120 research outputs found
Topological metal behavior in GeBi2Te4 single crystals
The metallic character of the GeBi2Te4 single crystals is probed using a
combination of structural and physical properties measurements, together with
density functional theory (DFT) calculations. The structural study shows
distorted Ge coordination polyhedra, mainly of the Ge octahedra. This has a
major impact on the band structure, resulting in bulk metallic behavior of
GeBi2Te4, as indicated by DFT calculations. Such calculations place GeBi2Te4 in
a class of a few known non-trivial topological metals, and explains why an
observed Dirac point lies below the Fermi energy at about -0.12eV. A
topological picture of GeBi2Te4 is confirmed by the observation of surface
state modulations by scanning tunneling microscopy (STM).Comment: 10 pages, 8 figure
New structural and magnetic aspects of the nanotube system Na2V3O7
We present new experimental results of low temperature x-ray synchrotron
diffraction, neutron scattering and very low temperature (mK-range) bulk
measurements on the nanotube system {\tube}. The crystal structure determined
from our data is similar to the previously proposed model (P. Millet {\it et
al.} J. Solid State Chem. , 676 (1999)), but also deviates from it in
significant details. The structure comprises nanotubes along the c-axis formed
by stacking units of two V-rings buckled in the -plane. The space group is
P and the composition is nonstoichiometric, Na(2-x)V3O7, x=0.17. The
thermal evolution of the lattice parameters reveals anisotropic lattice
compression on cooling. Neutron scattering experiments monitor a very weak
magnetic signal at energies from -20 to 9 meV. New magnetic susceptibility,
specific heat measurements and decay of remanent magnetization in the 30 mK -
300 mK range reveal that the previously observed transition at ~76 mK is
spin-glass like with no long-range order. Presented experimental observations
do not support models of isolated clusters, but are compatible with a model of
odd-legged S=1/2 spin tubes possibly segmented into fragments with different
lengths
Complex hydrides for energy storage
In the past decades, complex hydrides and complex hydrides-based materials have been thoroughly investigated as materials for energy storage, owing to their very high gravimetric and volumetric hydrogen capacities and interesting cation and hydrogen diffusion properties. Concerning hydrogen storage, the main limitations of this class of materials are the high working temperatures and pressures, the low hydrogen absorption and desorption rates and the poor cyclability. In the past years, research in this field has been focused on understanding the hydrogen release and uptake mechanism of the pristine and catalyzed materials and on the characterization of the thermodynamic aspects, in order to rationally choose the composition and the stoichiometry of the systems in terms of hydrogen active phases and catalysts/destabilizing agents. Moreover, new materials have been discovered and characterized in an attempt to find systems with properties suitable for practical on-board and stationary applications. A significant part of this rich and productive activity has been performed by the research groups led by the Experts of the International Energy Agreement Task 32, often in collaborative research projects. The most recent findings of these joint activities and other noteworthy recent results in the field are reported in this paper
Tetrahedra system Cudaca: high-temperature manifold of molecular configurations governing low-temperature properties
The Cudaca system composed of isolated Cu2+ S=1/2 tetrahedra with
antiferromagnetic exchange should exhibit properties of a frustrated quantum
spin system. ab initio density functional theory calculations for electronic
structure and molecular dynamics computations suggest a complex interplay
between magnetic exchange, electron delocalization and molecular vibrations.
Yet, extensive experimental characterization of Cudaca by means of synchrotron
x-ray diffraction, magnetization, specific heat and inelastic neutron
scattering reveal that properties of the real material can be only partly
explained by proposed theoretical models as the low temperature properties seem
to be governed by a manifold of molecular configurations coexisting at high
temperatures.Comment: 15 figure
Boron-nitrogen based hydrides and reactive composites for hydrogen storage
Hydrogen forms chemical compounds with most other elements and forms a variety of different chemical bonds. This fascinating chemistry of hydrogen has continuously provided new materials and composites with new prospects for rational design and the tailoring of properties. This review highlights a range of new boron and nitrogen based hydrides and illustrates how hydrogen release and uptake properties can be improved
Solving crystal structures of metal and chemical hydrides
The methods of structural characterization of metal and chemical hydrides are reviewed. The existing difficulties and problems are outlined and possible solutions presented. It is shown that powder diffraction, and especially the Direct Space Method, is essential component of hydride research. Crystal structures containing as many as 55 independent atoms (including hydrogen) have been fully characterized using powder diffraction. This is of great importance, because rapid collection of powder data thanks to modem synchrotron and neutron time-of-flight sources opens the possibility for fast in-situ studies, mapping of phase transitions induced by the temperature, pressure, hydrogen content, and chemical reactions. The progress in structural characterization of hydrides goes hand in hand with the progress in the powder diffraction methodology
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