Ca25Co22O56(OH)28: A layered misfit compound

The high pressure synthesis, structure and magnetic properties of Ca₂₅Co₂₂O₅₆(OH)₂₈ are reported. The compound has a misfit structure, consisting of double, square calcium oxide hydroxide rock-salt-like layers between hexagonal CoO₂ layers. The misfit compound crystallizes in the monoclinic space group C2/m, and can be characterized by the coexistence of two subsystems with common a = 4.893(5) Å, c = 8.825(9) Å and β = 95.745(8) parameters, and different b parameters: b_RS = 4.894(5) Å, and b_HEX = 2.809(3) Å, for the rock-salt and hexagonal type planes, respectively. The compound shows Curie–Weiss paramagnetism with an antiferromagnetic Weiss temperature of −43 K and a reduced Co moment. Substantial deviations from Curie–Weiss behavior are seen below 50 K with no indication of magnetic ordering. No superconductivity was observed down to a temperature of 2 K.     

Neutron diffraction and magnetic study of the low-temperature transitions in SrMo1−xFexO3−δ

SrMo_1?xFe_xO_3?δ (x = 0, 0.1, 0.2 and 0.3) perovskites have recently been described as performing anode materials in solid-oxide fuel cells. In this work, we describe the structural phase transitions they undergo below room-temperature (RT), studied “in-situ” from neutron powder diffraction data and DSC measurements. At RT all the studied compositions are cubic, space group Pm-3m, with unit-cell parameters that decrease with Fe doping. Upon cooling the samples, two structural phase transitions are identified: one to a tetragonal structure with I4/mcm space group (around T₁ = 240 K), and the second one to an orthorhombic Imma phase below T₂ = 100 K. The magnetic properties have also been evaluated; the Fe substitution drives an evolution from a Pauli-paramagnetic state (x = 0) to a weak ferromagnetic state combined with antiferromagnetic interactions; the susceptibility and the saturation magnetization increases monotonically with increasing the Fe-doping content.     

Structure and magnetism in Pr3RuO7

The crystal structure of the ordered fluorite, Pr₃RuO₇, was refined from powder neutron diffraction data in Cmcm. An interesting structural feature is the presence of relatively well separated zig-zag chains of corner sharing RuO₆ octahedra, Ru–Ru interchain distance 6.61 Å vs. Ru–Ru intrachain distance of 3.76 Å. Magnetic susceptibility data show a Curie–Weiss behavior for T>225 K with C=5.96(4) emu K mol⁻¹ and θ_c=+11(2) K. In an attempt to separate the contributions of Pr(3+) and Ru(5+), the properties of isostructural Pr₃TaO₇ were also measured, yielding C=4.63(3) emu K mol⁻¹. Thus, the contribution of Ru(5+), 4d³, S=3/2, to the measured Curie constant is estimated to be 1.33 emu K mol⁻¹, not far from the spin-only value of 1.87 emu K mol⁻¹. This supports the view that the Ru 4d electrons are localized and magnetic, not itinerant. A susceptibility maximum at about 50 K is attributed to long-range magnetic order and this is substantiated by neutron diffraction data. There is little evidence for one-dimensional antiferromagnetic correlations in this material but behavior characteristic of short-range ferromagnetic correlations attributed to Pr–Ru exchange interactions are found in the temperature range 50–200 K, consistent with the positive θ_c.     

High pressure neutron diffraction study of the magnetoresistive 1222-type ruthenocuprate,RuSr2Nd0.9Y0.2Ce0.9Cu2O10

The crystal structure of the 1222-type ruthenocuprate RuSr₂Nd_0.9Y_0.2Ce_0.9Cu₂O₁₀ has been studied by time-of-flight neutron diffraction at temperatures 100–160 K and pressures up to 5 GPa. The structure has tetragonal I4/mmm symmetry throughout (e.g. a = 3.8104(2) Å and c = 28.125(3) Å at 160 K and 5.1 GPa) with no significant distortions observed at the 140 K Ru spin ordering transition. The strongly bonded Cu–O and Ru–O network leads to a bulk modulus of 145 GPa which is high for layered cuprates, with a low anisotropy in the cell compressibility (k_c/k_a = 1.32). The Cu–O–Cu buckling angle and the tilting of the CuO₅ square pyramids decreases with pressure, but the in-plane rotation of the RuO₆ octahedra increases.     

