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Register a new userElectronic Spin transition in FeO$_{2}$: evidence for Fe(II) with peroxide O$_{2}^{2-}$
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The discovery of FeO$_{2}$ containing more oxygen than hematite (Fe$_{2}$O$_{3}$) that was previously believed to be the most oxygen rich iron compounds, has important implications on the study of the deep lower mantle compositions. Compared to other iron compounds, there are limited reports on FeO$_{2}$ making studies of its physical properties of great interest in fundamental condensed matter physics and geoscience. Even the oxidation state of Fe in FeO$_{2}$ is the subject of debate in theoretical works and there have not been reports from experimental electronic and magnetic properties measurements. Here, we report the pressure-induced spin state transition from synchrotron experiments and our computational results explain the underlying mechanism. Using density functional theory and dynamical mean field theory, we calculated spin states of Fe with volume and Hubbard interaction $U$ change, which clearly demonstrate that Fe in FeO$_{2}$ consists of Fe(II) and peroxide O$_{2}^{2-}$. Our study suggests that localized nature of both Fe 3$d$ orbitals and O$_{2}$ molecular orbitals should be correctly treated for unveiling the structural and electronic properties of FeO$_{2}$.
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Results of magnetic field and temperature dependent neutron diffraction and magnetization measurements on oxy-arsenate Rb$_{2}$Fe$_{2}$O(AsO$_{4}$)$_{2}$ are reported. The crystal structure of this compound contains pseudo-one-dimensional [Fe$_{2}$O$_{6}$]$^infty$ sawtooth-like chains, formed by corner sharing isosceles triangles of $Fe^{3+}$ ions occupying two nonequivalent crystallographic sites. The chains extend infinitely along the crystallographic $b$-axis and are structurally confined from one another via diamagnetic (AsO$_{4}$)$^{3-}$ units along the $a$-axis, and Rb$^+$ cations along the $c$-axis direction. Neutron diffraction measurements indicate the onset of a long range antiferromagnetic order below approximately 25 K. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other. Within each chain, one of the two Fe sites carries a moment which lies along the emph{b}-axis, while the second site bears a canted moment in the opposite direction. Externally applied magnetic field induces a transition to a ferrimagnetic state, in which the coupling between the sawtooth chains becomes ferromagnetic. Magnetization measurements performed on optically-aligned single crystals reveal evidence for an uncompensated magnetization at low magnetic fields that could emerge from to a phase-segregated state with ferrimagnetic inclusions or from antiferromagnetic domain walls. The observed magnetic states and the competition between them is expected to arise from strongly frustrated interactions within the sawtooth chains and relatively weak coupling between them.
We describe the local structural properties of the iron oxychalcogenides, La$_2$O$_2$Fe$_2$O$M_2$ ($M$ = S, Se), by using pair distribution function (PDF) analysis applied to total scattering data. Our results of neutron powder diffraction show that $M$ = S and Se possess similar nuclear structure at low and room temperatures. The local crystal structures were studied by investigating deviations in atomic positions and the extent of the formation of orthorhombicity. Analysis of the total scattering data suggests that buckling of the Fe$_2$O plane occurs below 100 K. The buckling may occur concomitantly with a change in octahedral height. Furthermore, within a typical range of 1-2 nm, we observed short-range orthorhombic-like structure suggestive of nematic fluctuations in both of these materials.
We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ ($M$ = S, Se). Elastic neutron scattering reveals that $M$ = S and Se have similar nuclear structures at room and low temperatures. We find that both materials obtain antiferromagnetic ordering at a Neel temperature $T_{N}$ 90.1 $pm$ 0.16 K and 107.2 $pm$ 0.06 K for $M$= Se and S, respectively. The magnetic arrangements of $M$ = S, Se are obtained through Rietveld refinement. We find the order parameter exponent $beta$ to be 0.129 $pm$ 0.006 for $M$ = Se and 0.133 $pm$ 0.007 for $M$ = S. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen exchange, the nature of the magnetic interactions is similar in these materials.
Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ (BFN) ceramics are considered to be a potential candidate for technological applications owing to their high dielectric constant over a wide range of temperature values. However, there exists considerable discrepancy over the structural details. We address this discrepancy through a comparative analysis of the earlier reported structures and combined X-Ray Diffraction (XRD) at room temperature and Neutron Powder Diffraction (NPD) measurements in the range of 5K up to room temperature. Our study reveals a cubic structure with space group $Pmbar{3}m$ at all measured temperatures. The local environment of the Fe ions is investigated using X-ray Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) technique. A detailed investigation of the electronic properties of the synthesized BFN ceramics is carried out by combination of theoretical and experimental tools: X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and density functional theory (DFT) within GGA$+U$. The bandgap is estimated using the diffuse reflectance measurements in the UV-Vis-NIR range and an appropriate value of the electron-electron correlation strength $U$ is estimated based on the UV-Vis-NIR and the XAS spectra.
Using complementary polarized and unpolarized single-crystal neutron diffraction, we have investigated the temperature-dependent magnetic structures of Eu$_{0.5}$Ca$_{0.5}$Fe$_{2}$As$_{2}$. Upon 50 % dilution of the Eu sites with isovalent Ca$^{2+}$, the Eu sublattice is found to be still long-range ordered below $mathit{T_{Eu}}$ = 10 K, in the A-typed antiferromagnetic (AFM) structure. The moment size of Eu$^{2+}$ spins is estimated to be as large as 6.74(4) $mu_{B}$ at 2.5 K. The Fe sublattice undergoes a spin-density-wave transition at $mathit{T_{SDW}}$ = 192(2) K and displays an in-plane AFM structure above $mathit{T_{Eu}}$. However, at 2.5 K, the Fe$^{2+}$ moments are found to be ordered in a canted AFM structure with a canting angle of 14(4){deg} out of the $mathit{ab}$ plane. The spin reorientation of Fe below the AFM ordering temperature of Eu provides a direct evidence of a strong interplay between the two magnetic sublattices in Eu$_{0.5}$Ca$_{0.5}$Fe$_{2}$As$_{2}$.
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