Abbie C. Mclaughlin

Colossal magnetoresistance in Mn2+ oxypnictides NdMnAsO(1-x)F(x).

Colossal magnetoresistance is a rare phenomenon in which the electronic resistivity of a material can be decreased by orders of magnitude upon application of a magnetic field. Such an effect could be the basis of the next generation of memory devices. Here we report CMR in the antiferromagnetic oxypnictide NdMnAsO(1-x)F(x) as a result of competition between an antiferromagnetic insulating phase and a paramagnetic semiconductor upon application of a magnetic field. Mn(2+) oxypnictides are relatively unexplored, and tailored synthesis of novel compounds could result in an array of materials for further investigation and optimization.

Doping studies of the ferromagnetic superconductorRuSr2GdCu2O8

Solid solutions of the ferromagnetic superconductor Ru1xa0−xa0xMxSr2GdCu2O8 have been prepared for Mxa0=xa0Sn nor Nb. Up to 7.5% Sn and 20% Nb have been incorporated into nthe structure, and powder diffraction and physical measurements show that nsubstitution occurs at the Ru site. A suppression of the ferromagnetic moment nin the ruthenate layers is observed upon Sn and Nb substitution and the Curie ntemperature drops from 136xa0K in the undoped material to 103xa0K nin Ru0.8Nb0.2Sr2GdCu2O8. nThe onset of superconductivity increases from 19xa0K in the 15% nNb doped sample to 38xa0K in the parent compound to 50xa0K in the n7.5% Sn doped sample, evidencing a tuning of the superconducting transition nby charge transfer. Structural changes upon M doping have been determined nfrom powder synchrotron X-ray diffraction and, for one sample, neutron ndiffraction using the high flux diffractometer GEM to compensate for the high nabsorption of natural Gd.

Oxide ion conductivity in the hexagonal perovskite derivative Ba3MoNbO8.5

Oxide ion conductors are important materials with a range of technological applications and are currently used as electrolytes for solid oxide fuel cells and solid oxide electrolyzer cells. Here we report the crystal structure and electrical properties of the hexagonal perovskite derivative Ba3MoNbO8.5. Ba3MoNbO8.5 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures. This is a new and so far unique crystal structure that contains a disordered distribution of (Mo/Nb)O6 octahedra and (Mo/Nb)O4 tetrahedra. Ba3MoNbO8.5 shows a wide stability range and exhibits predominantly oxide ion conduction over a pO2 range from 10-20 to 1 atm with a bulk conductivity of 2.2 × 10-3 S cm-1 at 600 °C. The high level of conductivity in a new structure family suggests that further study of hexagonal perovskite derivatives containing mixed tetrahedral and octahedral geometry could open up new horizons in the design of oxygen conducting electrolytes.

The synthesis, structure and magnetic properties of Pb2Sr2Cu2RuO8Cl, a new layered ruthenocuprate

Abstract A new ruthenocuprate Pb 2 Sr 2 Cu 2 RuO 8 Cl has been synthesised in a mixed phase sample. It has a tetragonal unit cell, space group P4/mmm ( a =3.86681(9), c =15.3699(8) A). A neutron diffraction study has been performed at 295 and 10xa0K, which reveals that the average crystal structure is tetragonal with no evidence of superstructure or orthorhombic distortion. Antiferromagnetic diffraction peaks are observed in the 10 K neutron diffraction pattern but weak ferromagnetism below T M =117xa0K is evidenced from DC magnetometry experiments. The magnetism of this material appears to be strikingly similar to that of the compound RuSr 2 GdCu 2 O 8 .

Electrical and Structural Characterization of Ba3Mo1–xNb1+xO8.5–x/2: The Relationship between Mixed Coordination, Polyhedral Distortion and the Ionic Conductivity of Ba3MoNbO8.5

The electrical and structural properties of the series Ba3Mo1-xNb1+xO8.5-x/2 (x = 0.0, 0.1, 0.2, 0.3) have been determined. Ba3Mo1-xNb1+xO8.5-x/2 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)O4 and (Mo/Nb)O6 units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10-6 S cm-1 for x = 0.0 to 1.1 × 10-7 S cm-1 for x = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba3MoNbO8 preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)O4 tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba3MoNbO8.5 system.

