Structure and electronic properties of a mu-oxo ruthenium bromide

The crystal structure of potassium μ-oxo-bis[pentabromoruthenate] K4Ru2OBr10, determined from synchrotron X-ray powder diffraction, is described. Each Ru atom is surrounded by five Br atoms and one O atom. Magnetic measurements show the complex to be diamagnetic as a result of strong Ru–Ru interactions facilitated by the linear Ru–O–Ru linkage

Neutron diffraction study of the tetragonal – monoclinic phase transition in NdNbO4 and NdTaO4

Phase transition and high-temperature properties of NdNbO4 and NdTaO4 were studied in situ using powder neutron diffraction methods. Both oxides undergo a reversible phase transition from a monoclinic I2/a phase at low temperatures to a tetragonal I41/a phase at high temperatures. The phase transition has been investigated through analysis of the spontaneous strains and symmetry distortion modes. Well below the transition temperature, Tc, the thermal evolution of the lattice parameters and symmetry modes suggest the transition is continuous, although a small discontinuity in both the spontaneous strains and symmetry distortion modes shows the transition is strictly first order. Analysis of the refined structures reveals that the Nb and Ta cations are best described as having a distorted 6-coordinate arrangement in the monoclinic structure, with four short and two long bonds. Breaking of the two long bonds at high temperatures, resulting in a transformation of the Nb(Ta) coordination to a regular tetrahedron, is believed to be responsible for the first order nature of the transition

Expanding the tunability and applicability of exchange-coupled/decoupled magnetic nanocomposites

CoFe2O4/Co-Fe magnetic composites are usually prepared through partial reduction of CoFe2O4, which often yields monoxides (i.e., FeO, CoO) as secondary phases. Since these compounds are paramagnetic at ambient conditions, the presence of a small amount of monoxide is generally downplayed in the literature, and the possible effects on the magnetic properties are simply ignored. However, the present study shows that even a low concentration of monoxide results in decoupling of the soft and hard magnetic phases, which inevitably leads to a deterioration of the magnetic properties. Additionally, it is confirmed that a partial reduction of CoFe2O4 is a suitable method to produce CoFe2O4/Co-Fe nanocomposites, provided that the treatment is well controlled with respect to duration, temperature and flow of reductant. A monoxide-free nanocomposite was produced and its magnetic properties evaluated both at room and low temperature. Our model system exemplifies the potential of exchange-coupling (and decoupling) as a tool to tune the magnetic properties of a material within a relatively wide range of values, thus widening its spectrum of potential applications

Structural And Magnetic Properties Of Some Vacancy Ordered Osmium Halide Perovskites

The structures and magnetic properties of the Os4+ (5d4) halides K2OsCl6, K2OsBr6, Na2OsBr6 and Na2OsBr6.6H2O are described. K2OsCl6 and K2OsBr6 have a cubic vacancy-ordered double perovskites structure but undergo different symmetry lowering structural phase transitions upon cooling associated with a combination of the relative size of the ions and differences in their chemical bonding. The structure of Na2OsBr6.6H2O has been determined for the first time and the thermal stability of this established using a combination of in-situ diffraction and TGA. Na2OsBr6.6H2O and Na2OsBr6 are isostructural with the analogous iridium chlorides, Na2IrCl6.6H2O and Na2IrCl6, dehydration proceeds via different intermediate phases. The magnetic moments of four compounds display Kotani-like behaviour consistent with a Jeff = 0 ground state, however the magnetic susceptibility measurements reveal unusual low temperature properties indicative of a weakly magnetic ground state

Expanding the tunability and applicability of exchange-coupled/decoupled magnetic nanocomposites

[EN] CoFe2O4/Co–Fe magnetic composites are usually prepared through partial reduction of CoFe2O4, which often yields monoxides (i.e., FeO, CoO) as secondary phases. Since these compounds are paramagnetic at ambient conditions, the presence of a small amount of monoxide is generally downplayed in the literature, and the possible effects on the magnetic properties are simply ignored. However, the present study shows that even a low concentration of monoxide results in decoupling of the soft and hard magnetic phases, which inevitably leads to a deterioration of the magnetic properties. Additionally, it is confirmed that a partial reduction of CoFe2O4 is a suitable method to produce CoFe2O4/Co–Fe nanocomposites, provided that the treatment is well controlled with respect to duration, temperature and flow of reductant. A monoxide-free nanocomposite was produced and its magnetic properties evaluated both at room and low temperature. Our model system exemplifies the potential of exchange-coupling (and decoupling) as a tool to tune the magnetic properties of a material within a relatively wide range of values, thus widening its spectrum of potential applications.C. G.-M. and A. Q. have contributed equally to this work. The authors would like to thank financial support from the European Commission through the AMPHIBIAN project (H2020-NMBP-2016-720853), the Danish National Research Foundation (Center for Materials Crystallography, DNRF-93), and the Spanish Ministerio de Ciencia, Innovación y Universidades (RTI2018-095303-A-C52). C. G.-M. acknowledges financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades through the Juan de la Cierva Program (FJC2018- 035532-I). Authors from Aarhus University gratefully acknowledge affiliation with the Center for Integrated Materials Research (iMAT) at Aarhus University. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

