Showing papers by "Christian Sternemann published in 2013"

Microscopic structure of water at elevated pressures and temperatures

Abstract: We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley’s K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6 at 600 °C and p = 134 MPa.

A Solid‐Solution Approach to Mixed‐Metal Metal–Organic Frameworks – Detailed Characterization of Local Structures, Defects and Breathing Behaviour of Al/V Frameworks

Abstract: Invited for the cover of this issue is the group of Roland Fischer at Ruhr University Bochum. The cover image shows that the structure/properties of materials vary as the complexity of the framework is increased and different metals are combined in single-phased MOFs.

Stress-induced stabilization of crystals in shape memory natural rubber.

Abstract: In contrast to all known shape memory polymers, the melting temperature of crystals in shape memory natural rubber (SMNR) can be greatly manipulated by the application of external mechanical stress. As shown previously, stress perpendicular to the prior programming direction decreases the melting temperature by up to 40 K. In this study, we investigated the influence of mechanical stress parallel to prior stretching direction during programming on the stability of the elongation-stabilizing crystals. It was found that parallel stress stabilizes the crystals, which is indicated by linear increase of the trigger temperature by up to 17 K. The crystal melting temperature can be increased up to 126.5 °C under constrained conditions as shown by X-ray diffraction measurements.

Structural changes in amorphous Ge(x)SiO(y) on the way to nanocrystal formation.

Abstract: Temperature induced changes of the local chemical structure of bulk amorphous GexSiOy are studied by Ge K-edge x-ray absorption near-edge spectroscopy and Si L2/3-edge x-ray Raman scattering spectroscopy. Different processes are revealed which lead to formation of Ge regions embedded in a Si oxide matrix due to different initial structures of as-prepared samples, depending on their Ge/Si/O ratio and temperature treatment, eventually resulting in the occurrence of nanocrystals. Here, disproportionation of GeOx and SiOx regions and/or reduction of Ge oxides by pure Si or by a surrounding Si sub-oxide matrix can be employed to tune the size of Ge nanocrystals along with the chemical composition of the embedding matrix. This is important for the optimization of the electronic and luminescent properties of the material.

X-ray Raman scattering: An exciting tool for the study of matter at conditions of the Earth's interior

Abstract: The study of minerals and melts at in situ conditions is highly relevant to understand the physical and chemical properties of the Earth's crust and mantle. Here, x-ray Raman scattering provides a valuable tool to investigate the local atomic and electronic structure of Earth materials consisting predominantly of low Z elements at high pressures and temperatures. The capabilities of x-ray Raman scattering to investigate silicate minerals, glasses, and melts are discussed and the application of the method to in situ studies of silicate melts using a hydrothermal diamond anvil cell is demonstrated.

Study of time and pressure dependent phenomena at the hard x-ray beamline BL9 of DELTA

Abstract: The beamline BL9 of DELTA (Dortmund ELecTron Accelerator) is a multi-purpose beamline operating in an energy range between 4 and 27 keV. A short overview of the beamline and the experimental endstation is given. Exemplarily three typical applications, namely x-ray diffraction from interfaces, small angle x-ray scattering under high hydrostatic pressure and fast x-ray reflectivity measurements, are discussed in some detail in order to demonstrate the capabilities of the beamline.

CsNaC2, CsKC2, CsRbC2 – Syntheses and Crystal Structures of Three New Acetylides

Abstract: Three new alkali metal acetylides CsNaC2, CsKC2, and CsRbC2 have been synthesized and characterized by means of synchrotron powder diffraction studies. As a new synthetic approach, the binary alkali metal acetylides were reacted at relatively low temperatures (200 °C). DSC measurements were performed to prove the general usability of this reaction. CsKC2 and CsRbC2 crystallize in a variant of the anti-PbCl2-type structure (Pnma, Z = 4), while for CsNaC2 a new structure type (Pbcm, Z = 4) is found.