D. Rios-Jara

Structural transitions in a TiNiFe shape memory alloy

Results for resistance versus temperature, differential scanning calorimetry (DSC), Moessbauer spectroscopy, and magnetic susceptibility measurements in a Ti[sub 50]Ni[sub 47]Fe[sub 3] sample are presented. The resistance versus temperature curve shows a sharp increase at 245K and then rises slowly until a maximum is attained at [approx]150K. The DSC curve also exhibits a peak at the former temperature. The room temperature Moessbauer spectrum consists of a single line. As the temperature is lowered to 270K, a small asymmetric quadrupole doublet appears. The isomer shift and quadrupole splitting values of this doublet increase with decreasing temperature until almost constant values are achieved at temperatures [approx]100K. The magnetic susceptibility curve shows a sharp decrease that begins at 255K and then an unexpected increase at 90K. The authors relate these observations with a charge density wave-driven premartensitic transition in this system, and with the final martensitic phase.

Local magnetic fields in the Cu sites of YBa2Cus−xFexOy detected by Mössbauer spectroscopy

Abstract The temperature dependence of the Mossbauer parameters of the spectra of an YBa 2 Cu s−x Fe x O y superconducting sample, with x =0.125, together with a strong temperature dependent asymmetry in one of the three observed quadrupole doublets, suggests the existence of a small magnetic field in the positions of the Fe atoms, which are assumed to be only the Cu(1) sites of the 1-2-3 structure.

Praseodymium 1-2-3: Intrinsic structure, oxygen concentration effects, and solid solutions with yttrium, calcium and zinc

Abstract PrBa 2 Cu 3 O 7−y is probably the most enigmatic member of the 1-2-3 family of compounds. It is the only non-superconducting example in this lanthanide series of isomorphic structures. We have studied its structural details, and dependence of its physical properties on oxygen concentration and solid solutions with Y, Ca and Zn, with the objective of understanding its peculiar behavior and the role Pr plays in suppressing superconductivity.

Anomalous behaviour of Moessbauer parameters of a YBa2Cu2.875Fe0.125Oδ superconductor around 110 K

Abstract Transmission Mo¨ssbauer spectra at various temperatures were obtained from YBa2Cu2.875Fe0.125Oδ powder samples. The spectra displayed three quadrupole doublets, one of which behaves differently from the other two. The experimental data allows to infer that the Fe atoms occupy only Cu(1) sites with definite valence states and in three different environments. The model put forward here is consistent with previous experiments.

On the Twin Formation in Orthorhombic Y 1 Ba 2 Cu 3 O 7-Y

Transmission and scanning electron microscopy were used to study some features of twins in the orthorhombic phase of Y 1 Ba 2 Cu 3 O 7-Y compounds. A model that explains the formation and proliferation of thene defectr, is proposed. Some conclusions about the kinetics of formation of these defects are stated, which agree with observed changes in twin densities produced by different cooling rates. Electron energy loss spectrometry (EELS) was ucod to measure relative changes in oxygen, which give some information about the relative value and the stability of oxygen content in twinned regions. A Braggs-Williams model of the order-disorder transition, for oxygen occupancies in the Cu-O 2 planes, predicts an enrichment of vacancies at the boundaries in these oxygen deficient perovskites. This could explain the observed deficiency in oxygen content with respect to the perfect Y 1 Ba 2 Cu 3 O 7 stoichio-metry.

LOW TEMPERATURE MAGNETIC BEHAVIOR OF NI-FE-AL-B SHAPE MEMORY ALLOYS - MAGNETIC SUSCEPTIBILITY AND MOSSBAUER SPECTRA

Magnetic susceptibility measurements and Mossbauer spectra (MS) were performed on a series of Ni x Fe y Al z B shape memory ribbons obtained by a melt spinning technique. The MS at room temperature show quadrupole splittings of two iron sites. At low temperatures (T

PREMARTENSITIC PHASE-TRANSFORMATION IN A TINIFE SHAPE MEMORY ALLOY STUDIED BY MOSSBAUER-SPECTROSCOPY

The temperature behaviour of Mössbauer spectra of a Ti50Ni47Fe3 alloy, and their corresponding parameters, are related to the structural changes associated with the premartensitic charge density wave (CDW) transitions in this system, and with the final martensitic phase.

Superconducting and structural properties of Er1−xRxBa2Cu3Oy compounds with R = Yb,Ho,Gd,Eu,La,Y

Samples of the Er1−xRxBa2Cu3Oy compounds, with R = Yb, Ho, Gd, La, Eu and Y were prepared. The structure of the compounds wa studied by X-ray powder diffractometry and the superconducting properties by resistivity and susceptibility measurements. The Tc was rather insensitive to the substitution of all the elements, except La, in spite of the fact that the changes in the values of lattice parameters of the 1:2:3 structure vary rather smoothly. The dependence of Tc on the La concentration x was analised and explained by a two-dimensional percolation model.

X-Ray Study of Superconducting Y1Ba2Cu3−xFexOy Compounds

Superconducting compounds of Y 1 B 2 C 3−x Fe x O y , with x=0.125, 0.0625, 0.125 and 0.300, were studied using X-ray powder diffractometry. Two sets of samples were prepared. Samples quenched from 950°C to liquid nitrogen temperature showed a tetragonal non-superconducting phase, whose lattice parameters depend on the Fe content and differ from those of the yttrium 1:2:3 tetragonal phase. Samples oxygenated at 450°C showed an orthorhombic to tetragonal phase transition for Fe contents higher than x=0.0625. The lattice parameters of this tetragonal phase were different from those of both tetragonal phases mentioned above. All these observations allow us to conclude that C(l) sites are occupied at least partially by Fe impurities.