Absence of colossal magnetoresistance in the oxypnictide PrMnAsO0.95F0.05.

We have recently reported a new mechanism of colossal magnetoresistance (CMR) in electron doped manganese oxypnictides NdMnAsO1-xFx. Magnetoresistances of up to -95% at 3 K have been observed. Here we show that upon replacing Nd for Pr, the CMR is surprisingly no longer present. Instead a sizable negative magnetoresistance is observed for PrMnAsO0.95F0.05 below 35 K (MR7T (12 K) = -13.4% for PrMnAsO0.95F0.05). A detailed neutron and synchrotron X-ray diffraction study of PrMnAsO0.95F0.05 has been performed, which shows that a structural transition, Ts, occurs at 35 K from tetragonal P4/nmm to orthorhombic Pmmn symmetry. The structural transition is driven by the Pr 4f electrons degrees of freedom. The sizable -MR observed below the transition most likely arises due to a reduction in magnetic and/or multipolar scattering upon application of a magnetic field.

Electronic and magnetic properties of Nd1-xSrxMnAsO oxyarsenides

The oxypnictides Nd1-xSrxMnAsO have been successfully synthesised with x up to 0.1. A synchrotron X-ray diffraction study demonstrates that there is no change in crystal symmetry upon doping with Sr. An expansion of the inter-layer distance between Nd-O-Nd and As-Mn-As blocks is observed with increasing x. Results from variable temperature neutron diffraction and resistivity measurements show that the local moment antiferromagnetic order of the Mn spins is preserved as the [MnAs]- layers are hole doped and the materials are driven metallic for x > 0.05. A sizeable positive magnetoresistance is observed at low temperature which demonstrates that multiple MR mechanisms are possible in LnMnAsO oxypnictides.

Partial frustration of magnetic order in synthetic angelellite, Fe4As2O11

The antiferromagnetic structure of Fe4As2O11 has been solved from neutron powder diffraction data below the Neel temperature of 159 K. A 2axa0×xa0bxa0×xa0c magnetic supercell is adopted and the two crystallographically distinct Fe3+ sites order with different critical exponents and with saturated magnetic moments of 3.34(2) and 4.37(2) μB. The large difference between these magnetic moments reflects differing degrees of frustration at the two sites. Structural refinement using high resolution powder neutron diffraction data confirms the structural model previously reported and no evidence for the presence of an OH group associated with a very short Fe–O distance of 1.76(2) A is found.

The Crystal Structure of Ba3Nb2O8 Revisited: A Neutron Diffraction and Solid-State NMR Study

The structure of Ba3Nb2O8 has been investigated using high resolution neutron powder diffraction. Our results show that, while the structure has some features in common with the 9R perovskite and palmierite structures, it is a new and distinct structure. It is shown to follow a (chh)(hhc)(chh) sequence with BaO3-δ packing layers and is a cation- and anion-deficient 9H perovskite polytype. Nb atoms occupy octahedral sites with vacancies between hexagonal close-packed layers. Isolated, corner-sharing and face-sharing Nb-O octahedra all occur within the unit cell. The identification of purely octahedral Nb is supported by solid-state 93Nb wideline NMR measurements. A two-component line shape was detected: a narrow featureless resonance with an isotropic chemical shift of δiso -928 ± 5 ppm consistent with regular Nb octahedra, and a much broader featureless resonance with an approximate isotropic chemical shift in the range δiso ∼ -944 to -937 ± 10 ppm consistent with Nb octahedra influenced by O vacancies. These are both characteristic of 6-fold oxo-coordinated Nb environments. The highly distorted octahedral environments in Ba3Nb2O8 make it a potential candidate for dielectric and photocatalytic applications.

A cyclic hexacopper(II) fluoro complex that encapsulates two fluoride anions

The complex [[Cu3(HpztBu)4(mu-pztBu)2(mu-F)2(mu 3-F)]2]F2 (HpztBu = 3[5]-tert-butylpyrazole) has a cyclic, C2v-symmetric hexacopper core. The two non-coordinated F- anions are encapsulated within cavities formed by three HpztBu ligands.