121 research outputs found
Structural Evolution of One-dimensional Spin Ladder Compounds Sr14-xCaxCu24O41 with Ca doping and Related Hole Redistribution Evidence
Incommensurate crystal structures of spin ladder series Sr14-xCaxCu24O41
(x=3, 7, 11, 12.2) were characterized by powder neutron scattering method and
refined using the superspace group Xmmm(00{\gamma})ss0 (equivalent to
superspace group Fmmm(0,0,1+{\gamma})ss0); X stands for non-standard centering
(0,0,0,0), (0,1/2,1/2,1/2), (1/2,1/2,0,0), (1/2,0,1/2,1/2)) with a modulated
structure model. The Ca doping effects on the lattice parameters, atomic
displacement, Cu-O distances, Cu-O bond angles and Cu bond valence sum were
characterized. The refined results show that the CuO4 planar units in both
chain and ladder sublattices become closer to square shape with an increase of
Ca doping. The Cu bond valence sum calculation provided new evidence for the
charge transfer from the chains to ladders (approximately 0.16 holes per Cu
from x=0 to 12.2). The charge transfer was attributed to two different
mechanisms: (a) the Cu-O bond distance shrinkage on the ladder; (b) increase of
the interaction between two sublattices, resulting in Cu-O bonding between the
chains and ladders. The low temperature structural refinement resulted in the
similar conclusion, with a slight charge backflow to the chains.Comment: 29 pages, 16 figures, submitted to physics review b, accepte
Structural properties in Sr0.61a0.39Nb2O6 in the temperature range 10 K to 500 K investigated by high-resolution neutron powder diffraction and specific heat measurements
We report high-resolution neutron powder diffraction on Sr0.61Ba0.39Nb2O6,
SBN61, in the temperature range 15-500 K. The results indicate that the
low-temperature anomalies (T<100K) observed in the dielectric dispersion are
due to small changes in the incommensurate modulation of the NbO6-octahedra, as
no structural phase transition of the average structure was observed. This
interpretation is supported by specific heat measurements, which show no latent
heat, but a glass-like behavior at low temperatures. Furthermore we find that
the structural changes connected with the ferroelectric phase transition at Tc
approx. 350K start already at 200K, explaining the anisotropic thermal
expansion in the temperature range 200-300K observed in a recent x-ray
diffraction study.Comment: Accepted by PRB (2006
Personal reflections on the history of aperiodic crystals from early days to the state of the art
International audienceAperiodic crystals occur as modulated, intergrowth and quasicrystal structures. Their characterization in terms of symmetries possibly requires not only a higher-dimensional description, but also non-Euclidean transformations (like scaling). These crystallographic symmetries, observed in nature, have been investigated but only partially classified. Turning points in the structure determination of aperiodic crystals are presented in historical perspective up to the present time, together with methods and programs. The WEB allows to get more information on all that, on additional tools and on databases. An attempt to present challanges towards lines of future research concludes the review
Navigating institutional pressure in state-socialist and democratic regimes: The case of movement brontosaurus
Using the case of Movement Brontosaurus, a Czech organization founded in state socialist times, this article investigates how civic associations and nongovernmental organizations seeking to promote alternatives to the status quo respond to institutional pressures in different political and social contexts. The case shows that under state socialism, Brontosaurus appeared to conform to state mandates and societal expectations. However, its formal structure was decoupled from many activities to obscure its oppositional intent.After the transition to democracy, the organization was only able to maintain its place in society after it aligned its structure and practices with each other and openly expressed its alternative agenda. The findings demonstrate how social change and alternative lifestyle organizations vary their responses to institutional pressure in ways that enable them to realize their values and pursue their missions while accounting for the political and social contexts in which they are embedded
Reactivity of Hydrogen-Helium and Hydrogen-Nitrogen Mixtures at High Pressures
Through a series of Raman spectroscopy studies, we investigate the behaviour
of hydrogen-helium and hydrogen-nitrogen mixtures at high pressure across wide
ranging concentrations. We find that there is no evidence of chemical
association, miscibility, nor any demixing of hydrogen and helium in the solid
state up to pressures of 250 GPa at 300 K. In contrast, we observe the
formation of concentration-dependent N-H van der Waals solids, which
react to form N-H bonded compounds above 50 GPa. Through this combined study,
we can demonstrate that the recently claimed chemical association of H-He
can be attributed to significant N contamination and subsequent formation
of N-H compounds.Comment: 16 pages, 8 figures including Supplementary Materia
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