M. T. Fernández-Díaz

k=0 Magnetic Structure and Absence of Ferroelectricity in SmFeO3

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675  K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

In situ high temperature neutron powder diffraction study of oxygen-rich La2NiO4+δ in air: correlation with the electrical behaviour

The knowledge of the thermal evolution of the crystal structure of a cathode material across the usual working conditions in solid oxide fuel cells is essential to understand not only its transport properties but also its chemical and mechanical stability in the working environment. In this regard, high resolution neutron powder diffraction (NPD) measurements have been performed in air from 25 to 700 °C on O2-treated (350 °C, 200 bar) La2NiO4+δ. A structural transition from the orthorhombic Fmmm to the tetragonal F4/mmm space group takes place at about 150 °C. The reversibility of this transition has been determined to be strongly dependent on the sample oxygen content. The structural data have been correlated with the transport properties of this layered perovskite. The electrical conductivity of O2-treated La2NiO4+δ exhibits a dirty-metal (high T)-to-semiconducting (low T) transition as a function of temperature, displaying a maximum value of 82 S cm−1 at around 400 °C. The largest conductivity corresponds, microscopically, to the shortest axial Ni–O2 distance (2.19(1) A), revealing a major anisotropic component for the electronic transport. The interstitial oxygens occupy the 16j and 16e positions in the low and high temperature phases, respectively. The refined oxygen occupancy from NPD data is in quite good agreement with the thermogravimetric data. Good thermal stability of the oxygen content has been observed in the studied temperature range, as required for practical applications.

Switching from ferro- to antiferromagnetism in A2CrSbO6 (A = Ca, Sr) double perovskites: a neutron diffraction study

Double perovskites Sr2CrSbO6 and Ca2CrSbO6 have been prepared by a solid-state procedure. The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the room-temperature crystal structure is monoclinic (space group P21/n), and contains an almost completely ordered array of alternating CrO6 and SbO6 octahedra sharing corners, tilted along the three pseudocubic axes according to the Glazer notation a−a−b+. The monoclinic distortion is larger in Ca2CrSbO6 than in Sr2CrSbO6, which is associated with the tilting of the CrO6 and SbO6 octahedra, displaying tilting angles φ = 13.5° and φ = 5.5°, respectively. Magnetization measurements and low-temperature NPD data show that Sr2CrSbO6 is an antiferromagnet with a Neel temperature of 12 K with an ordered magnetic moment of 1.64(4) μB per Cr3+. The propagation vector is k = 0. Ca2CrSbO6 exhibits ferromagnetic long-range order below TC = 16 K, with a saturation magnetization of 2.36 μB at 5 K. In the ferromagnetic arrangement, the Cr3+ spins are aligned approximately along the [110] direction with an ordered magnetic moment of 2.6(2) μB. To our knowledge, this is the first example of a ferromagnetic double perovskite containing a non-magnetic element in the B positions of the perovskite structure.

Magnetic evolution of the antiferromagnetic Co2−xCux(OH)PO4 (0 ≤x≤ 2) solid solution. A neutron diffraction study

The Co2−xCux(OH)PO4 (0 ≤ x ≤ 2) solid solution was prepared from hydrothermal synthesis. Neutron powder diffraction patterns show that the Co2+ and Cu2+ ions are simultaneously present in both the [MO4(OH)2] octahedra and the [MO4(OH)] distorted trigonal bipyramid topologies. The evolution of the lattice parameters follows Vegard’s law in the whole range of substitution. This study allowed us to determine correctly the a and b crystallographic parameters of the Cu2(OH)PO4 phase which were interchanged in the literature. The magnetic behaviour in the cobalt–copper compounds indicates the existence of overall antiferromagnetic interactions as predominant. Three-dimensional magnetic ordering with critical temperatures of 69, 64, 60 and 47 K for x = 0.1, 0.3, 0.5 and 1 respectively is observed. The magnetic study of Co1.9Cu0.1(OH)PO4 suggests a spin-glass like state below 10 K. AC measurements obtained at different frequencies and applied fields confirm the freezing process in Co1.9Cu0.1 and the long range interactions in the Co2−xCux(OH)PO4 (x ≤ 1) phases. For x > 1, the magnetic dimensionality decreases with the increase of Cu(II) amount being of short range for x = 2. These results are attributed to the presence of the unpaired electron in the dx2−y2 orbital and the absence of overlap between neighbour ions. Specific-heat measurements confirm the evolution to a short range magnetic system with the Cu(II) amount. From low-temperature neutron diffraction data, it can be observed that the existence of antiferromagnetic order for x ≤ 1 is originated by the antiparallel ordering of both ferromagnetic linear octahedral chains and trigonal bipyramidal dimers. The propagation vector is k = [0,0,0] and the magnetic moments are aligned in the z direction. The values of the main magnetic exchange pathways [M–O–M] are characteristic of ferro- and antiferromagnetic couplings with a superexchange ferromagnetic angle, M(1)–O(3)–M(2) of 107–109°, which plays an important role in the competition of the freezing process. These results are explained on the basis of both the electronic configuration and the correlations between structural and magnetic properties.