Structural,thermal, spectroscopic and magnetic studies of the (C2N2H10)0.5[FexV1−x(HPO3)2] (x = 0.26, 0.52, 0.74) solid solution

The (C₂N₂H₁₀)_0.5[Fe_xV_1−x(HPO₃)₂] (x = 0.26, 0.52 0.74) compounds have been obtained by mild solvothermal conditions in the form of micro-crystalline powder with brown color. The crystal structures were refined by X-ray powder diffraction data using the Rietveld method. The compounds crystallize in the monoclinic system, space group P2/c with the unit-cell parameters, a = 9.262(5) Å, b = 8.823(5) Å, c = 9.714(6) Å, β = 120.84(3)°; a = 9.245(1) Å, b = 8.823(1) Å, c = 9.698(1)Å, β = 120.80(1)° and, a = 9.254(4)Å, b = 8.822(4)Å, c = 9.702(4)Å, β = 120.73(3)° for (C₂N₂H₁₀)_0.5[Fe_0.26V_0.74 (HPO₃)₂] (1), (C₂N₂H₁₀)_0.5[Fe_0.52V_0.48(HPO₃)₂] (2), and (C₂N₂H₁₀)_0.5[Fe_0.74V_0.26(HPO₃)₂] (3). The compounds show an open crystalline structure with three-dimensional character, whose formula for the anionic inorganic skeleton is [M(HPO₃)₂]²⁻. The inorganic framework is formed by [MO₆] octahedra inter-connected by phosphite groups. The structure of the compounds exhibits channels extended along the [1 0 0] and [0 0 1] directions and the ethylendiammonium cations are located inside these channels, linked through hydrogen bonds and ionic interactions. The infrared spectra show the bands corresponding to the stretching (P–H) vibration of the phosphite group and the band corresponding to the deformation mode of the ethylendiammonium cation, δ(NH₃⁺). The thermal and thermodiffractometric behavior show that the compounds are stable up to approximately 300 °C, at higher temperatures the decomposition of the crystal structure by calcination of the organic cation starts. The diffuse reflectance spectra show bands of the V³⁺ ion (d²), and a band of the Fe³⁺ ion (d⁵), in a slightly distorted octahedral symmetry. The values of the Dq and Racah parameters (B and C) have been calculated for the V(III) cation. Magnetic measurements were performed on a powdered sample from 5 to 300 K at magnetic fields 1000, 500 and 100 G, in the ZFC and FC modes. At the magnetic field of 1000 G antiferromagnetic interactions were observed, but at 100 G have been detected higher values of the χ_m in the FC mode than those observed in the ZFC one, indicating the existence of a dominant ferromagnetic component at low temperature. The magnetization measurements show hystheresis loops at 5 K, with values of the remanent magnetization and coercive field of 1.91 emu/mol and 23 Gauss for (1), 25 emu/mol and 300 Gauss for (2), and 3 emu/mol and 50 Gauss for the compound (3).     

The LiyNi0.2Mn0.2Co0.6O2 electrode materials: A structural and magnetic study

Layered LiNi_0.2Mn_0.2Co_0.6O₂ phase, belonging to a solid solution between LiNi_1/2Mn_1/2O₂ and LiCoO₂ most commercialized cathodes, was prepared via the combustion method at 900 °C for a short time (1 h). Structural and magnetic properties of this material during chemical extraction were investigated. The powders adopted the α-NaFeO₂ structure with almost none of the well-known Li/Ni cation disorder. The analysis of the magnetic properties in the paramagnetic domain agrees with the combination of Ni²⁺ (S = 1), Co³⁺ (S = 0) and Mn⁴⁺ (S = 3/2) spin-only values. X-ray analysis of the chemically delithiated Li_yNi_0.2Mn_0.2Co_0.6O₂ reveals no structural transition. The process of lithium extraction from and insertion into LiNi_0.2Mn_0.2Co_0.6O₂ was discussed on the basis of ex situ EPR experiments and magnetic susceptibility. Oxidation of Ni²⁺ (S = 1) to Ni³⁺ (S = 1/2) and to Ni⁴⁺ (S = 0) was observed upon lithium removal.     

