A peak has been observed in the critical current density plot and can be explained on the basis of field induced pins. The influence of oxygen annealing in the structure and properties of this material are presented and discussed. This seems to be the first case of a SG-SC transformation following an oxidation reaction in cuprates.