Magnetic structures of (Co2-xNix)(OH)PO4 (x = 0.1, 0.3) spin glass-like state in antiferromagnetically ordered phases

Compounds of the general formula Co2?xNix(OH)PO4 (x = 0.1, 0.3) have been synthesized under mild hydrothermal conditions. Neutron powder diffraction, susceptibility and heat capacity measurements were carried out on polycrystalline samples. The cobalt?nickel compounds are ordered as three-dimensional antiferromagnets with ordering temperatures of 70 and 64?K for x = 0.1 and x = 0.3, respectively. The magnetic study shows a spin glass-like state below 11 and 5?K for Co1.9Ni0.1(OH)PO4 and Co1.7Ni0.3(OH)PO4, respectively. Specific heat data present peaks at 68 and 61?K for Co1.9Ni0.1 and Co1.7Ni0.3, respectively. These peaks show broad shoulders between approximately 15 and 40?K. The lack of any distinguishable anomaly below 10?K supports the spin glass nature of the low temperature transitions. Refinement of room temperature neutron diffraction data indicates that the Ni(II) ions are in octahedral co-ordination with the practical absence of these ions in the trigonal bipyramidal sites. The magnetic structures of Co2?xNix(OH)PO4 consist of ferromagnetic arrangements between the octahedral chains and trigonal bipyramidal dimers within the xz plane with the magnetic moments along the z axis. The ferromagnetic layers are disposed antiparallel to one another along the y direction establishing the three-dimensional antiferromagnetic order (TN?70?K for Co1.9Ni0.1 and ?64?K for Co1.7Ni0.3). The different exchange pathways, the anisotropy of the Co(II) ions and the frustration of the magnetic moments in the trigonal bipyramidal geometry could be responsible for the freezing process.

A Magnetic Ionic Liquid Based on Tetrachloroferrate Exhibits Three‐Dimensional Magnetic Ordering: A Combined Experimental and Theoretical Study of the Magnetic Interaction Mechanism

A new magnetic ionic liquid (MIL) with 3D antiferromagnetic ordering has been synthetized and characterized. The information obtained from magnetic characterization was supplemented by analysis of DFT calculations and the magneto-structural correlations. The result gives no evidence for direct iron-iron interactions, corroborating that the 3D magnetic ordering in MILs takes place via super-exchange coupling containing two diamagnetic atoms intermediaries.

Oxyhalides: A new class of high-TC multiferroic materials

Researchers discover a new class of magnetoelectric multiferroics with high critical temperature. Magnetoelectric multiferroics have attracted enormous attention in the past years because of their high potential for applications in electronic devices, which arises from the intrinsic coupling between magnetic and ferroelectric ordering parameters. The initial finding in TbMnO3 has triggered the search for other multiferroics with higher ordering temperatures and strong magnetoelectric coupling for applications. To date, spin-driven multiferroicity is found mainly in oxides, as well as in a few halogenides. We report multiferroic properties for synthetic melanothallite Cu2OCl2, which is the first discovery of multiferroicity in a transition metal oxyhalide. Measurements of pyrocurrent and the dielectric constant in Cu2OCl2 reveal ferroelectricity below the Néel temperature of ~70 K. Thus, melanothallite belongs to a new class of multiferroic materials with an exceptionally high critical temperature. Powder neutron diffraction measurements reveal an incommensurate magnetic structure below TN, and all magnetic reflections can be indexed with a propagation vector [0.827(7), 0, 0], thus discarding the claimed pyrochlore-like “all-in–all-out” spin structure for Cu2OCl2, and indicating that this transition metal oxyhalide is, indeed, a spin-induced multiferroic material.