(C2N2H10)[FexV1−x(HPO3)F3] (x = 0.44, 0.72): Two new organically templated phosphites: Solvothermal synthesis and structural,thermal, spectroscopic and magnetic studies

(C₂N₂H₁₀)[Fe_xV_1−x(HPO₃)F₃] (x = 0.44, 0.72) have been synthesized using mild solvothermal conditions under autogenous pressure and the ethylenediamine molecule as templating agent. The crystal structures have been determined from X-ray single-crystal diffraction data. The compounds crystallize in the orthorhombic P2₁2₁2₁ space group with Z = 4 and unit-cell parameters a = 12.8494(9), b = 9.5430(6), c = 6.4372(5) Å, and a = 12.8578(1), b = 9.5342(1), c = 6.4370(7) Å for (C₂N₂H₁₀)[Fe_0.44V_0.56(HPO₃)F₃] and (C₂N₂H₁₀)[Fe_0.72V_0.28(HPO₃)F₃], (1) and (2), respectively. These isostructural compounds exhibit a monodimensional crystal structure formed by pillared double anionic chains with the formula [M(HPO₃)F₃]²⁻, extended along the [0 0 1] direction. These doubled ionic chains are the result of the linking of two simple chains in which there are alternating octahedral [MO₃F₃] and tetrahedral groups [HPO₃]. The ethylendiammonium cations are placed in the space delimited by three different chains. The metallic ions are interconnected by the pseudo-pyramidal (HPO₃)²⁻ phosphite oxoanions, adopting a slightly distorted octahedral geometry. The IR spectra show bands corresponding to the phosphite oxoanion and the ethylendiamonium cation at 2400 and 1600 cm⁻¹, respectively. The thermogravimetric analyses show that these phases are stable up to ca. 280 °C, at higher temperatures, the decomposition of the crystal structure begins by calcination of the organic cation and the elimination of the fluoride anions. The diffuse reflectance spectra show bands of the V³⁺ ion (d²) in octahedral symmetry. The values of the Dq (1540, 1540 cm⁻¹), and Racah parameters, B (560, 535 cm⁻¹) and C (3055, 3140 cm⁻¹) for (1) and (2), respectively, correspond with those usually found for octahedrically coordinated V(III) compounds. Magnetic measurements, performed on a powered sample from 5.0 to 300 K at 1000 G, in the ZFC and FC modes, indicate the existence of antiferromagnetic interactions.     

Low temperature synthesis of LnOF rare-earth oxyfluorides through reaction of the oxides with PTFE

A low temperature solid-state synthesis route, employing polytetrafluoroethylene (PTFE) and the rare-earth oxides, for the formation of the LnOF rare-earth oxyfluorides (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er), is reported. With the exception of LaOF, which forms in a tetragonal variant, rhomobohedral LnOF is found to be the major product of the reaction. In the case of PrOF, a transition from the rhombohedral to the cubic fluorite phase is observed on heating in air to 500 °C. X-ray diffraction shows the expected lanthanide contraction in the lattice parameters and bond lengths. Magnetic susceptibility measurements show antiferromagnetic-like ordering in TbOF, T_m = 10 K, with a metamagnetic transition at a field μ₀H_t = 1.8 T at 2 K. An antiferromagnetic transition, T_N = 4 K, is observed in GdOF. Paramagnetic behavior is observed above 2 K in PrOF, NdOF, DyOF, HoOF and ErOF. The magnetic susceptibility of EuOF is characteristic of Van Vleck paramagnetism.     

