J. N. Reimers

Effects of Impurities on the Electrochemical Properties of LiCoO2

LiCoO[sub 2] is currently used as a cathode material in commercial, high energy density lithium ion cells. Li[sub x] CoO[sub 2] undergoes two successive phase transitions near x = 1/2, which are clearly seen in the voltage profile, when Li is intercalated or deintercalated. The temperature dependence of these phase transitions determines a phase diagram. The phase transitions are very sensitive to impurity levels and can be used as a quality control check on the material. These transitions are also an indicator of composition and absolute cathode potential (vs. Li metal) in Li ion cells which have no fixed voltage reference.

Crystal structure, short range and long range magnetic ordering in CuSb2O6

CuSb2O6 crystallizes in a monoclinically distorted trirutile structure. Atomic positions were determined by profile refinement of neutron powder diffraction data, space group P21n, a = 4.6349(1), b = 4.6370(1), c = 9.2931(1), β = 91.124(2). Magnetic susceptibility data exhibit a broad maximum at about 60 K and an abrupt transition at 8.5 K. The high temperature data can be fitted to a Curie-Weiss Law giving μeff = 1.758 and θ = −48 K. Although the crystal structure indicates a nearly square planar Cu2+ lattice as in other trirutiles, a CuOOCu superexchange pathway seems to be dominant, giving rise to short range correlations which are approximately one dimensional. The high-temperature susceptibility is explained well by the 1-d Heisenberg model with Jk = −43.1 K. Analysis by Oguchis method gives a ratio of interchain to intrachain coupling constants of about 2 × 10−3.

Defect ordering in YBa2Cu3O6.5 and YBa2Cu3O6.6: Synthesis and characterization by neutron and electron diffraction

Abstract Polycrystalline samples of YBa2Cu3O6.5 and YBa2Cu3O6.6 were prepared by oxygen titration of YBa2 Cu3O6.0 at 450°C followed by slow cooling to room temperature. Both samples showed evidence for thea′ = 2a supercell in individual grains by electron diffraction as reported previously. In addition the superlattice was observed in neutron powder diffraction indicating that the bulk material is also well ordered. In this study the YBa2Cu3O6.6 phase showed longer correlation lengths for ordering along botha* andb* than YBa2Cu3O6.5. For the former compound the powder-averaged, sample-averageda* correlation distance is 26A˚from neutron diffraction. Analysis of electron diffraction profiles on selected single crystals give correlation lengths alonga*, b*, andc* of 100, 200, and 50A˚, respectively. Dark field imaging discloses the presence of striped, ordered domains elongated alongb* with a distribution of sizes. Both neutron diffraction and dark field imaging indicate that the volume fraction of the ordered domains is about 50%. A correlation is noted between the Meissner Effect and the extent of defect ordering in the bulk samples of the two phases.

Short-range ordering in a three-dimensionally frustrated magnet, Tb2Mo2O7, by wide- and small-angle neutron diffraction

Tb2Mo2O7 is an ordered crystalline oxide with the pyrochlore structure. This material undergoes a magnetic transition at 25 K which is similar to that of a spin glass. Neutron diffraction data indicate the presence of short range magnetic order via broad, diffuse reflections which persist above 25 K. Below 25 K the diffuse peaks increase in intensity but do not narrow. Fourier analysis of the data yield an approximation to the radial distribution function which is interpreted in terms of spin–spin correlations up to fourth neighbors. The first, third, and fourth neighbor interactions are ferromagnetic but the second neighbor interaction is strongly antiferromagnetic and involves only a Tb‐Mo coupling. SANS data show magnetic scattering only at large Q values, reenforcing the view that antiferromagnetic interactions dominate. The Q‐integrated magnetic SANS intensities show a temperature dependence similar to that for the correlation functions.

Experimental and theoretical evidence for a new universality class in FeF3: A 3D lattice with frustrated Heisenberg spins (abstract)

The pyrochlore form of FeF3 is a Heisenberg antiferromagnet [TN=15.2(1)K], in which the Fe3+ atoms form a high‐symmetry lattice of corner‐sharing tetrahedra. The low‐temperature magnetic structure is noncollinear with four sublattices oriented 109° from each other. Neutron diffraction was used to determine the critical exponent, β=0.17(2), which does not correspond to any known universality class. Monte Carlo simulations with finite‐size scaling analysis confirm the measured value of β and also provide the exponent ν=0.34(4). The existence of this new universality class supports Kawamura’s hypothesis that critical properties of a system depend on the symmetry properties (and not just the dimensionality) of the order parameter.

Frustration and short range magnetic correlations in the pyrochlore Y2Mn2O7 (abstract)

The bulk magnetic susceptibility of Y2Mn2O7 is suggestive of ferromagnetic order, θc=+40 K, apparent Tc=25 K. However, both dc and ac susceptibility data show a broad maximum near 6 K which is frequency and field dependent. Heat capacity studies to 2 K show no evidence for long range magnetic order. Also, neutron scattering data indicate the presence of short range correlations. These results are compared to mean field theory predictions for the highly frustrated 16c sites in the pyrochlore lattice.

Neutron scattering from the geometrically frustrated pyrochlore antiferromagnet Tb2Mo2O7 (abstract)

The magnetic metal ions in the cubic pyrochlore Tb2Mo2O7 form an infinite three‐dimensional network of corner‐sharing tetrahedra with a very high potential for frustration in the presence of antiferromagnetism. Previous bulk susceptibility measurements show strong irreversibilities below a temperature of Tf∼25 K. We have performed neutron scattering measurements that show short range spatial correlations that develop continuously with decreasing temperature, while the characteristic time scale for the fluctuating moments decreases dramatically below Tf. Further, the results indicate extremely slow spin dynamics are relevant near Tf. The low temperature state of this pure material, which exhibits frustration that is solely geometrical in origin, closely resembles that of a spin glass.

Frustration and absence of long range order on the corner sharing‐tetrahedron lattice within mean field theory (abstract)

The metal atoms in the pyrochlore system of compounds (A2B2O7, where A and B are metals) form an infinite 3D network of corner sharing tetrahedra with cubic symmetry. For antiferromagnetic nearest neighbor interactions and only B atoms magnetic, there is a very high degree of frustration, and no long range order is predicted in the absence of further neighbor interactions. Mean field calculations for n‐component classical vector spins, show that the wave vector dependent susceptibility diverges, at the same temperature Tc, for 2N normal modes of the system where N is the number of unit cells. In some cases further neighbor interactions will stabilize the q=0 or incommensurate modes. A comparison will be made with long range order known to exist in the pyrochlore form of FeF3.

Crystal structure and short‐range and long‐range order in CuSb2O6 (abstract)

CuSb2O6 crystallizes in a monoclinically distorted trirutile structure. Atomic positions were determined by profile refinement of neutron powder diffraction data. An important feature of the structure is the presence of nearly square planar Cu2+ layers. The Cu‐O‐Cu superexchange pathways are such that short‐range interactions are approximately one‐dimensional. Evidence for short‐range order is found from a susceptibility maximum at 60 K. Long‐range order occurs near 25 K and an additional, first‐order transition occurs at 8 K.