Anion−π and Halide–Halide Nonbonding Interactions in a New Ionic Liquid Based on Imidazolium Cation with Three-Dimensional Magnetic Ordering in the Solid State

We present the first magnetic phase of an ionic liquid with anion-π interactions, which displays a three-dimensional (3D) magnetic ordering below the Néel temperature, TN = 7.7 K. In this material, called Dimim[FeBr4], an exhaustive and systematic study involving structural and physical characterization (synchrotron X-ray, neutron powder diffraction, direct current and alternating current magnetic susceptibility, magnetization, heat capacity, Raman and Mössbauer measurements) as well as first-principles analysis (density functional theory (DFT) simulation) was performed. The crystal structure, solved by Patterson-function direct methods, reveals a monoclinic phase (P21 symmetry) at room temperature with a = 6.745(3) Å, b = 14.364(3) Å, c = 6.759(3) Å, and β = 90.80(2)°. Its framework, projected along the b direction, is characterized by layers of cations [Dimim](+) and anions [FeBr4](-) that change the orientation from layer to layer, with Fe···Fe distances larger than 6.7 Å. Magnetization measurements show the presence of 3D antiferromagnetic ordering below TN with the existence of a noticeable magneto-crystalline anisotropy. From low-temperature neutron diffraction data, it can be observed that the existence of antiferromagnetic order is originated by the antiparallel ordering of ferromagnetic layers of [FeBr4](-) metal complex along the b direction. The magnetic unit cell is the same as the chemical one, and the magnetic moments are aligned along the c direction. The DFT calculations reflect the fact that the spin density of the iron ions spreads over the bromine atoms. In addition, the projected density of states (PDOS) of the imidazolium with the bromines of a [FeBr4](-) metal complex confirms the existence of the anion-π interaction. Magneto-structural correlations give no evidence for direct iron-iron interactions, corroborating that the 3D magnetic ordering takes place via superexchange coupling, the Fe-Br···Br-Fe interplane interaction being defined as the main exchange pathway.

Neutron structural characterization and transport properties of oxidized and reduced La0.5Sr0.5M0.5Ti0.5O3 (M = Mn, Fe) perovskites: Possible electrode materials in solid-oxide fuel cells

Oxygen-stoichiometric La0.5Sr0.5M0.5Ti0.5O3 (M = Mn, Fe) perovskites and the corresponding reduced specimens, of La0.5Sr0.5M0.5Ti0.5O3-δ composition, have been prepared and characterized by x-ray diffraction and neutron powder diffraction (NPD), in complement with thermal analysis, electrical conductivity, and thermal expansion measurements. NPD data show that these perovskites are all orthorhombic, space group Pbnm (No. 62). The total reduction of M3+ to M2+ in the reduced phases is accompanied with the occurrence of oxygen vacancies, which was confirmed by thermogravimetric analysis (TGA). Above room-temperature, these phases undergo two structural phase transitions studied in situ from NPD data; the former to a tetragonal (I4/mcm) structure, and the second one to a cubic (Pm-3m) phase. All the oxides display a semiconductor-like behavior with a maximum conductivity value of 15 S·cm−1 for the oxidized La0.5Sr0.5Mn0.5Ti0.5O3 phase at 850 °C. The measured thermal expansion coefficients perfectly match wit...

Magnetic Interactions in the Double Perovskites R2NiMnO6 (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction

R2NiMnO6 (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P21/n space group, with an almost complete order between Ni(2+) and Mn(4+) cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La2NiMnO6, with a ferromagnetic structure involving Mn(4+) and Ni(2+) moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (TCM) decreases abruptly as the size of the rare earth decreases, since TCM is mainly influenced by the superexchange interaction between Ni(2+) and Mn(4+) (Ni(2+)-O-Mn(4+) angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below TCM.