During the past decade, CoFe2O4 (hard)/Co-Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material

Experimental and Computational Insights into the Anomalous Thermal Expansion of NH¬4ReO4

The temperature dependence of the structure and the ground state properties of scheelite type NH4ReO4 have been studied using neutron powder diffraction (NPD) and Density Functional Theory (DFT), respectively. Despite the large incoherent background in the experimental NPD, associated with the presence of hydrogen, accurate and precise structural parameters were obtained. Comparison of the results of the NPD and DFT studies shows that the observed anomalous thermal contraction in NH4ReO4 is a consequence of thermally induced rotational disorder of the NH4 groups. Comparing the experimentally determined and optimized structures reveals deformation of the NH4 tetrahedra that is responsible for the unusual tetragonal distortion of this material. The Raman Spectra of NH4ReO4 is presented and the modes are assigned based on the DFT calculations

Beyond the Ionic Radii: A Multifaceted Approach to Understand Differences between the Structures of LnNbO4 and LnTaO4 Fergusonites

Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid tantalates, LnTaO4, at room temperature. Three different structures are observed, depending on the size of the Ln cation: P21/c (Ln = La, Pr), I2/a (Ln = Nd-Ho), and P2/c (Ln = Tb-Lu). BVS analysis indicated that TaV is six-coordinate in these structures, with four short bonds and two longer bonds. Synchrotron X-ray powder diffraction methods were also used to observe the impact of Ta doping on the orthoniobates, Ln(Nb1-xTax)O4 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb and Lu). Where both the niobate and tantalate oxide were isostructural (fergusonite structure, space group I2/a), complete solid solutions were prepared. In these solid solutions, the unit cell volume decreases as the Ta content increases. The subtle interaction evident between the LnO8 and BO6 sublattices in the fergusonite-type oxides was not observed in the related pyrochlore oxides. A combined synchrotron X-ray and neutron powder diffraction study of the series Ho(Nb1-xTax)O4 was used to determine accurate atomic positions of the anions, and hence, bond lengths. This revealed a change in the (Nb/Ta)-O bond lengths, reflective of the difference in the valence orbitals of Nb(4d) and Ta(5d). Examination of the partial density of states demonstrates differences in the electronics between Nb and Ta, leading to a difference in the bandgap. This study highlights the importance of the long B-O contacts in the fergusonite structures, and its potential impact on the I2/a to I41/a phase transition

Magnetic domains in SrFe12O19/Co hard/soft bilayers

ESRF (The european Synchrotron) User Meeting 2022, 7 - 9 February, 2022 . -- online meeting . -- https://www.esrf.fr/fr/home/events/conferences/2022/user-meeting-2022.html .-- Youtube access: https://www.youtube.com/playlist?list=PLsWatK2_NAmyYnkC-bXhvT70wsYaTmojqThe nature of the magnetic coupling between a SrFe12O19 particle (hard phase) and a Co layer grown on top (soft phase) has been studied by means of photoemission electron microscopy (PEEM) and spatially-resolved x-ray absorption (XAS) and magnetic circular dichroism (XMCD) at CIRCE, ALBA synchrotron (Spain). Our study reveals the soft metallic overlayer presents an in-plane magnetization despite the strong out-of-plane magnetocrystalline anisotropy of the hard platelet. Thus, the two phases show completely uncorrelated magnetic domain patterns. Micromagnetic simulations seem to indicate the degree of exchange-coupling is low or null, although the conditions for rigid coupling are a priori well met

Cation and lone pair order-disorder in the polymorphic mixed metal bismuth scheelite Bi3FeMo2O12

The Bi3FeMo2O12 system is examined as a rare example of a transition metal oxide which, upon heating, undergoes a symmetry lowering and 2:1 ordering of the transition metal cations. The compound was synthesised in the tetragonal scheelite structure (S.G. #88: I41/a) by a sol-gel method and converted into the monoclinic polymorph (S.G. #15: C2/c) by calcination above 500 °C. The structure of both polymorphs was analysed using a combination of X-ray and neutron diffraction data, and the temperature-dependent phase transition between these was investigated in situ using variable temperature neutron powder diffraction and scanning transmission electron microscopy. The results show that the structural phase transition takes place at low temperature (~500 °C) and is 1st order in nature, as evident from the coexistence of both structures. The transition from tetragonal to monoclinic results in reduction of the equivalent unit cell volume. The role of the Bi3+ 6s lone pairs in the temperature-driven phase transition has been studied using neutron pair distribution function analysis. Local structure analysis via neutron total scattering revealed the Bi3+ 6s lone pairs to be stereochemically active in both structures, with short correlation lengths in the tetragonal structure and long correlation lengths in the monoclinic structure, leading to the facile phase conversion and to a more efficient packing density with highly correlated lone pairs in the monoclinic structure. Magnetization isotherms of the tetragonal structure collected at 1.8 K exhibit ferromagnetic behavior, suggesting that the interplay between the observed short-range monoclinic order, defects and surface-to-bulk effects alters the magnetic interaction, leading to short range ferromagnetic interactions, which is highly unexpected given the low temperature antiferromagnetic order observed in the monoclinic structure