Mikhail Sofin

Wigner crystallization in Na3Cu2O4 and Na8Cu5O10 chain compounds

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.

Structures of incommensurate and commensurate composite crystals NaxCuO2 (x = 1.58, 1.6, 1.62)

NaxCuO2 (x approximately 1.6) has been synthesized for different compositions x, resulting in both commensurate and incommensurate composite crystals. The crystal structures are reported for two incommensurate compounds (x=1.58 and 1.62) determined by Rietveld refinements against X-ray powder diffraction data. The incommensurate compounds and commensurate Na8Cu5O10 (x=1.6) are found to possess similar structures, with valence fluctuations of Cu2+/Cu3+ as the origin of the modulations of the CuO2 subsystems; the displacive modulations of Na being defined by the closest Na-O contacts between the subsystems. A comparison of the structure models obtained from single-crystal X-ray diffraction, synchrotron-radiation X-ray powder diffraction and X-ray powder diffraction with Cu Kalpha1 radiation indicates that single-crystal X-ray diffraction is by far the most accurate method, while powder diffraction with radiation from an X-ray tube provides the least accurate structure model.

Incommensurate spin-density modulation in a copper oxide chain compound with commensurate charge order.

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.

Magnetic excitations and phonons in the spin-chain compound Na Cu 2 O 2

We report an inelastic light scattering study of single-crystalline

New Route of Preparation and Properties of NaNiO2

\mathrm{Na}{\mathrm{Cu}}_{2}{\mathrm{O}}_{2}

Wigner Crystallization inNa3Cu2O4andNa8Cu5O10Chain Compounds

, 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.

Synthesis and Characterisation of Na5[CoO2]CO3

NaNiO2 was prepared through oxidation of a Na2NiO2/ NiO mixture (1:1) in dried oxygen at 500 °C. Single crystals have been grown by annealing of NaNiO2 powder at 600 °C for 83 d in a flow of dried oxygen. According to the X-ray analysis of the crystal structure (C2/m, Z = 2, a = 5.3177(2), b = 2.8458(1), c = 5.5819(3) Å , β = 110.409(2)°, R1(all) = 3.4%, 185 independent reflections), the Jahn-Teller distorted NiO6 octahedra, sharing edges, build up layers lying parallel to the ab plane. A phase transition (associated with an energy of 2.9 kJ/mole) to a high temperature rhombohedral form (R3̅m, Z = 3, a = 2.958(1), c = 15.748(2) Å , at 300 °C) was observed by Guinier and DSC measurements at 195 °C. The magnetic susceptibility of the monoclinic phase can be described by the Curie-Weiss law between 100 and 330 K: μ = 2.01μB (g = 2.32), Θ = 37 K, indicating the dominance of ferromagnetic interactions of S = 1/2 spins within the NiO2 layers. Antiferromagnetic interlayer interactions produce an overall antiferromagnetically ordered structure below 18 K.

Helicoidal magnetic order in a clean copper oxide spin chain compound

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.

Ein neuartiges Polyoxocobaltat(II)-Anion in Rb2Co2O3†

Na5[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 °C and annealed at this temperature for 50 h in silver crucibles. Single crystals have been grown by subsequent annealing of the powder at 500 °C 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) Å ). Na5[CoO2]CO3 is isostructural with Na5[NiO2]CO3 and contains Co1+, which is coordinated by two oxygen atoms forming a dumb-bell. Na5CoCO5 decomposes at 600 °C to Na3CoO2 and Na2CO3.