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ABSTRACT: The crystal structure and magnetic ordering pattern of PdAs2O6 were
investigated by neutron powder diffraction. While the magnetic structure of
PdAs2O6 is identical to the one of its isostructural 3d-homologue NiAs2O6, its
N\'{e}el temperature (140 K) is much higher than the one of NiAs2O6 (30 K).
This is surprising in view of the long distance and indirect exchange path
between the magnetic Pd$^{2+}$ ions. Density functional calculations yield
insight into the electronic structure and the geometry of the exchange-bond
network of both PdAs2O6 and NiAs2O6, and provide a semi-quantitative
explanation of the large amplitude difference between their primary exchange
interaction parameters.
03/2012;
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ABSTRACT: Single-crystal neutron diffraction has been used to determine the incommensurate magnetic structure of NaCu2O2, a compound built up of chains of edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice symmetry were identified by a group-theoretical analysis, and their magnetic structure factors were compared to the experimentally observed Bragg intensities. In conjunction with other experimental data, this analysis yields an elliptical helix structure in which both the helicity and the polarization plane alternate among copper-oxide chains. This magnetic ground state is discussed in the context of the recently reported multiferroic properties of other copper-oxide chain compounds.
04/2010;
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ABSTRACT: Neutron diffraction has been used to determine the magnetic structure of Na8Cu5O10, a stoichiometric compound containing chains based on edge-sharing CuO4 plaquettes. The chains are doped with 2/5 hole per Cu site and exhibit long-range commensurate charge order with an onset well above room temperature. Below TN=23 K, the neutron data indicate long-range collinear magnetic order with a spin-density modulation whose propagation vector is commensurate along, and incommensurate perpendicular to, the chains. Competing interchain exchange interactions are discussed as a possible origin of the incommensurate magnetic order.
Physical Review Letters 08/2008; 101(4):047202. · 7.37 Impact Factor
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ABSTRACT: We report an inelastic light scattering study of single-crystalline NaCu$_2$O$_2$, a spin-chain compound known to exhibit a phase with helical magnetic order at low temperatures. Phonon excitations were studied as a function of temperature and light polarization, and the phonon frequencies are compared to the results of ab-initio lattice dynamical calculations, which are also reported here. The good agreement between the observed and calculated modes allows an assignment of the phonon eigenvectors. Two distinct high-energy two-magnon features as well as a sharp low-energy one-magnon peak were also observed. These features are discussed in terms of the magnon modes expected in a helically ordered state. Their polarization dependence provides evidence of substantial exchange interactions between two closely spaced spin chains within a unit cell. At high temperatures, the spectral features attributable to magnetic excitations are replaced by a broad, quasielastic mode due to overdamped spin excitations.
11/2005;
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ABSTRACT: We report susceptibility, specific heat, and neutron diffraction measurements on NaCu2O2, a spin-1∕2 chain compound isostructural to LiCu2O2, which has been extensively investigated. Below 12 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu2O2. In contrast to the Li analog, substitutional disorder is negligible in NaCu2O2. We can thus rule out that the helix is significantly influenced by impurities. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.
Physical Review B 05/2005; 71:140402. · 3.69 Impact Factor
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ABSTRACT: We report susceptibility, specific heat, and neutron diffraction measurements on NaCu$_2$O$_2$, a spin-1/2 chain compound isostructural to LiCu$_2$O$_2$, which has been extensively investigated. Below 13 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu$_2$O$_2$. In contrast to the Li analogue, substitutional disorder is negligible in NaCu$_2$O$_2$. We can thus rule out that the helix is induced by impurities, as was claimed on the basis of prior work on LiCu$_2$O$_2$. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.
Physical Review B 04/2005; 71:140402. · 3.69 Impact Factor
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ABSTRACT: We report the synthesis of novel edge-sharing chain systems Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10), which form insulating states with commensurate charge order. We identify these systems as one-dimensional Wigner lattices, where the charge order is determined by long-range Coulomb interaction and the number of holes in the d shell of Cu. Our interpretation is supported by x-ray structure data as well as by an analysis of magnetic susceptibility and specific heat data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems allow for a distinction between the (classical) Wigner lattice and the 4k(F) charge-density wave of quantum mechanical origin.
Physical Review Letters 03/2005; 94(7):076403. · 7.37 Impact Factor
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ABSTRACT: We report the synthesis of novel edge-sharing chain systems Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10), which form insulating states with commensurate charge order. We identify these systems as one-dimensional Wigner lattices, where the charge order is determined by long-range Coulomb interaction and the number of holes in the d-shell of Cu. Our interpretation is supported by X-ray structure data as well as by an analysis of magnetic susceptibility and specific heat data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems allow for a distinction between the (classical) Wigner lattice and the 4k_F charge-density wave of quantum mechanical origin.
02/2005;
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ABSTRACT: Rb2Co2O3 was prepared via the azide/nitrate route. Mixtures of the precursors Co3O4, RbN3 and RbNO3 in the molar ratios 6:17:1 were heated in a special regime up to 450 degreesC and annealed at this temperature for 50 It in silver crucibles. Single crystals have been grown by subsequent annealing of prepared powder at 450 degreesC for 500 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure (Pnma, Z = 8, 11.729(2), 6.058 (1), 8.004(1) Angstrom) cobalt is trigonal planar coordinated by oxygen atoms. The CoO3-units share through all corners and build up an infinite two-dimensional (infinityCo2O3)-Co-2-network.
Zeitschrift für anorganische und allgemeine Chemie, v.628, 2697-2700 (2002).
