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.

The structural, magnetic and electronic properties of the Ruddlesden-Popper material Sr3CoRuO7 have been investigated. Examination of the dc and ac magnetic susceptibility demonstrates that Sr3CoRuO7 exhibits a spin glass transition at 40 K and a magnetic transition at 140 K. Variable-temperature neutron powder diffraction experiments showed no evidence of magnetic diffraction peaks down to 5 K. This suggests the transition at 140 K is a result of short range magnetic order or magnetic clustering of the Co/Ru spins. Sr3CoRuO7 is semiconducting and Mott variable range hopping behaviour is observed below 240 K.Short range antiferromagnetic order is observed below ∼ 140 K in Sr3CoRuO7 followed by spin glass behaviour at 40 K.Download high-res image (33KB)Download full-size image

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 solid solution Sr1−xCaxMoO3 has been synthesised.•Structural phase transitions are observed.•Discontinuities were observed in the cell parameters and bond lengths and angles.•Upon increasing x from 0 to 0.17 the band gap reduces from 2.20 eV to 2.10 eV.The solid solution Sr1−xCaxMoO3 (x=0.00, 0.05, 0.10, 0.13, 0.15 and 0.17) has successfully been synthesised and X-ray Powder diffraction has revealed the occurrence of structural phase transitions, from cubic Pm−3m to tetragonal I4/mcm, and then to orthorhombic Imma as the value of x increased. Discontinuities were observed in the cell parameters and bond lengths and angles at the transition from tetragonal to orthorhombic symmetry as a result of the switching of the octahedral rotation axis at the tetragonal to orthorhombic transition. The increased octahedral tilting could also be linked to the decrease in the band gap from 2.20 eV to 2.10 eV as x increased from 0 to 0.17.Table of Contents Figure Caption: Ultraviolet-visible absorbance spectra for Sr1−xCaxMoO3 showing a reduction in band gap upon increasing x as a result of increased octahedral tilting.

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.

A study of the magnetic and structural properties of the double perovskite Ba2GdMoO6 has been performed. The crystal structure distorts from the ideal cubic (Fmm) structure to the tetragonal space group I4/m at 220 K, before undergoing a second distortion to a triclinic system (I) at 80 K. The phase transition to triclinic symmetry is also evident in magnetic susceptibility measurements. The variable temperature synchrotron powder X-ray diffraction results reveal that Ba2GdMoO6 is ferroelastic, with the onset of ferroelastic domain formation occurring at the cubic–tetragonal phase transition. A number of Rietveld refinement techniques for modelling diffuse scattering from ferroelastic domain boundaries have been explored.

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 NdMnAsO1–xFx as a result of competition between an antiferromagnetic insulating phase and a paramagnetic semiconductor upon application of a magnetic field. Mn2+ oxypnictides are relatively unexplored, and tailored synthesis of novel compounds could result in an array of materials for further investigation and optimization.

Ba2LnMoO6 double perovskites have been recently shown to display a wide range of interesting magnetic and structural properties; Ba2154SmMoO6 exhibits simultaneous antiferromagnetic order and a Jahn–Teller distortion. Here we report a high temperature neutron diffraction study of Ba2154SmMoO6 from 353 to 877 K. The results evidence a tetragonal to cubic phase transition at 423 K. Above this temperature the thermal displacement parameters of the oxygen atoms are modelled anisotropically as a result of a transverse vibration of the bridging oxygen. A smooth increase in the cell parameter a is observed with temperature for Ba2154SmMoO6.Graphical abstractThe high temperature crystal structure of Ba2154SmMoO6 evidencing a transverse oxygen vibration.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A high temperature neutron diffraction study has been performed on an isotopically enriched sample of Ba2154SmMoO6. ► A cubic–tetragonal phase transition occurs below 423 K. ► The thermal displacement parameters of the bridging oxygens are modelled anisotropically. ► There is a transverse vibration of the bridging oxygen.

A sizeable negative magnetoresistance (MR) has been observed for oxypnictides LnOMnAs (Ln = La,Nd). MR up to −24% is observed at 200 K for LaOMnAs which is unprecedented for divalent Mn2+. Both materials are weak ferromagnets with transition temperatures above room temperature.

An unexpected enhancement of the large negative magnetoresistance (MR) observed in RuSr2Nd0.95Y0.15Ce0.9Cu2O10−δ up to −47% at 4 K and 9 T is evidenced upon dilution of the Ru magnetic order by substitution of Ta for Ru; this enhancement of −MR scales with the cell volume.

A magnetic, electronic and structural study of the double perovskites Ba2REMoO6 (RE=Sm, Eu, Gd, Dy) has been performed. All materials crystallise in the cubic Fm3¯m symmetry space group and the cell volume decreases as RE varies from Sm to Dy in accordance with Vegard's law. An antiferromagnetic transition is observed below TN=130 and 112 K for RE=Sm and Eu, respectively. The Néel temperatures of these ordered rare earth molybdenum double perovskites are much higher than previously observed in double perovskites containing Eu or Sm and a 4d or 5d transition metal arranged in an ordered rock salt configuration. The high Néel temperatures arise due to a strong superexchange magnetic interaction via the Mo–O–RE–O–Mo pathway. All of the phases are electronically insulating and there is no evidence of magnetoresistance at any temperature.

A structural, magnetic and electronic study of the cobaltocuprate CoSr2Y2−xCexCu2O9±δ   (x=0.5–0.8x=0.5–0.8) has been performed. All materials crystallise in the orthorhombic Cmcm symmetry space group in which chains of corner linked CoO4 tetrahedra run parallel to the 1 1 0 direction. An antiferromagnetic transition is observed for x=0.5–0.8x=0.5–0.8; TM increases with x  . A change in the dimensionality of the magnetic order occurs at x=0.8x=0.8 as the interchain distance increases to a critical value. There is charge transfer between the cuprate planes and cobaltate layer as Ce doping increases, so that Co3+ is partially oxidised to Co4+ with a concomitant reduction in the valence of Cu. Superconductivity is not observed in any of the samples and a crossover from Mott to Efros and Shklovskii variable range hopping behaviour is evidenced as x increases from 0.5 to 0.8.The variation of magnetic susceptibility with temperature for CoSr2Y2−xCexCu2O9±δ   (x=0.5x=0.5, 0.6, 0.7, 0.8).

A study of the magnetic and structural properties of the double perovskite Ba2GdMoO6 has been performed. The crystal structure distorts from the ideal cubic (Fmm) structure to the tetragonal space group I4/m at 220 K, before undergoing a second distortion to a triclinic system (I) at 80 K. The phase transition to triclinic symmetry is also evident in magnetic susceptibility measurements. The variable temperature synchrotron powder X-ray diffraction results reveal that Ba2GdMoO6 is ferroelastic, with the onset of ferroelastic domain formation occurring at the cubic–tetragonal phase transition. A number of Rietveld refinement techniques for modelling diffuse scattering from ferroelastic domain boundaries have been explored.

A sizeable negative magnetoresistance (MR) has been observed for oxypnictides LnOMnAs (Ln = La,Nd). MR up to −24% is observed at 200 K for LaOMnAs which is unprecedented for divalent Mn2+. Both materials are weak ferromagnets with transition temperatures above room temperature.

An unexpected enhancement of the large negative magnetoresistance (MR) observed in RuSr2Nd0.95Y0.15Ce0.9Cu2O10−δ up to −47% at 4 K and 9 T is evidenced upon dilution of the Ru magnetic order by substitution of Ta for Ru; this enhancement of −MR scales with the cell volume.