14 research outputs found
Direct observation of nanoscale interface phase in the superconducting chalcogenide KFeSe with intrinsic phase separation
We have used scanning micro x-ray diffraction to characterize different
phases in superconducting KFeSe as a function of temperature,
unveiling the thermal evolution across the superconducting transition
temperature (T32 K), phase separation temperature (T520 K)
and iron-vacancy order temperature (T580 K). In addition to the
iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic
minority filamentary phase, we have found a clear evidence of the interface
phase with tetragonal symmetry. The metallic phase is surrounded by this
interface phase below 300 K, and is embedded in the insulating texture.
The spatial distribution of coexisting phases as a function of temperature
provides a clear evidence of the formation of protected metallic percolative
paths in the majority texture with large magnetic moment, required for the
electronic coherence for the superconductivity. Furthermore, a clear
reorganization of iron-vacancy order around the T and T is found
with the interface phase being mostly associated with a different iron-vacancy
configuration, that may be important for protecting the percolative
superconductivity in KFeSe.Comment: 6 pages, 4 figure
Design and Characterization of fast X-ray Cameras for X-ray Photon Correlation Spectroscopy Experiments
Evolution of Size and Optical Properties of Upconverting Nanoparticles during High Temperature Synthesis
We
investigated the growth of β-phase NaYF4:Yb3+,Er3+ upconversion nanoparticles synthesized by
the thermal decomposition method using a combination of in situ and
offline analytical methods for determining the application-relevant
optical properties, size, crystal phase, and chemical composition.
This included in situ steady state luminescence in combination with
offline time-resolved luminescence spectroscopy as well as small-angle
X-ray scattering (SAXS) transmission electron microscopy (TEM), X-ray
diffraction analysis (XRD), and inductively coupled plasma optical
emission spectrometry (ICP-OES). For assessing the suitability of
our optical monitoring approach, the in situ-collected spectroscopic
data, which reveal the luminescence evolution during nanocrystal synthesis,
were compared to measurements done after cooling of the reaction mixture
of the as-synthesized particles. The excellent correlation of the
in situ and time-resolved upconversion luminescence with the nanoparticle
sizes determined during the course of the reaction provides important
insights into the various stages of nanoparticle growth. This study
highlights the capability of in situ luminescence monitoring to control
the efficiency of UCNP synthesis, particularly the reaction times
at elevated temperatures and the particle quality in terms of size,
shape, and crystal structure, as well as luminescence lifetime and
upconversion quantum yield
Colloidal crystallite suspensions studied by high pressure small angle x-ray scattering
We report on high pressure small angle x-ray scattering on suspensions of colloidal crystallites in water. The crystallites made out of charge-stabilized poly-acrylate particles exhibit a complex pressure dependence which is based on the specific pressure properties of the suspending medium water. The dominant effect is a compression of the crystallites caused by the compression of the water. In addition, we find indications that also the electrostatic properties of the system, i.e. the particle charge and the dissociation of ions, might play a role for the pressure dependence of the samples. The data further suggest that crystallites in a metastable state induced by shear-induced melting can relax to a similar structural state upon the application of pressure and dilution with water. X-ray cross correlation analysis of the two-dimensional scattering patterns indicates a pressure-dependent increase of the orientational order of the crystallites correlated with growth of these in the suspension. This study underlines the potential of pressure as a very relevant parameter to understand colloidal crystallite systems in aqueous suspension