Weak spin interactions in Mott insulating La2O2Fe2OSe2

Identifying and characterizing the parent phases of iron-based superconductors is an important step towards understanding the mechanism for their high-temperature superconductivity. We present an investigation into the magnetic interactions in the Mott insulator La2O2Fe2OSe2. This iron oxyselenide adopts a 2-k magnetic structure with low levels of magnetic frustration. This magnetic ground state is found to be dominated by next-nearest-neighbor interactions J2 and J2′ and the magnetocrystalline anisotropy of the Fe2+ site, leading to 2D-Ising-like spin S=2 fluctuations. In contrast to calculations, the values are small and confine the spin excitations below ∼25 meV. This is further corroborated by sum rules of neutron scattering. This indicates that superconductivity in related materials may derive from a weakly coupled and unfrustrated magnetic structure

Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi2Sr2Nb2.5Fe0.5O12 is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe3+ and Nb5+ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr2+ and Bi3+ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie-Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi3-xSrxNb2FeO12, Bi2Sr2-xLaxNb2FeO12 and Bi2Sr2Nb3-xFexO12 were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi2Sr2Nb3-xFexO12 system. © 2008 Elsevier Inc. All rights reserved

Synthesis and Structural and Magnetic Characterization of Mixed Manganese−Coppern= 1 Ruddlesden−Popper Phases

New n = 1 Ruddlesden-Popper phases containing manganese and copper have been synthesized. Their structure has been investigated by X-ray and neutron powder diffraction and found to be tetragonal (14/mmm) at temperatures down to 10 K. The series of materials LaxSr2-xMn 0.5Cu0.5O4 (x = 0.75, 1, 1.25, 1.5) displays significant Jahn-Teller distortions of the B site cations, with the degree of distortion increasing with x. The materials LaSrMn0.5Cu 0.5O4, PrSrMn0.5Cu0.5O4, and NdSrMn0.5Cu0.5O4 show interesting magnetic behavior with antiferromagnetic ordering, but evidence of some ferromagnetic interactions. LaSrMn0.5Cu0.5O4 has long-range antiferromagnetic ordering and La0.5Sr0.5Mn 0.5Cu0.5O4 shows more complex behavior due to competing interactions, possibly resulting in a canted magnetic ground state. © 2006 American Chemical Society

Structural and magnetic characterisation of Bi2Sr1.4La0.6Nb2MnO12and its relationship to “Bi2Sr2Nb2MnO12”

A new Aurivillius phase (generic formula M2An-IB nO3n+3) has been synthesized with n = 3 and containing manganese, Bi2Sr1.4La0.6Nb2MnO 12. The structure has been investigated by X-ray and neutron powder diffraction and found to be tetragonal (I4/mmm) at temperatures down to 2 K, with a= 3.89970(7) Ã?, c = 32.8073(9) Ã?at 2 K. There is significant cation disorder between Bi3+ (predominantly on the M sites) and Sr2+ and La3+ which prefer the A sites: 19(2)% of Bi 3+ occupy the A sites. This disorder, leading to occupancy of M sites by Sr2+, is thought to relieve strain due to size-mismatch between the fluorite-like and perovskite-like blocks. A high level of order exists between Mn and Nb on the B sites, with Mn located predominantly (76.1 (6)%) in the central B site whilst Nb preferentially occupies the lower symmetry, outer B site, where it undergoes an out-of-centre displacement towards the fluorite-like blocks. Magnetic measurements indicate that this material displays spin-glass behaviour on cooling. Synthesis of the Mn4+ analogue Bi2Sr2Nb2MnO12 was unsuccessful, possibly due to the small size of the Mn4+ cation

Structural and magnetic characterization of iron oxyselenides Ce2O2Fe2OSe2 and Nd2O2Fe2OSe2

We present here an investigation of the magnetic ordering in the Mott insulating oxyselenide materials Ln2O2Fe2OSe2 (Ln=Ce, Nd). Neutron powder diffraction data are consistent with a noncollinear multi-k ordering on the iron sublattice structure and analysis indicates a reduced magnetic correlation length perpendicular to the [Fe2O]2+ layers. The magnetic role of the Ln3+ cations is investigated and Ce3+ moments are found to order at T≤16 K

Formation of disordered and partially ordered LixCo1−xO

Analysis of XRD, TG, XANES and density measurement data show that a new, partially-ordered, rock salt-like phase, W, exists for LixCo 1-xO, 0.075 â?¤ x â?¤ 0.24(2)-0.31(2), depending on temperature, with doping mechanism: 2Co2+ â?? Li+ + Co3+. Phase W forms as ordered domains within a cation-disordered rock salt solid solution based on CoO; domain size depends on temperature and composition. Binary phase diagrams of LixCo1-xO and phase W solid solutions in air and N2 are presented. © 2009 The Royal Society of Chemistry

Partial cation-order and early-stage, phase separation in phase W, Li x Co 1− x O: 0.075≤ x ≤0.24−0.31

We report the characterization using X-ray and neutron powder diffraction, transmission electron microscopy and extended X-ray absorption fine structure of a new, partially ordered rock-salt-like solid solution phase Li xCo1-xO: 0.075�x�0.24-0.31. The cation stacking sequence along [111] consists of alternating planes of Co and Co/Li. Nano-sized domains of this cation-ordered phase appear alongside disordered regions; domain size increases from 2 to 8 nm with increasing Li content. Compositions of ordered and disordered regions are Li- and Co-rich, respectively, and, therefore, the phase exhibits frozen-in, incipient phase separation. This microstructure could be considered as a precursor to precipitation of fully ordered, rhombohedral LiCoO2. © 2009 The Royal Society

Crystal structure and electrical characterisation of Bi2NbO5F: an Aurivillius oxide fluoride

Structural characterisation of Bi2NbO5F was performed using X-ray and neutron powder diffraction and electron diffraction. Structural refinements show that the material adopts a distorted structure with significant rotation of the octahedra around two axes. The physical properties of the material have been investigated and show no evidence of polar symmetry. The structure is therefore best described by space group Pbca (a = 5.428(1) �, b = 5.426(1) �, c = 16.656(1) �). Bond valence sum calculations suggest that ordering of the anions in the structure is favourable with the fluorine atoms preferring the apices of the NbX6 (X = O/F) octahedra. © The Royal Society of Chemistry 2007

Magnetism of the Fe2+ and Ce3+ sublattices in Ce2O2FeSe2: A combined neutron powder diffraction, inelastic neutron scattering, and density functional study

The discovery of superconductivity in the 122 iron selenide materials above 30 K necessitates an understanding of the underlying magnetic interactions. We present a combined experimental and theoretical investigation of magnetic and semiconducting Ce 2 O 2 FeSe 2 composed of chains of edge-linked iron selenide tetrahedra. The combined neutron diffraction and inelastic scattering study and density functional calculations confirm the ferromagnetic nature of nearest-neighbor Fe-Se-Fe interactions in the ZrCuSiAs-related iron oxyselenide Ce 2 O 2 FeSe 2 . Inelastic measurements provide an estimate of the strength of nearest-neighbor Fe-Fe and Fe-Ce interactions. These are consistent with density functional theory calculations, which reveal that correlations in the Fe-Se sheets of Ce 2 O 2 FeSe 2 are weak. The Fe on-site repulsion U Fe is comparable to that reported for oxyarsenides and K 1−x Fe 2−y Se 2 , which are parents to iron-based superconductors