Partitioning the Two-Leg Spin Ladder in Ba2Cu1 - x Zn x TeO6: From Magnetic Order through Spin-Freezing to Paramagnetism.

Ba2CuTeO6 has attracted significant attention as it contains a two-leg spin ladder of Cu2+ cations that lies in close proximity to a quantum critical point. Recently, Ba2CuTeO6 has been shown to accommodate chemical substitutions, which can significantly tune its magnetic behavior. Here, we investigate the effects of substitution for non-magnetic Zn2+ impurities at the Cu2+ site, partitioning the spin ladders. Results from bulk thermodynamic and local muon magnetic characterization on the Ba2Cu1 - x Zn x TeO6 solid solution (0 ≤ x ≤ 0.6) indicate that Zn2+ partitions the Cu2+ spin ladders into clusters and can be considered using the percolation theory. As the average cluster size decreases with increasing Zn2+ substitution, there is an evolving transition from long-range order to spin-freezing as the critical cluster size is reached between x = 0.1 to x = 0.2, beyond which the behavior became paramagnetic. This demonstrates well-controlled tuning of the magnetic disorder, which is highly topical across a range of low-dimensional Cu2+-based materials. However, in many of these cases, the chemical disorder is also relatively strong in contrast to Ba2CuTeO6 and its derivatives. Therefore, Ba2Cu1 - x Zn x TeO6 provides an ideal model system for isolating the effect of defects and segmentation in low-dimensional quantum magnets.

Site-Selective d10/d0 Substitution in an S = 1/2 Spin Ladder Ba2CuTe1-xWxO6 (0 ≤ x ≤ 0.3).

Isovalent nonmagnetic d10 and d0 B″ cations have proven to be a powerful tool for tuning the magnetic interactions between magnetic B' cations in A2B'B″O6 double perovskites. Tuning is facilitated by the changes in orbital hybridization that favor different superexchange pathways. This can produce alternative magnetic structures when B″ is d10 or d0. Furthermore, the competition generated by introducing mixtures of d10 and d0 cations can drive the material into the realms of exotic quantum magnetism. Here, Te6+ d10 was substituted by W6+ d0 in the hexagonal perovskite Ba2CuTeO6, which possesses a spin ladder geometry of Cu2+ cations, creating a Ba2CuTe1-xWxO6 solid solution (x = 0-0.3). We find W6+ is almost exclusively substituted for Te6+ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. The site-selective doping directly tunes the intraladder, Jrung and Jleg, interactions. Modeling the magnetic susceptibility data shows the d0 orbitals modify the relative intraladder interaction strength (Jrung/Jleg) so the system changes from a spin ladder to isolated spin chains as W6+ increases. This further demonstrates the utility of d10 and d0 dopants as a tool for tuning magnetic interactions in a wide range of perovskites and perovskite-derived structures.

Li-ion diffusion in Li intercalated graphite C6Li and C12Li probed by µ+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 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.

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.

Jahn-Teller distorted frameworks and magnetic order in the Rb-Mn-P-O system.

Two previously uncharacterized members of the Rb-Mn-P-O system, RbMnP(2)O(7) and beta-RbMnHP(3)O(10), have been synthesized using a phosphoric acid flux synthetic route and their crystal and magnetic structures determined using neutron powder diffraction. The crystal structure of RbMnP(2)O(7) (space group P2(1)/c, a = 7.3673(2) A, b = 9.6783(2) A, c = 8.6467(2) A, and beta = 105.487(1) degrees) was found to be isostructural with RbFeP(2)O(7). The polymorph beta-RbMnHP(3)O(10) was also isolated as a single phase and found to crystallize in the space group C2 (a = 12.2066(5) A, b = 8.5243(3) A, c = 8.8530(4) A, beta = 107.233(2) degrees). Both structures consist of frameworks of corner-sharing MnO(6) octahedra linked together by condensed phosphate anions, with Rb(+) cations located in the intersecting channels. In both cases the Mn(3+) octahedra exhibit unusual Jahn-Teller distortions indicative of a plasticity effect driven by the steric requirements of the condensed phosphate anions, and this causes a strong violet coloration similar to that observed in the manganese violet pigment; the structure of this has yet to be determined. Magnetic susceptibility measurements show that both RbMnP(2)O(7) (T(N) = 20 K) and beta-RbMnHP(3)O(10) (T(N) = 10 K) undergo a phase transition at low temperatures to an antiferromagnetically ordered state. Low-temperature neutron powder diffraction studies show that the magnetic ground states of each of these materials involve both ferromagnetic and antiferromagnetic super-superexchange interactions between orbitally ordered Mn(3+), which are mediated by PO(4) tetrahedra. These interactions are compared and discussed.

Fluorination of perovskite-related phases of composition SrFe(1-x)Sn(x)O(3-δ).

Perovskite-related compounds of composition SrFe(1-x)Sn(x)O(3-δ) (x = 0.31, 0.54) have been prepared. X-ray powder diffraction shows that the materials adopt orthorhombic unit cells. The lattice parameters increase with the incorporation of increasing amounts of tin, which is shown by x-ray absorption near edge structure investigation to be present as Sn(4+). (57)Fe Mössbauer spectroscopy indicates that iron in these phases is present as Fe(5+) and Fe(3+) and that the materials adopt the compositions SrFe(0.69)Sn(0.31)O(2.94) and SrFe(0.46)Sn(0.54)O(2.88). We propose that the disproportionation of Fe(4+) in SrFeO(3-δ) to Fe(5+) and Fe(3+) in SrFe(1-x)Sn(x)O(3-δ) is driven by the reduction of local lattice strain. The materials have been fluorinated by reaction with poly(vinylidene fluoride) to give products of composition SrFe(0.69)Sn(0.31)O(2.31)F(0.69) and SrFe(0.46)Sn(0.54)O(2.54)F(0.46). The increased iron to oxygen or fluorine distances as revealed by the extended x-ray absorption fine structure are associated with the reduction of Fe(5+) to Fe(3+) as evidenced by (57)Fe Mössbauer spectroscopy. The (57)Fe Mössbauer spectra recorded from the fluorinated materials at low temperature show the coexistence of magnetic sextet and non-magnetic doublet components corresponding to networks of Fe(3+) coupled through oxide ions. The Sn(4+) ions disrupt the coupling and the size of the networks. The magnetic susceptibility measurements and Mössbauer spectra recorded between 4.2 and 300 K are used to model the magnetic properties of these materials, with the larger networks appearing to possess random spin orientations consistent with spin glass-type materials.