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PubMed2000

Structures of high-density and low-density water

The three site-site partial structure factors for water have been measured as a function of pressure, using neutron diffraction, at a temperature of 268 K. It is found that the measured structure functions imply a continuous transformation with increasing pressure from a low-density form of water ( rho(L) approximately 0.0295 molecules/A(3)), with an open, hydrogen-bonded tetrahedral structure, to a high-density form of water ( rho(H) approximately 0.0402 molecules/A(3)), with nontetrahedral O-O-O angles and a collapsed second coordination shell, which implies broken hydrogen bonds between the first and second coordination shells.

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PubMed2000

Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C

We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron diffraction. The ordering vector is Q(A) = (0.48,0,0) and the phase appears above a critical in-plane magnetic field of 0.9 T. The field was applied in order to test the assumption that the zero-field magnetic structure at Q(F) = (0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity were destroyed. We present theoretical calculations which show that two effects are important: a suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase and a reduction of the superconducting condensation energy due to the periodic modulation of the moments at Q(A).

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PubMed2000

Structure of liquid PEO-LiTFSI electrolyte

The structure of a polymer electrolyte, P(EO)7.5LiN(SO 2CF (3))(2), has been determined by neutron diffraction with isotropic substitution. The Li ions are bonded on average to five ether oxygens belonging to pairs of PEO coils. These are arranged with a considerable degree of extended-range order providing pathways for the Li ion conduction. The lack of ion pairing in this system below 4.8 A is reminiscent of that observed in the remarkable structure of P(EO)6LiAsF (6) and implies that anions and cations are free to migrate independently.

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PubMed2000

Difference between blocking and Néel temperatures in the exchange biased Fe3O4/CoO system.

The blocking temperature T(B) has been determined as a function of the antiferromagnetic layer thickness in the Fe3O4/CoO exchange biased system. For CoO layers thinner than 50 A, T(B) is reduced below the Néel temperature T(N) of bulk CoO (291 K), independent of crystallographic orientation or film substrate ( alpha-Al2O3, SrTiO3, and MgO). Neutron diffraction studies show that T(B) does not track the CoO ordering temperature and, hence, that this reduction in T(B) does not arise from finite-size scaling. Instead, the ordering temperature of the CoO layers is enhanced above the bulk T(N) for layer thicknesses approximately less than or equal to 100 A due to the proximity of magnetic Fe3O4 layers.

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PubMed2000

Pressure-induced hydrogen bonding: structure of D2S phase I'

The full structure of the high-pressure cubic phase I' of hydrogen sulfide has been solved using neutron diffraction data. The molecules are partially rotationally disordered about the <111> axes, as in phase II at ambient pressure but with markedly greater nonuniformity of the toroidal D distribution. The changes in structure at the II-->I' transition signal the onset of significant pressure-induced hydrogen bonding.

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PubMed2000

Origin of low-frequency local vibrational modes in high density amorphous ice.

Incoherent-inelastic neutron scattering data are obtained from 5-80 K for high-density amorphous (hda) ice in the region 0-135 cm(-1). An excess contribution to the vibrational density of states is identified near 20 cm(-1). The origin of these vibrations has been identified by lattice dynamics calculations on an "experimental" structure derived from reverse Monte Carlo analysis of hda ice neutron diffraction data. An interpretation that localized oscillations of short chains and isolated water molecules are responsible for the excess low-frequency modes is consistent with our data.

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PubMed2001

Anomalous temperature dependence of the magnetic field induced antiferromagnetic moment in the antiferroquadrupolar ordered state of CeB6.

The magnetic field induced antiferromagnetic moment M(AF) at low magnetic fields in the antiferroquadrupolar (AFQ) ordered phase of CeB6 was investigated by elastic neutron diffraction experiments for H parallel [110]. The peak intensity at the AF magnetic reciprocal point (1 / 2,1 / 2,1 / 2) corresponding to M(2)(AF) increases with decreasing temperature below the AFQ ordering temperature T(Q), and exhibits a broad maximum at T approximately 3 K and decreases with a further decrease of temperature. This unusual behavior of M(AF) at low fields is explained as a result of the competition between the AF-octupolar and AF-exchange interactions in the O(xy) type AFQ ordered state.

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PubMed2001

Weak Ferromagnetism and Field-Induced Spin Reorientation in K2V3O8.

Magnetization and neutron diffraction studies of the 2D S = 1/2 antiferromagnet, K2V3O8, indicate an ordered state exhibiting weak ferromagnetism and field-induced spin reorientations. Of particular interest is the behavior in a basal plane magnetic field where a unique spin reorientation is observed in which the spins rotate from the easy c axis to the basal plane while remaining normal to the applied field. The experimental observations are well described by a two spin exchange model incorporating Heisenberg and Dzyaloshinskii-Moriya interactions with an additional c-axis anisotropy.

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PubMed2001

Field-induced three- and two-dimensional freezing in a quantum spin liquid.

Field-induced commensurate transverse magnetic ordering is observed in the Haldane-gap compound Ni(C(5)D(14)N(2))2N(3)(PF(6)) by means of neutron diffraction. Depending on the direction of applied field, the high-field phase is shown to be either a three-dimensional ordered Néel state or a short-range ordered state with dominant two-dimensional spin correlations. The structure of the high-field phase is determined, and properties of the observed quantum phase transition are discussed.

