Fiona C. Coomer

Spin-liquid behavior in a kagomé antiferromagnet: Deuteronium jarosite

The spin dynamics of the frustrated quasi–two-dimensional spin-5/2 Heisenberg antiferromagnetic kagome compound deuteronium jarosite, (D3O)Fe3(SO4)2(OD)6, has been studied using inelastic neutron scattering. The dynamic magnetic susceptibility shows a clear spin-liquid behavior with strong dynamic short-range correlations with staggered chirality, as seen from the wave vector dependence at finite-energy transfers. The magnetic scattering at low temperatures is gapless and extends out to at least 20 meV. The dynamics shows a non-exponential relaxation behavior but no dynamical ω/T scaling. The characteristic energy (inverse relaxation rate) increases linearly with temperature.

Structural and magnetic properties of Ba2LuMoO6: a valence bond glass

We report here the synthesis of the site ordered double perovskite Ba(2)LuMoO(6). Rietveld refinement of room temperature powder x-ray diffraction measurements indicates that it crystallizes in the cubic space group Fm3m, with a = 8.3265(1) Å. Powder neutron diffraction data indicate that, unusually, this cubic symmetry is maintained down to 2 K, with [Formula: see text], Mo(5+) ions situated on the frustrated face-centred cubic lattice. Despite dc-susceptibility measurements showing Curie-Weiss behaviour with strong antiferromagnetic interactions at T ≥ 200 K, there is no evidence of long range magnetic ordering at 2 K. At T ≤ 50 K, susceptibility measurements indicate a loss in moment to ∼18% of the expected value, and there is a corresponding loss in the magnitude of the magnetic exchange. The structural and magnetic properties of this compound are compared with the related compound Ba(2)YMoO(6), which is a valence bond glass.

Inelastic neutron scattering study of magnetic excitations in the kagome antiferromagnet potassium jarosite

We report an inelastic neutron scattering study of coherent magnetic excitations in powder and single-crystal samples of the model kagome antiferromagnet potassium iron jarosite, KFe3(OH)6(SO4)2. Initial measurements on a natural single crystal using a triple-axis spectrometer revealed a mode with a zone-centre gap of 7 meV that showed little dispersion within the kagome layers, as well as some indication of a mode with a zone-boundary energy of approximately 20 meV. However, the high background from hydrogen in the sample made it very difficult to search for other excitations. In the absence of suitable deuterated crystals, measurements were performed on deuterated powders using time-of-flight neutron spectrometers over a range of temperatures that include . This confirmed the flat 7 meV mode as well as dispersive modes that reached to higher energies. The origin of these modes is discussed in relation to the most likely Hamiltonian for the magnetic degrees of freedom in this material, and estimates are made of the strength of the nearest-neighbour exchange, J1, and contributions from a Dzyaloshinsky–Moriya interaction or single-ion anisotropy arising from a crystal field.

Structural and magnetic study of order−disorder behavior in the double perovskites Ba2Nd1−xMnxMoO6

The synthesis and structural and magnetic characterization of the site-ordered double perovskites, Ba2Nd1-xMnxMoO6, 0 < x ≤ 1, are reported in order to show the effect of doping Jahn-Teller active, S = 1/2, Mo(5+) into the structure of Ba2MnMoO6, which exhibits anomalous long-range antiferromagnetic order. Rietveld refinements against room temperature neutron powder diffraction data indicate that the tetragonal distortion present in the Ba2NdMoO6 end member persists to x ≤ 0.3. This is predominantly manifested as a tilting of the MO6 octahedra, and there is no evidence of any structural phase transitions on cooling to 1.5 K. For x > 0.3, no deviation from the ideal cubic Fm3̅m symmetry is observed. Furthermore, dc-susceptibility measurements confirm that Mn(2+) is being doped onto the Nd(3+) site, and the associated oxidation of Mo(5+) to Mo(6+). For all compositions, the Curie-Weiss paramagnetic behavior above 150 K indicates negative Weiss constants that range from -24(2) and -85(2) K. This net antiferromagnetic interaction is weakest when x ≈ 0.5, where the disorder in cation site occupancy and competition with ferromagnetic interactions is the greatest. Despite these strong antiferromagnetic interactions, there is no evidence in the dc-susceptibility of a bulk cancellation of spins for x > 0.05. Low-temperature neutron diffraction measurements indicate that there is no long-range magnetic order for 0.1 ≤ x < 0.9. Ba2Nd0.10Mn0.90MoO6 exhibits additional Bragg scattering at 2 K, indicative of long-range antiferromagnetic ordering of the Mn(2+) cations, with a propagation vector k = (1/2, 1/2, 1/2). The scattering intensities can be modeled using a noncollinear magnetic structure with the Mn(2+) moments orientated antiferromagnetically along the four different ⟨111⟩ directions.

