103 research outputs found
Pressure-induced lattice instabilities and superconductivity in YBa2Cu4O8 and optimally doped YBa2Cu3O7-{\delta}
Combined synchrotron angle-dispersive powder diffraction and micro-Raman
spectroscopy are used to investigate the pressure-induced lattice instabilities
that are accompanied by T anomalies in YBaCuO, in comparison with the optimally doped YBaCuO and the non-superconducting PrBaCuO. In the first two superconducting systems there is a clear
anomaly in the evolution of the lattice parameters and an increase of lattice
disorder with pressure, that starts at as well as
irreversibility that induces a hysteresis. On the contrary, in the Pr-compound
the lattice parameters follow very well the expected equation of state (EOS) up
to 7 GPa. In complete agreement with the structural data, the micro-Raman data
of the superconducting compounds show that the energy and width of the A phonons show anomalies at the same pressure range where the lattice
parameters deviate from the EOS and the average Cu2-O bond length
exhibits a strong contraction and correlate with the non-linear pressure
dependence of T. This is not the case for the non superconducting Pr
sample, clearly indicating a connection with the charge carriers. It appears
that the cuprates close to optimal doping are at the edge of lattice
instability.Comment: 6 pages, 7 figure
Local lattice distortions vs. structural phase transition in NdFeAsO1-xFx
The lattice properties at low temperatures of two samples of NdFeAsO1-xFx
(x=0.05 and 0.25) have been examined in order to investigate possible
structural phase transition that may occur in the optimally doped
superconducting sample with respect to the non-superconducting low-F
concentration compound. In order to detect small modifications in the ion
displacements with temperature micro-Raman and high resolution synchrotron
powder diffraction measurements were carried out. No increase of the width of
the (220) or (322) tetragonal diffraction peaks and microstrains could be found
in the superconducting sample from synchrotron XRD measurements. On the other
hand, the atomic displacement parameters deviate from the expected behavior, in
agreement with modifications in the phonon width, as obtained by Raman
scattering. These deviations occur around 150 K for both F dopings, with
distinct differences among the two compounds, i.e., they decrease at low doping
and increase for the superconducting sample. The data do not support a hidden
phase transition to an orthorhombic phase in the superconducting compound, but
point to an isostructural lattice deformation. Based on the absence of magnetic
effects in this temperature range for the superconducting sample, we attribute
the observed lattice anomalies to the formation of local lattice distortions
that, being screened by the carriers, can only acquire long-range coherence by
means of a structural phase transition at low doping levels
Lattice anomalies in the FeAs tetrahedra of the NdFeAsO superconductor that disappear at T
High resolution synchrotron X-ray powder diffraction (SXRPD) was used to
study the temperature dependence of the oxygen deficient NdFeAsO
superconducting compound. By employing a dense temperature sampling we have
managed to reveal unnoticed structural modifications that start around
180K, and disappear at the transition temperature. The data show minor
changes of the structural characteristics in the Nd-O charge reservoir layer
while in the superconducting Fe-As layer the FeAs tetrahedron shows
gradual modifications below 180K, which suddenly disappear at T
strongly indicating a connection with superconductivity.Comment: 4 pages, 1 table, 5 figure
Structure Determination Feasibility of Three-Dimensional Electron Diffraction in Case of Limited Data
During the last two decades, three-dimensional electron diffraction (3D ED) has undergone a renaissance, starting with the introduction of precession (Precession Electron Diffraction Tomography, PEDT) that led to variations on the idea of collecting as much of the diffraction space as possible in order to solve crystal structures from sub-micron sized crystals. The most popular of these acquisition methods is based on the continuous tilting/rotation of the crystal (so-called Microcrystal Electron Diffraction, MicroED) akin to the oscillating crystal method in X-ray crystallography, which was enabled by the increase of sensitivity and acquisition speed in electron detectors. While 3D ED data is more complex than the equivalent X-ray data due to the higher proportion of dynamical scattering, the same basic principles of what is required in terms of data quality and quantity in order to solve a crystal structure apply; high completeness, high data resolution and good signal-to-noise statistics on measured reflection intensities. However, it may not always be possible to collect data in these optimum conditions, the most common limitations being the tilt range of the goniometer stage, often due to a small pole piece gap or the use of a non-tomography holder, or the position of the sample on the TEM grid, which may be too close to a grid bar and then the specimen of interest becomes occluded during tilting. Other factors that can limit the quality of the acquired data include the limited dynamic range of the detector, which can result on truncated intensities, or the sensitivity of the crystal to the electron beam, whereby the crystallinity of the particle is changing under the illumination of the beam. This limits the quality and quantity of the measured intensities and makes structure analysis of such data challenging. Under these circumstances, traditional approaches may fail to elucidate crystal structures, and global optimization methods may be used here as an alternative powerful tool. In this context, this work presents a systematic study on the application of a global optimization method to crystal structure determination from 3D ED data. The results are compared with known structure models and crystal phases obtained from traditional ab initio structure solution methods demonstrating how this strategy can be reliably applied to the analysis of partially complete 3D ED data
Characterization of defects in mono-like silicon for photovoltaic applications using X-ray Bragg diffraction imaging
Reply to comment on: “Local lattice distortions vs. structural phase transition in NdFeAsO1-xFx”
M. Calamiotou and E. Liarokapis discuss the reply of researchers on the comment on a research study entitled �Local lattice distortions vs. structural phase transition in NdFeAsO1-xFx�. The NdFeAsO1-xFx system is studied using high reso- lution synchrotron powder diffraction (SXRPD) to investigate the possible structural phase transition that may occur in the optimally doped superconducting sample. Two F concentrations have been examined and the results have been compared with those from micro-Raman measurements concerning the phonon widths. The main conclusion of this study is that a precursor local lattice instability occurs in the NdFeAsO1-xFx sys tem around 150 K for both F dopings
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