R. Escamilla

Synthesis by molten salt method of the AFeO3 system (A=La, Gd) and its structural, vibrational and internal hyperfine magnetic field characterization

Abstract Polycrystalline samples of LaFeO3 and GdFeO3 were synthesized by the molten salt method. Some properties and the quality of the resulting compounds were investigated. The crystal structure and purity of the samples were determined through X-ray diffraction and Rietveld analysis. The vibrational properties were characterized by Raman and IR spectroscopy. Mossbauer spectroscopy was used to determine the ionic state of the Fe ions and the internal hyperfine magnetic fields. Considerable reduction of the heat treatment (temperature and time) for the reaction to take place was achieved without detriment of the quality of the compounds.

Crystalline structure and the superconducting properties of NbB2+x

The effect of boron excess in the structure and superconducting properties of NbB2 is reported. Rietveld refinements of the x-ray diffraction patterns indicate that boron excess induces significant changes in the Nb–B bond length, increasing the c-axis. In contrast, the B–B bond length remains essentially constant. Magnetization behaviour was studied in the temperature range from 2 to 15 K. We found that for (B/Nb)exp 2.20(2) of boron excess samples display superconductivity with a maximum TC of about 9.8 K at (B/Nb)exp = 2.34(1) .H igh pressure measurements in samples with two different boron contents reveal that TC decreases at different ratios, dTC/dP. Superconducting parameters were determined, indicating that NbB2+x is a type II superconductor. We correlated the change of TC with the evolution of th es tructural parameters and found that it coincides with theoretical predictions of band structure. (Some figures in this article are in colour only in the electronic version)

X-ray photoelectron spectroscopy studies of non-stoichiometric superconducting NbB2+x

Polycrystalline samples of NbB2+x with nominal composition (B/Nb) = 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5 were studied by x-ray photoelectron spectroscopy (XPS). The spectra revealed Nb and B oxides on the surface of the samples, mainly B2O3 and Nb2O5. After Ar ion etching the intensity of Nb and B oxides decreased. The Nb 3d5/2 and B 1s core levels associated with the chemical states (B/Nb) were identified and they do not change with etching time. The binding energies of the Nb 3d5/2 and B 1s core levels increase as boron content increases, suggesting a positive chemical shift in the core levels. On the other hand, analysis of valence band spectra showed that the contribution of the Nb 4d states slightly decreased while the contribution of the B 2pπ states increased as the boron content increased. As a consequence, the electronic and superconducting properties were substantially modified, in good agreement with band-structure calculations.

Elastic properties of superconducting NbB2+x obtained from first-principles calculations

First-principles calculations have been performed on NbB2+x to obtain its elastic constants using the plane wave pseudopotential method based on density functional theory, within the generalized gradient approximation. We have derived the bulk moduli, shear moduli, Youngs moduli, Poissons ratios and elastic anisotropies for NbB2+x polycrystalline aggregates. The elastic properties obtained here show that the structure becomes less symmetrical as the boron content increases. The higher elastic anisotropies AK and AG were associated with the higher superconducting transition temperature Tc reported for this material (Escamilla et al 2004 J. Phys.: Condens. Matter 16 5979). Using the Debye temperature, θD, and Tc, the electron–phonon coupling constant λ was obtained from the McMillan equation. For the concentrations from (B/Nb) = 2.20(1) to 2.34(1), the values of λ increase from 0.497 to 0.572; this suggests that NbB2+x is a weakly coupled Bardeen–Cooper–Schrieffer type superconductor.

Study of the crystal structure, superconducting and magnetic properties of Ru1−xFexSr2GdCu2O8

Samples from the Ru1−xFexSr2GdCu2O8 system with x = 0, 0.025, 0.05, 0.075, 0.1 and 0.2, were prepared and their structural, superconducting and magnetic properties were studied. Rietveld refinement of the x-ray diffraction patterns shows that the Fe substitution occurs at both Ru and Cu sites. An increase of Fe concentration produces no significant changes in the Ru–O(3)–Ru bond angle, which is a measure of the rotation of the RuO6 octahedra around the c-axis, or in the Cu–O(1)–Ru bond angle , which is a measure of the canting of the RuO6 octahedra. On the other hand, the Cu–O(2)–Cu bond angle, which is a measure of the buckling of the CuO2 layer, has a slight tendency to decrease with increase of the Fe content. We found that both ferromagnetic and superconducting transition temperatures are reduced with increase of the Fe concentration. Analysis related to the decay of the superconducting and ferromagnetic states is presented.

Site occupancy and Tc degradation in iron substituted YBa2(Cu1−xFex)4O8+δ

Abstract A series of YBa2(Cu1−xFex)4O8+δ samples with x=0.00625, 0.0125, 0.01875, 0.025 and 0.05, were synthesized and characterized by resistance vs. temperature measurements, X-ray diffraction and by Mossbauer spectroscopy. In order to make the site assignment for the Fe atoms, a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible oxygen environments around the two Cu sites was carried out, taking into account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyramidal coordination, with the apical oxygen atom placed along the a-axis between the double chains of the unit cell. The presence of these extra oxygen atoms in the crystal structure could be related to the rapid Tc reduction as iron concentration increases.

XPS study of the electronic density of states in the superconducting Mo2B and Mo2BC compounds

The electronic structure of the Mo

Crystal Structure and Ferroelectric Properties of SBT Doped with Praseodymium

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