Poly([1,4]Dithiino[2,3-c]Furan): the synthesis, electrochemistry, and optoelectronic properties of a furan-containing polymer

The chemical synthesis of a novel polyfuran, poly(2,3-bis(hexylthio)-[1,4]dithiino[2,3-c]furan) (PBDF), substituted at the 2,3-positions with an S-alkylated dithiin unit, is reported. The new polymer has been characterized in terms of its electronic absorption, electrochemical, and thermal properties. Employment of the dithiin moiety provides intrinsic additional electroactivity, as well as a functional handle for substitution with alkyl groups, enhancing the processability of the polymer. The new polymer is compared with the closely related and well-established literature compounds PEDOT and PEDTT as well-studied, highly chalcogenated polythiophenes.

Insulating to metallic behaviour in the cation ordered perovskites Ba2Nd1−xFexMoO6

The series of cation ordered double perovskites Ba2Nd1−xFexMoO6 undergo a compositionally-driven transition from localised to delocalised electronic behaviour, as exhibited in the end members Ba2NdMoO6 and Ba2FeMoO6 respectively. Rietveld structural analyses against neutron diffraction data indicate that all compounds are stoichiometric in oxygen and show replacement of Nd3+ with Fe3+ on the larger of the two octahedral sites in the cation-ordered structure. A tetragonal distortion persists up to x = 0.25 and Ba2Nd0.9Fe0.1MoO6 shows freezing of magnetic moments at 5 K. Neutron scattering indicates an absence of long range magnetic ordering suggesting the formation of a spin glass phase below this temperature. Ba2Nd0.75Fe0.25MoO6 shows high electrical resistivity with a temperature dependence indicative of fully localised electronic behaviour. Despite the Fe3+ occupation (0.25) being above the percolation limit (0.195) for the face centred cubic lattice, this compound shows no magnetic ordering at 2 K. Compositions in the range 0.30 ≤ x ≤ 0.85 give a mixture of two perovskite phases with lattice parameters of ca. 8.4 and 8.1 A. The single phase compositions Ba2Nd0.10Fe0.90MoO6 and Ba2Nd0.05Fe0.95MoO6 form face centred cubic structures with long range magnetic ordering of the Fe3+ moments below ferrimagnetic ordering transitions of 270 and 285 K respectively. Neutron diffraction shows almost complete parallel alignment of the Fe3+ moments and, combined with conductivity measurements showing delocalised electronic behaviour in Ba2Nd0.10Fe0.90MoO6, indicate ferrimagnetic ordering of Fe3+ and delocalised Mo5+.

Li-ion diffusion in Li intercalated graphite C6Li and C12Li probed by mu+SR

In order to study a diffusive behavior of Li+ in Li intercalated graphites, we have measured muon spin relaxation (μ+SR) spectra for C6Li and C12Li synthesized with an electrochemical reaction between Li and graphite in a Li-ion battery. For both compounds, it was found that Li+ ions start to diffuse above 230 K and the diffusive behavior obeys a thermal activation process. The activation energy (Ea) for C6Li is obtained as 270(5) meV, while Ea = 170(20) meV for C12Li. Assuming a jump diffusion of Li+ in the Li layer of C6Li and C12Li, a self-diffusion coefficient DLi at 310 K was estimated as 7.6(3) × 10-11 (cm2 s-1) in C6Li and 14.6(4) × 10-11 (cm2 s-1) in C12Li.

Structural and magnetic study of Yb{sup 3+} in the perovskites Sr{sub 2}YbMO{sub 6} (M=Nb, Ta, Sb)

The compounds Sr2YbNbO6, Sr2YbTaO6 and Sr2YbSbO6 have been prepared using solid state methods by heating pelleted reagents in air at temperatures up to 1400°C. Rietveld refinement against room temperature neutron powder diffraction data show that all three compounds crystallise with a cationordered variant of the perovskite structure in the P21/n space group. Complete cation ordering occurs between M5+ and Yb3+ over two octahedrally-coordinated sites in the structure and all compounds are stoichiometric in oxygen. The Sb-O bond lengths are similar to related perovskite compounds but differ slightly from those indicated by bond valence sums. Magnetic susceptibility data resemble Curie-Weiss paramagnetic behaviour, but can be better understood as arising from the effect of the octahedral crystal field on the 2F5/2 ground state of Yb3+ leading to a temperature dependent magnetic moment on this ion below 100 K.