104 research outputs found

    Structural Evolution of One-dimensional Spin Ladder Compounds Sr14-xCaxCu24O41 with Ca doping and Related Hole Redistribution Evidence

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    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

    Navigating institutional pressure in state-socialist and democratic regimes: The case of movement brontosaurus

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    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

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    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 N2_2-H2_2 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 H2_2-He can be attributed to significant N2_2 contamination and subsequent formation of N2_2-H2_2 compounds.Comment: 16 pages, 8 figures including Supplementary Materia

    Anisotropic temperature-field phase diagram of single crystalline β

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    Solution and refinement of magnetic structures with Jana2006

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    The program Jana2006 allows the solution and refinement of regular, twinned, modulated, and composite structures against different diffraction data sets. Recently a new option for solving and refining magnetic structures from powder and single crystal neutron diffraction data has been developed

    Modulated Structure of η´-Cu 3+x

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