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ABSTRACT: Na-5[CoO2]CO3 was prepared via the azide/nitrate route. Stoichiometric mixtures of the precursors (Co3O4, NaN3, NaNO3 and Na2CO3) were heated in a special regime up to 500 degreesC and annealed at this temperature for 50 h in silver crucibles. Single crystals have been grown by subsequent annealing of the powder at 500 degreesC for 2000 h in silver crucibles, which were sealed in glass ampoules under dry Ar. According to the X- ray analysis of the crystal structure (P4/mmm, Z = 1, a = 4.6467(4), c = 8.2577(6) Angstrom). Na-5[CoO2]CO3 is isostructural with Na-5[NiO2]CO3 and contains Co1+, which is coordinated by two oxygen atoms forming a dumb-bell. Na5CoCO5 decomposes at 600 degreesC to Na3CoO2 and Na2CO3.
Zeitschrift für Naturforschung B, v.57, 1461-1463 (2002).
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Zeitschrift für anorganische und allgemeine Chemie, v.628, 2500-2504 (2002).
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Angewandte Chemie International Edition, v.42, 3527-3529 (2003).
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ABSTRACT: Nuclear and magnetic structures of sodium ruthenate (VI) have been studied by neutron powder diffraction in the temperature range 1.5–200 K. Na 2 RuO 4 crystallizes in the monoclinic structure, with space group P 2 1 /c. The structure consists of apical corner sharing RuO 5 trigonal bipyramids forming infinite chains running along the b axis. These infinite [ RuO 3 O 2/2 ] chains form a pseudo hexagonal close packing of rods with Ru–Ru distances of 3.51 Å within the chains and 5.30–5.47 Å between the chains. At T N =37.2 K a magnetic transition leads to an antiferromagnetic state. The Ru 6+ magnetic moments are ordered antiferromagnetically along the chains (b-axis), while the inter-chain interaction is ferromagnetic. A classical infinite chain model was fitted to the magnetic susceptibility data in order to estimate the strength of the nearest-neighbor exchange interactions along and between the chains, resulting in an intrachain coupling parameter of 2J=-86 K, and an interchain parameter J ⊥ with $\vert $ 2J ⊥ $\vert $ = 3 K. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2006
The European Physical Journal B - Condensed Matter and Complex Systems. 52(3):371-376.
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ABSTRACT: New experiments on V2O5 have been performed using large volume high pressure and high temperature devices, to pressures up to 29 GPa and temperatures up to 1500 °C. Post-temperature and pressure quench, samples have been structurally analysed using X-ray diffraction and Raman spectroscopy. Data obtained confirms earlier experimental results at elevated pressures and temperatures, and reveals the stability range of the high-pressure phases far beyond the previously known region. Rietveld refinement of the δ-phase of V2O5 shows that the phase crystallizes in space group C12/c1 (no. 15) with a = 11.9719(2) Å, b = 4.7017(1) Å, c = 5.3253(1) Å, β = 104.41(0)°, V = 290.32(7) Å3 and Z = 4. The tentative mapping of phase boundaries is also in agreement with our theoretical predictions for pressure-induced transitions at absolute zero temperature.
Journal of Alloys and Compounds 429:87-98. · 2.29 Impact Factor
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ABSTRACT: We have investigated SeO2 at high pressures and high temperatures. Two new phases (β-SeO2 and γ-SeO2) and the boundary separating them have been found, following experimental runs performed at pressures up to 15 GPa and temperatures up to 820°C. The two phases crystallize in the orthorhombic system in space group Pmc21 (no. 26) with a=5.0722(1) Å, b=4.4704(1) Å, c=7.5309(2) Å, V=170.760(9) Å3 and Z=4 for the β-phase, and with a=5.0710(2) Å, b=4.4832(2) Å, c=14.9672(6) Å, V=340.27(3) Å3 and Z=8 for the γ-phase. Both phases are stable at ambient pressure and temperature below −30°C. At ambient temperature the phases return to the starting phase (α-SeO2) in a few days. We discuss our findings in relation to a previous report of in-situ measurements at high pressures and ambient temperature.
Journal of Solid State Chemistry. 177:1631-1638.
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ABSTRACT: We report an inelastic light scattering study of single-crystalline NaCu2O2, a spin-chain compound known to exhibit a phase with helical magnetic order at low temperatures. Phonon excitations were studied as a function of temperature and light polarization, and the phonon frequencies are compared to the results of ab initio lattice dynamical calculations, which are also reported here. The good agreement between the observed and calculated modes allows an assignment of the phonon eigenvectors. Two distinct high-energy two-magnon features as well as a sharp low-energy one-magnon peak were also observed. These features are discussed in terms of the magnon modes expected in a helically ordered state. At high temperatures, the spectral features attributable to magnetic excitations are replaced by a broad, quasielastic mode due to overdamped spin excitations.
Phys. Rev. B. 73(9).
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ABSTRACT: The crystal structure and magnetic ordering pattern of PdAs2O6 were investigated by neutron powder diffraction. While the magnetic structure of PdAs2O6 is identical to that of its isostructural 3d homologue NiAs2O6, its Néel temperature (140 K) is much higher than that of NiAs2O6 (30 K). This is surprising in view of the long distance and indirect exchange path between the magnetic Pd2+ ions. Density-functional calculations yield insight into the electronic structure and the geometry of the exchange-bond network of both PdAs2O6 and NiAs2O6, and provide a semiquantitative explanation of the large amplitude difference between their primary exchange interaction parameters.
Phys. Rev. B. 85(11).