Magnetic moments and exchange interaction in Sm(Co, Fe)5 from first-principles

The electronic structure, magnetization and exchange interaction in Sm(Co_1?xFe_x)₅ with x = 0–1 are studied from a first-principles density functional calculation. The dependence of the magnetization on Fe content shows a 3d-like Slater–Pauling relationship in these alloys. As the Fe content x increases from 0 to 1.0, the magnetization increases from 7.8 μ_B to 10.6 μ_B (x = 0.8) and then decreases to 10.0 μ_B (x = 1). The effective exchange interaction parameters show a peak value around x = 0.6, which is ascribed to the exchange parameters between Fe and Co being larger than those for Co–Co and Fe–Fe pairs. The estimated T_C from the calculated exchange parameters range between 890 K and 1357 K in Sm(Co_1?xFe_x)₅ using a multi-sublattices mean field model.     

Facile syntheses,crystal structures and magnetic properties of NdBaMnCoO5 and NdBaMnCoO6

NdBaMnCoO₅ has been synthesized using a mass-flow controlled reducing reaction, followed by low (800 °C) temperature oxidation to produce NdBaMnCoO₆. The structures and properties have been analyzed using powder neutron diffraction and magnetic susceptibility measurements. At room temperature both phases are tetragonal with unit cells a_p × a_p × 2a_p (a_p = perovskite subcell length) showing full order of the A-site cations and no order of either B-site cations or charge. Low temperature neutron diffraction shows a fully antiferromagnetic (G-type) structure for NdBaMnCoO₅ (cell √2a_p × √2a_p × 2a_p) and an orthorhombic cell (2a_p × 2a_p × 2a_p) for NdBaMnCoO₆.     

Electron paramagnetic resonance (EPR) investigations of the local environment around Co2+ ions doped in PbMoO4 single crystals – Correlation with optical studies

The electron paramagnetic resonance (EPR) measurements of PbMoO₄ single crystals doped with Co²⁺ ions (0.5 wt.%) carried out at temperature range 10–12 K are reported. The observed spectra reveal evidently two major groups of EPR spectra, each group consists of eight hyperfine structure components associated with the Co nuclear spin I = 7/2. Additionally, EPR spectra of the unintentional impurities have been observed, which have been assigned to Nd³⁺ ions in PbMoO₄. The analysis of the various characteristic features of the EPR spectra as well as our earlier optical absorption studies of Co²⁺:PbMoO₄ indicate that the Co²⁺ ions are located at two crystallographically distinct complexes, denoted Co²⁺(α) and Co²⁺(β). The fictitious ‘spin’ S′ = ½ has been assigned to the observed ground Kramers doublet state of each of the two Co²⁺ complexes tentatively identified as located at the Mo sites in PbMoO₄. The spin Hamiltonian parameters for Co²⁺ (S′ = ½) ions in PbMoO₄, including the components of the Zeeman and hyperfine structure tensors g_ij and A_ij, respectively, are experimentally determined for the first time. Comparison of our results with the values of g_ij and A_ij for Co²⁺ ions in other structurally similar compounds further strengthens this conclusion. Correlation of the present results with those obtained earlier from optical studies is discussed. Alternative assignments of the ground states and options for the origin of the ‘spin’ S′ = ½ for Co²⁺ ions in PbMoO₄ will be considered in a subsequent paper.     

Microwave dielectric properties of BaO–TeO2 binary compounds

A series of BaO–TeO₂ binary ceramic compounds were explored for microwave dielectric applications with ultra-low processing temperatures. During the calcination of mixed BaCO₃ and TeO₂ raw powders, BaTe₄O₉, BaTe₂O₆, BaTeO₃, and Ba₂TeO₅ phases were obtained through the sequential phase formations from Te-rich to Ba-rich phases at temperatures ranging from 500 to 850 °C. Sintering temperatures were as low as only 550 °C for the Te-rich phases. Barium tellurate ceramics exhibited excellent microwave dielectric properties with intermediate dielectric permittivities and high quality factors (Q). The dielectric properties at microwave frequencies were ε_r = 10–21, Q × f = 34,000–55,000 GHz, and TCf = − 51 to − 124 ppm/°C, depending on compositions.     

