F. García-Alvarado

Infrared and Raman spectra of the MBa2Cu3O7-type high-Tc superconductors

Abstract Infrared and Raman spectra of superconducting MBa2Cu3O7 (M = Y, Sm, Eu, Gd, Ho) have been measured at several temperatures between 4 and 300 K. Contrary to the Raman spectra, the ir spectra display upon cooling below 80 K structure which seems characteristic of the superconducting gap starting at ∼210 cm-1 and below. A number of phonons are observed in both spectra, mostly as Davydov pairs. They are attributed to stretching and bending vibrations of Cu-O bonds and vibrations of the Ba and M atoms along the x, y, and z directions. The Cu-O bonds also yield one set of ir-active stretching and bending modes without Davydov conjugate. The vibrations of the M atom along x, y, z yield only ir-active modes. These results are compared with those for the non-superconducting tetragonal YBa2Cu3O6 compound.

Hollandite-type TiO2: a new negative electrode material for sodium-ion batteries

The electrochemical properties of TiO2 with the hollandite structure (TiO2(H)) as a negative electrode material for sodium-ion batteries are reported. TiO2(H) was obtained from hollandite K0.21TiO2 by an oxidation–ion extraction process. Na/TiO2(H) cells exhibit a large first discharge capacity of 280 mA h g−1 down to 0.2 V. After the first discharge the Na/TiO2(H) cells develop a reversible charge–discharge capacity of 85 mA h g−1 at C/8 rate in the 2.5–0.2 V voltage range; this corresponds to the reversible insertion of 0.25 Na per TiO2(H) formula unit. Chronoamperometry and potentiostatic intermittent titration techniques were used to further characterize the electrochemical reaction mechanism. Structural changes in the TiO2(H) electrode upon sodium insertion and extraction have been studied by ex situ XRD and high resolution in situ synchrotron diffraction techniques, for which appropriately modified coin-type cells were used. It is seen that sodium insertion into TiO2(H) is commenced with a single-phase solid solution followed by a structural transition from tetragonal I4/m to monoclinic I2/m symmetry, in which the skeleton framework is retained. The reversible transition includes few structural changes with a small volume change of only 1.1%. Fourier difference maps deduced from SXRD patterns revealed the location of Na ions in 4i sites in the tunnel space. The coordination arrangement around Na ions is distorted capped trigonal prisms formed by seven oxygen atoms. Although still far from the theoretical capacity (335 mA h g−1), the cycling properties at a low insertion potential together with the host framework stability indicate the feasibility of TiO2 with the hollandite structure as a negative electrode material for Na-ion batteries.

Excess electrical conductivity in polycrystalline Bi-Ca-Sr-Cu-O compounds and thermodynamic fluctuations of the amplitude of the superconducting order parameter

Abstract Measurements of the rounding effects on the electrical resistivity above the superconducting transition in Bi-Ca-Sr-Cu-O polycrystalline compounds are reported, to our knowledge for the first time in this HTSC system. These effects are analyzed in terms of thermodynamic fluctuations of the amplitude of the superconducting order parameter (SCOPF). In the mean-field-like region, the experimental critical exponent seems to be compatible with an order parameter of two components (2d) fluctuating in two dimensions (2D). This contrasts with previous results for A-Ba-Cu-O ( A = Y , Ln ) and Ln-M-Cu-O ( M = Ba , Sr ) superconductors, where SCOPF seem to be 2d-3D in all the different dynamic critical regions.

Temperature dependence of the Raman and infrared phonons of MBa2Cu3O7−δ-type superconductors

An anomalous temperature dependence of the ∼340 cm-1 Raman phonons of YBa2Cu3O7-δ has been recently reported by Macfarlane et al. Here we show that this phenomenon, a phonon softening with decreasing T for T < Tc, is common to many materials of the family (M = Y, Sm, Eu, Ho, Gd). We also show the ir-active Davidov counterpart of this phonon, and another ir-active phonon at slightly lower frequency, also exhibit similar effects. These phonons are attributed to bending vibrations of the Cu2-02 and Cu3-03 bonds. All other ir and Raman phonons observed exhibit a normal temperature dependence (i.e., hardening with decreasing T even for T < Tc). The 310 cm-1 phonon of the tetragonal non-superconducting YBa2Cu3O6 also shows a normal behavior vs. T.

