Emilio Morán

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.

Microwave-assisted synthesis, microstructure, and physical properties of rare-earth chromites.

The full rare-earth (RE) chromites series (RE)CrO(3) with an orthorhombic distorted (Pnma) perovskite structure and the isostructural compound YCrO(3) can be synthesized through a simple microwave-assisted technique, yielding high-quality materials. Magnetization measurements evidence that the Néel temperature for antiferromagnetic Cr(3+)-Cr(3+) ordering strongly depends on the RE(3+) ionic radius (IOR), and a rich variety of different magnetic spin interactions exists. Dielectric spectroscopy on sintered pellets indicates electronic inhomogeneity in all samples as manifested by the presence of at least two dielectric relaxation processes associated with grain boundary and grain interior bulk contributions. X-ray diffraction, Raman spectroscopy, and temperature-dependent dielectric permittivity data do not indicate potential noncentrosymmetry in the crystal or concomitant ferroelectricity. Strong correlations between the magnetic and dielectric properties were not encountered, and microwave-synthesized (RE)CrO(3) may not be classified as magnetoelectric or multiferroic materials.

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).

Towards Lead-Free Piezoceramics: Facing a Synthesis Challenge

The search for electroceramic materials with enhanced ferro-pyro-piezoelectric properties and revealing the perovskite type structure has been the objective of a significant number of manuscripts reported in the literature. This has been usually carried out by proposing the synthesis and processing of new compounds and solid solution series. In this work, several methods to obtain ferro-pyro-piezoelectric families of materials featuring the well-known ABO3 perovskite structure (or related) such as BaTiO3, Ba1–xCaxTi1–yZryO3, (Bi0.5Na0.5)TiO3, (K0.5Na0.5)NbO3 and their solid solutions with different cations either in the A or B positions, are presented. For this kind of materials, the challenge for obtaining a single phase compound with a specific grain size and morphology and, most importantly, with the adequate stoichiometry, will also be discussed. The results reviewed herein will be discussed in terms of the tendency of working with softer conditions, i.e., lower temperature and shorter reaction times, also referred to as soft-chemistry.

In situ reduction of (100) SrTiO3

Abstract Low energy electron diffraction (LEED) and photoelectron spectroscopy (PES) have been employed to study the (100) SrTiO3 surface annealed in ultra high vacuum. The annealing was performed up to 830°C. After annealing at 830°C, we have observed from PES spectra the appearance of the conduction band and from LEED patterns the ( 5 × 5 )R 26.6° superstructure. Both features seem to have the same origin: the ordering of oxygen vacancies.

Ba2PrCu3O7: Crystal growth, structure and magnetic properties

Abstract A novel procedure for growing crystals of the title phase has been found by dissolving Pr 2 CuO 4 in a mixture of CuO and BaO 2 . The crystal structure determination shows this solid to be isostructural with, but of higher symmetry than “Ybacuo”, being tetragonal with unit cell parameters a =3.8918(2) A , c =11.6484(8) A . Magnetic susceptibility measurements together with crystallochemical considerations allow us to estimate the oxidation states of the cations in this material which can be formulated as Ba 2 Pr 4+ 3 Pr 3+ .7 Cu 3+ .7 Cu 2+ 2.3 O 7 . In spite of having the same oxygen stoichiometry than the best superconducting “Ybacuo”, this material does not show any superconducting transition down to 4.2 K.

A new high temperature superconductor: Ba2SmCu3O9−x

Abstract A new high temperature superconducting oxide Ba2SmCu3O9−x (x≈-2.0) has been synthesized and characterized by means of X-ray and electron diffraction as a single phase with a perovskite related structure (acx acx 3ac, ac=perovskite unit cell parameter). The superconducting D.C. transition occurs at Tc=96.5 K with a transition width (10–90%) of 1.7 K. X-ray photoemission experiments reveal a surface concentration of Cu3+ cations of 36%.

High pressure driven structural and electrochemical modifications in layered lithium transition metal intercalation oxides

High pressure–high temperature (HP/HT) methods are utilized to introduce structural modifications in the layered lithium transition metal oxides LiCoO2 and Li[NixLi1/3−2x/3Mn2/3−x/3]O2 where x = 0.25 and 0.5. The electrochemical property to structure relationship is investigated combining computational and experimental methods. Both methods agree that the substitution of transition metal ions with Li ions in the layered structure affects the compressibility of the materials. We have identified that following high pressure and high temperature treatment up to 8.0 GPa, LiCoO2 did not show drastic structural changes, and accordingly the electrochemical properties of the high pressure treated LiCoO2 remain almost identical to the pristine sample. The high pressure treatment of LiNi0.5Mn0.5O2 (x = 0.5) caused structural modifications that decreased the layered characteristics of the material inhibiting its electrochemical lithium intercalation. For Li[Li1/6Ni1/4Mn7/12]O2 more drastic structural modifications are observed following high pressure treatment, including the formation of a second layered phase with increased Li/Ni mixing and a contracted c/a lattice parameter ratio. The post-treated Li[Li1/6Ni1/4Mn7/12]O2 samples display a good electrochemical response, with clear differences compared to the pristine material in the 4.5 voltage region. Pristine and post-treated Li[Li1/6Ni1/4Mn7/12]O2 deliver capacities upon cycling near 200 mA h g−1, even though additional structural modifications are observed in the post-treated material following electrochemical cycling. The results presented underline the flexibility of the structure of Li[Li1/6Ni1/4Mn7/12]O2; a material able to undergo large structural variations without significant negative impacts on the electrochemical performance as seen in LiNi0.5Mn0.5O2. In that sense, the Li excess materials are superior to LiNi0.5Mn0.5O2, whose electrochemical characteristics are very sensitive to structural modifications.

XRD study of ZrW2O8versus temperature and pressure

Abstract ZrW 2 O 8 samples have been prepared from ZrO 2 and WO 3 precursors by a ceramic method. Different portions of this material were subjected to non cumulative pressure treatments within the 2–65 Kbar range; the metastable high-pressure phase, γ-ZrW 2 O 8 , has been isolated by quenching from 6 Kbar. Further pressure increases up to 10 Kbar produced the volume reduction of the γ-phase; a minimum volume was found even after having released the pressure. Otherwise, heating above 600°C, followed by quenching at 77 K leads to the high-temperature phase, β-ZrW 2 O 8 , which is stable at room temperature. Finally, when γ-ZrW 2 O 8 was subjected to pressure and temperature, γ-ZrW 2 O 8 could also be quenched with a still larger volume reduction at 6 Kbar and 600°C.

Synthesis of CuO, La2O3, and La2CuO4 by the Thermal-Decomposition of Oxalates Precursors Using a New Method

A new procedure to prepare oxides materials from oxalates complexes precursors is presented. Detailed description of the method is reported, illustrated with examples of the preparation of oxides. A solid-solid reaction between the oxalic acid and nitrate salt of metal leads to the formation of the corresponding oxalate. These oxalates are used as precursors and are heated to form the corresponding oxides by thermal-decomposition. This article relates the first examples of oxides prepared by this new method: the synthesis of the copper oxide CuO, the hexagonal phase of lanthanum oxide La2O3, and the lanthanum cuprate La2CuO4. All the samples are characterized by SEM and XRD. Thermal analysis TGA and DTA show the expected thermo-decomposition behavior for both copper and lanthanum oxalates. This study reveals also the formation mechanism of the cuprate La2CuO4 from the used precursor.