Controlled synthesis and magnetic properties of Co1−xNix/MWCNT nanocomposites by microwave irradiation

Co_1?xNi_x alloy nanoparticles (x = 0.2, 0.5, 0.6, and 0.8) with the diameter 15–28 nm attached on the surface of multi-walled carbon nanotubes (MWCNTs) were prepared to form Co_1?xNi_x/MWCNT nanocomposites by microwave irradiation. Experimental results demonstrated that Co_1?xNi_x alloy nanoparticles with quasi-spherical and face-centered cubic structure had been attached on the MWCNTs, the composition and size of Co_1?xNi_x alloy nanoparticles could be controlled through adjusting the atomic ratios of metal Co to Ni in the mixed acetate solution, the microwave power and microwave irradiation time, respectively. Both the coercivity and the saturation magnetization of Co_1?xNi_x alloy nanoparticles increased with increasing Co concentration from x = 0.8 to 0.5, and decreased when Co concentration was increased from x = 0.5 to 0.2. These confirm that microwave synthesis is promising for fabricating alloy nanoparticles attached on MWCNTs for magnetic storage and ultra high-density magnetic recording applications.     

Structure investigation of TeO2–BaO glass employing ultrasonic study

Tellurite glasses of the system (100 − x)TeO₂–xBaO for different compositions of BaO (x = 9, 12, 15, 18, 20 and 22 wt.%) have been prepared by rapid quenching method. A decrease in ultrasonic velocities and density, which is followed by an increase in attenuation with a progressive increase in BaO content has been observed at room temperature and also over a wide range of temperatures. The above results reveal the softening of the glass network due to the progressive transformation of the rigid framework characteristics of TeO₂ resulting in the formation of distorted TeO_3 + 1 units followed by the creation of regular TeO₂ (4 → 3 + 1 → 3) sites and the formation of ionic character bond (NBO).     

Thermoelectric and magnetic properties of Ca3Co4–xCuxO9 + δ with x = 0.00, 0.05, 0.07, 0.10 and 0.15

Ca₃Co_4–xCu_xO_9 + δ (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples were prepared by conventional solid-state synthesis and their thermoelectric properties were systematically investigated. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. Ca₃Co_3.85Cu_0.15O_9 + δ had the highest power factor of 2.17 μW cm⁻¹ K⁻² at 141 K, representing an improvement of about 64.4% compared to undoped Ca₃Co₄O_9 + δ. Magnetization measurements indicated that all the samples exhibit a low-spin state of cobalt ions. The observed effective magnetic moments decreased with increasing copper content.     

Synthesis and magnetocaloric properties of La0.85K0.15MnO3 nanoparticles

In this paper, La_0.85K_0.15MnO₃ nanoparticles were successfully synthesized at relatively low calcinated temperature from a polyaminocarboxylate complex precursor with diethylenetriaminepentaacetic acid (H₅DTPA) as ligand, and the magnetocaloric properties were investigated. The phase transformation, chemical composition, and microstructure of La_0.85K_0.15MnO₃ nanoparticles were characterized by X-ray diffraction (XRD), thermogravimetric (TG), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and electron diffraction (ED). The results revealed that La_0.85K_0.15MnO₃ nanoparticles calcined at temperatures within the range of 600–1000 °C are of pure single-phase rhombohedral structure and the grain sizes were precisely controlled by varing the calcined temperature. The relationship between magnetocaloric properties and the calcined temperature of La_1?xK_xMnO₃ nanoparticles was also investigated systematically. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered that the Curie temperature T_C is 274.5 K and is independent of the calcined temperature. From the measurements and calculation of isothermal magnetization at different temperatures, the maximum magneticentropy changes close to T_C (274 K) of the samples calcined at 600 °C, 800 °C and 1000 °C are 2.02, 3.06 and 3.56/kg K at H = 2T, respectively. Also La_0.85K_0.15MnO₃ nanoparticle displays a second-order phase transition. These results suggest that this material is a candidate for use as an active for magnetic refrigerent around the room temperature.     