Raman scattering in the high Tc superconductors MBa2Cu3O7-x

Abstract We report Raman scattering measurements of MBa 2 Cu 3 O 7 ( M = Y , Sm , Eu ) polycrystalline superconductors in the region of Cu-O stretching vibrations. Four peaks are seen. On the basis of the factor group analysis given here they are assigned to two Raman allowed and two ir-allowed LO modes. The latter are probably induced by disorder (e.g. O-vacancies).

Electrochemical lithium intercalation in Li2Ti3O7-Ramsdellite structure

Electrochemical lithium intercalation has been carried out in the Ramsdellite-type structure of Li2Ti3O7. The intercalation of 2.24 lithium ions at a very low potential in this compound leads to the formation of a new phase Li4.24Ti3O7. From powder X-ray diffraction we deduce that the structure of Li4.24Ti3O7 is closely related to the parent Ramsdellite. Both the large reversibility and the relatively high specific capacity (235 Ah Kg−1) make this compound a promising material as the negative electrode for secondary lithium cells.

Structural Factors That Enhance Lithium Mobility in Fast-Ion Li1+xTi2–xAlx(PO4)3 (0 ≤ x ≤ 0.4) Conductors Investigated by Neutron Diffraction in the Temperature Range 100–500 K

Structural features responsible for lithium conductivity in Li(1+x)Ti(2-x)Al(x)(PO4)3 (x = 0, 0.2, and 0.4) samples have been investigated by Rietveld analysis of high-resolution neutron diffraction (ND) patterns. From structural analysis, variation of the Li site occupancies and atomic thermal factors have been deduced as a function of aluminum doping in the temperature range 100-500 K. Fourier map differences deduced from ND patterns revealed that Li ions occupy M1 sites and, to a lower extent, M3 sites, disposed around ternary axes. The occupation of M1 sites by Li ions is responsible for the preferential expansion of the rhombohedral R3c unit cell along the c axis with temperature. The occupation of less symmetric M3 sites decreases electrostatic repulsions among Li cations, favoring ion conductivity in Li(1+x)Ti(2-x)Al(x)(PO4)3 compounds. The variations detected on long-range lithium motions have been related to variations of the oxygen thermal factors with temperature. The information deduced by ND explains two lithium motion regimes deduced previously by (7)Li NMR and impedance spectroscopy.

Lithium intercalation in Ag2V4O11

Abstract The electrochemical intercalation of lithium in Ag 2 V 4 O 11 and Ag 2 V 4 O 11− y has been performed. We found that 7 a 5.7 lithium ions can respectively be intercalated into these compounds through a multiphase intercalation process. Preliminary electrochemical evaluation of the performances of both vanadates as positive electrode materials for room temperature rechargeable lithium batteries indicates that among the two compounds, the oxygenated Ag 2 V 4 O 11 phase, with a theoretical specific capacity of 300 Ah/g of vanadate, is the most attractive.

New electrode materials for lithium rechargeable batteries

Abstract In this paper, a contribution to the search of new electrode materials for lithium batteries is reported. First, we will present the electrochemical behavior of an Aurivillius-type phase with the composition Bi 4 V 2 O 11 . This oxide may be used as cathode material (390 A h/kg) if the voltage is not so low (average voltage 1.6 V). On the other hand, if Bi 4 V 2 O 11 is reduced down to 0.5 V, it reacts with 28 Li ions per formula unit. Considering only the low voltage region, Li 28 Bi 4 V 2 O 11 could be a candidate to anode material (360 A h/kg at 0.7 V). Our research has also been directed towards the applications of several types of titanium oxides in lithium batteries. Among these compounds, we present the results for K x Ti 8 O 16 and Li 2 Ti 3 O 7 . The best results are those obtained from the ramsdellite Li 2 Ti 3 O 7 : the large reversibility, low polarization and relatively high capacity (235 A h/kg) make this compound a promising material as negative electrode for lithium ion cells. However, the relatively high average potential, close to 1.4 V, would reduce considerably the performance of a rocking-chair battery using this ramsdellite instead of carbon.

The rare-earth H.T.S.C. family Ba2(RE)Cu3O7; structural, electrical and magnetic studies (RE=Y,Nd,Sm,Eu,Gd,Dy,Ho,Er,Tm)

Abstract A systematic study of the title family has been performed in the light of structural, electric and magnetic properties. All samples are orthorhombic and the unit cell parameters show an interesting linear relationship with the lanthanide ion dimension.