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PubMed2001

Magnetic ordering and phase transition in MnO embedded in a porous glass.

We present the results of a neutron diffraction study of the antiferromagnet MnO embedded in a porous glass. The type of magnetic ordering and the structural distortion are similar to those of the bulk, but the ordered magnetic moment of 3.84(4)muB/ion is strongly reduced and the Néel temperature is enhanced. The magnetic transition is second order, in contrast to the first order transition of the bulk. The size of the magnetic region is smaller than the average size of the nanoparticles. The reasons for this behavior are discussed.

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PubMed2001

Partially disordered antiferromagnetic phase in Ca(3)CoRhO(6).

Neutron diffraction experiments are reported on Ca(3)CoRhO(6) which consists of ferromagnetic Ising spin chains on a triangular lattice. It was first confirmed from temperature dependence of the (110) peak intensity that Ca(3)CoRhO(6) realizes a partially disordered antiferromagnetic state, where 2/3 of the ferromagnetic chains order antiferromagnetically with each other and the remaining 1/3 are left incoherent with the other chains. The 1/3 incoherent ferromagnetic Ising chains freeze to maintain a disordered state at lower temperatures. This compound is successfully discussed as a candidate of a nonequilibrium one-dimensional Ising model.

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PubMed2002

First-order transition in the spin dynamics of geometrically frustrated Yb2Ti2O7.

Using neutron diffraction, 170Yb Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb2Ti2O7, where the Yb3+S' = 1/2 ground state has planar anisotropy. Below approximately 0.24 K, the temperature of the known specific-heat lambda transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime.

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PubMed2002

Chemical isomerism as a key to explore free-energy landscapes in disordered matter.

The effects of a minor chemical modification on the microscopic structure of a material in its glass and crystal phases are investigated by the concurrent use of neutron diffraction and computer simulation. Significant changes in short-, intermediate-, and long-range order are found, resulting from the change in molecular structure. These differences are explainable by a shift in the balance between directional and excluded-volume interactions.

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PubMed2002

Magnetic structure, phase diagram, and a new type of spin-flop transition dominated by higher order interaction in a localized 5f system U3Pd20Si6.

The magnetic structure of the localized-5f uranium intermetallic compound U3Pd20Si6 has been determined by means of a neutron diffraction experiment. Our data demonstrate that this compound has a collinear coupling of the sublattice ordering of the uranium spins on the 4a and 8c sites. We conclude that higher-order exchange and/or quadrupole interactions are necessary to stabilize this unique collinear structure. We discovered a new type of spin-flop transition against the uniaxial anisotropy induced by this collinear coupling.

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PubMed2002

Zener polaron ordering in half-doped manganites.

We have refined the crystal structures of a Pr(0.60)Ca(0.40)MnO(3) single crystal from neutron diffraction data. The result at low temperature gives a superstructure that cannot be interpreted as Mn(3+)/Mn(4+) charge ordering. The pattern of atom displacements suggests the trapping of electrons within pairs of Mn sites, involving both a local double exchange and a polaronic-like distortion. The two mechanisms act together to form vibronic localized electronic states: Zener polarons. We have confirmed this picture by showing how it elucidates the unconventional paramagnetic behavior of half-doped manganites.

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PubMed2002

Unconventional metallic magnetism in LaCrSb3.

Neutron-diffraction measurements in LaCrSb3 show a coexistence of ferromagnetic and antiferromagnetic sublattices below T(C)=126 K, with ordered moments of 1.65(4) and 0.49(4)mu(B)/formula unit, respectively (T=10 K), and a spin-reorientation transition at approximately 95 K. No clear peak or step was observed in the specific heat at T(C). Coexisting localized and itinerant spins are suggested.

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PubMed2002

Temperature evolution of sodium nitrite structure in a restricted geometry.

The NaNO2 nanocomposite ferroelectric material in porous glass was studied by neutron diffraction. For the first time, the details of the crystal structure including positions and anisotropic thermal parameters were determined for the solid material, embedded in a porous matrix, in ferro- and paraelectric phases. It is demonstrated that in the ferroelectric phase the structure is consistent with bulk data, but above transition temperature the giant growth of amplitudes of thermal vibrations is observed, resulting in the formation of a "premelted state." Such a conclusion is in good agreement with the results of dielectric measurements published earlier.

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PubMed2002

Ab initio molecular dynamics investigation of the structure and the noncollinear magnetism in liquid oxygen: occurrence of O4 molecular units.

We modeled liquid oxygen using ab initio molecular dynamics in which both the atomic structure and the noncollinear magnetic structure evolve without constraints. The atomic structure shows preference for parallel alignment of first-neighbor molecules and is supported by an excellent agreement between theoretical and experimental nuclear structure factors. The magnetic structure shows short-range antiferromagnetic correlations in agreement with spin-polarized neutron diffraction data. The observed correlations primarily result from appropriate trajectories of colliding O2 molecules. The simulation provides evidence for the occurrence of long-living O4 molecular units.

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