Magnetic properties of nanocrystalline ɛ-Fe3N and Co4N phases synthesized by newer precursor route

Nanocrystalline ɛ-Fe₃N and Co₄N nitride phases are synthesized first time by using tris(1,2-diaminoethane)iron(II) chloride and tris(1,2-diaminoethane)cobalt(III) chloride precursors, respectively. To prepare ɛ-Fe₃N and Co₄N nitride phases, the synthesized precursors were mixed with urea in 1:12 ratio and heat treated at various temperatures in the range of 450–900 °C under the ultrapure nitrogen gas atmosphere. The precursors are confirmed by FT-IR study. The ɛ-Fe₃N phase crystallizes in hexagonal structure with unit cell parameters, a = 4.76 Å and c = 4.41 Å. The Co₄N phase crystallizes in face centred cubic (fcc) structure with unit cell parameters, a = 3.55 Å. The estimated crystallite size for ɛ-Fe₃N and Co₄N phases are 29 nm and 22 nm, respectively. The scanning electron microscopy (SEM) studies confirm the nanocrystalline nature of the materials. The values of saturation magnetization for ɛ-Fe₃N and Co₄N phases are found to be 28.1 emu/g and 123.6 emu/g, respectively. The reduction of magnetic moments in ultrafine materials compared to bulk materials have been explained by spin pairing effect, lattice expansion, superparamagnetic behaviour and canted spin structures at the surface of the particles.     

Crystal structure,oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.1M0.1O3−δ (M = Fe,Cr, Mn,Zr) oxygen-permeable membranes

Oxygen-permeable ceramic membrane materials of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) and partially Fe-substituted Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr, Mn, Zr) were synthesized by solid-state reaction method. These materials possess purely cubic perovskite structure with the exception of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn, Zr), in which minor impurities exist. Oxygen permeability across these dense membrane disks were measured under an air/He oxygen partial pressure gradient in the temperature range of 973–1123 K. The results demonstrated that the oxygen permeation fluxes of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Mn, Zr) membranes increased in the following order: Fe (BSCFO) > Cr > Zr > Mn. The corresponding activation energies for oxygen permeation of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Zr) membranes were calculated to be similar (53 ± 4 kJ/mol), which was remarkably lower than that (99 ± 3 kJ/mol) of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn) membrane. In addition, good oxygen permeation stability of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr) membrane was achieved at the temperature lower than 1123 K. The X-ray diffraction (XRD) and differential thermal analysis (DTA) experiments showed that the structural stability of BSCFO could be significantly improved when Fe ions in the BSCFO material were partially substituted by Cr, Mn or Zr ions.     

Structural and magnetic study of thin films based on anisotropic ternary alloys FeNiPt2

L1₀ ordered (Fe–Ni)₅₀Pt₅₀ alloy films with perpendicular magnetic anisotropy were successfully prepared by interdiffusing FePt(0 0 1) and NiPt(0 0 1) layers co-deposited on MgO(0 0 1) substrates by MBE. The [0 0 1] growth direction corresponds to the epitaxy of the alloy on the substrate and is the interesting growth orientation to get a perpendicular magnetization. The X-ray diffraction shows a high L1₀ chemical order (S = 0.7 ± 0.1). The easy magnetization direction is perpendicular for all samples. The MFM images display highly interconnected stripes corresponding to up and down orientations of the magnetization. Large uniaxial magnetic anisotropy (K_u = 9.10⁵ J/m³) and suitable magnetic transition temperature (T_C = 400 K) are obtained. The addition of Ni changes the spin–orbit interaction in the FePt compound system, hence causes a decrease of anisotropy, saturation magnetization and coercivity.