Theoretical study of bimetallic magnetic nanostructures: Co<Subscript>n</Subscript>Pd<Subscript>N-n</Subscript>, n=0,1,...N,N=3,5,7,13

The geometrical structure, atomic ordering, electronic and magnetic properties of closed shell Co_n Pd_N-n for n=0, 1, ...,N, and N=3, 5, 7 and 13, have been studied. The whole set of values for N and n, as well as most of the isomers were analyzed. The electronic, magnetic, and structural characteristics were calculated within the density functional theory by means of the SIESTA code using the generalized gradient approximation of Perdew-Burke-Ernzerhof for the exchange and correlation potential. The lowest energy geometries obtained were the triangle, hexahedron, decahedron, and icosahedron. We also performed calculations assuming noncollinear magnetic moment arrangements, but the lowest energy configurations correspond to the collinear geometry. Finally we analyze in detail the chemical order between the two components in the cluster and its influence in the magnetic order.     

Fluorimetry Studies of Oscillating Chemilumnescence in the Luminol= H<Subscript>2</Subscript>O<Subscript>2</Subscript>= KSCN= CuSO<Subscript>4</Subscript>= TMAOH System

Oscillating chemical reactions are complex systems, involving a large number of chemical species. In oscillating chemical reaction, some species, usually a reaction intermediate, exhibit fluctuation in its concentration. In this report, visible oscillating chemiluminescence produced by the addition of luminol (3-aminophthalhydrazide) to the oscillating system of H₂O₂=KSCN=CuSO₄=TMAOH was investigated using spectrofluorimetry method. The effects of ingredient concentration of the oscillating system and complexing agents like citric acid and cysteine on the behavior of the oscillating system were investigated. Moreover, the influence of nonaqueous solvents such as ethanol and ethylene glycol has been studied.     

Structural,dielectric, electrical and piezoelectric properties of Ba<Subscript>4</Subscript>SrRTi<Subscript>3</Subscript>V<Subscript>7</Subscript>O<Subscript>30</Subscript> (R=Sm,Dy) ceramics

Polycrystalline samples of Ba₄SrRTi₃V₇O₃₀ (R=Sm and Dy), members of the tungsten-bronze family, were prepared using a high-temperature, solid-state reaction technique and studied their electrical properties (using complex impedance spectroscopy) in a wide range of temperature (31–500°C) and frequency (1 kHz-1 MHz). Preliminary structural (XRD) analyses of these compounds show the formation of single-phase, orthorhombic structures at room temperature. The scanning electron micrographs (SEM) provided information on the quality of the samples and uniform distribution of grains over the entire surface of the samples. Detailed studies of the dielectric properties suggest that they have undergone ferroelectric-paraelectric phase transition well above the room temperatures (i.e., 432 and 355°C for R= Sm and Dy, respectively, at frequency 100 kHz). Measurements of electrical conductivity (ac and dc) as a function of temperature suggest that the compounds have semiconducting properties much above the room temperature, with negative temperature coefficient of resistance (NTCR) behavior. The existence of ferroelectricity in these compounds was confirmed from a polarization study.      

Crystal-structure and magnetic phase transformations in solid solutions of BiFeO<Subscript>3</Subscript>-AFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (A = Ca,Sr, Ba,Pb)

Solid solutions of Bi_{1 ? x} A _x (Fe_{1 ? x/2}Nb _x/2)O₃, where A = Ca, Ba, and Pb, are obtained and their crystal structure and magnetic properties are investigated. It is shown that for A = Ca and x ≈ 0.15, the symmetry of the unit cell changes from rhombohedral (space group R3c) to orthorhombic (Pbnm). The transformation leads to the emergence of spontaneous magnetization due to the Dzyaloshinskii-Moriya interaction. Solid solutions with A = Pb remain rhombohedral up to a concentration of x = 0.3. Spontaneous magnetization sharply increases in the compound with x ≈ 1 at low temperatures and is due to the formation of the spin-glass component.     

Magnetic behaviour of doped dimer compounds Sr<Subscript>3</Subscript>Cr<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>8</Subscript> (M = V,Mn)

Wilson chains, based on a logarithmic discretization of a continuous spectrum, are widely used to model an electronic (or bosonic) bath for Kondo spins and other quantum impurities within the numerical renormalization group method and other numerical approaches. In this short note we point out that Wilson chains can not serve as thermal reservoirs as their temperature changes by a number of order ΔE when a finite amount of energy ΔE is added. This proves that for a large class of non-equilibrium problems they cannot be used to predict the long-time behavior.     

Thermal capacity,diffusion, and conductivity of Nd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>Mno<Subscript>3</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0.45 and 0.5) manganites

Temperature dependences (77–300 K) of the thermal capacity, diffusion, and conductivity are investigated for the Nd _0.5 Sr _0.5 MnO ₃ and Nd _0.55 Sr _0.45 MnO ₃ polycrystalline samples. The examined characteristics show anomalous behavior in the magnetic phase transition and transition to the charge-ordered state. It is demonstrated that the main reason for a sharp decrease in the thermal conductivity during Nd _0.5 Sr _0.5 MnO ₃ transition into the antiferromagnetic charge-ordered state is a change in the phonon spectrum caused by the lattice compression. A temperature dependence of the free phonon path is calculated for the examined temperature interval based on the thermal diffusion obtained and the literature data on the sound propagation velocity. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 72–75, April, 2007.     

Preparation of RE<Subscript>2</Subscript>O<Subscript>2</Subscript>SO<Subscript>4</Subscript> (RE=La,Pr-Lu) microspheres from rare-earth-based infinite coordination polymers

Rare-earth-based infinite coordination polymer (RE-ICP) spheres with diameters ranging from 50 nm to 2 μm have been prepared using meso-2,3-dimercaptosuccinic acid (DMSA) as ligand under hydrothermal conditions. RE₂O₂SO₄ microspheres with similar morphology were obtained by calcining the corresponding RE-ICP spheres. However, as for Ce-ICP and Sc-ICP, CeO₂ and Sc₂O₃ were obtained. The products were characterized using X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, infrared spectroscopy, laser Raman spectrometry, and energy-dispersive X-ray spectrum. Elemental analysis and inductive coupled plasma atomic emission spectrometer were adopted to study the composition of the Eu-ICP. To explore their potential applications, several samples of the products were selected and their properties were investigated. The Eu-ICP and Eu₂O₂SO₄ microspheres give strong red emissions when excited with a 394-nm ultraviolet light. Furthermore, the Eu-ICP displays a high selectivity for Fe(III). The obtained CeO₂ has a strong absorption in the UV region and the Gd₂O₂SO₄ microspheres show paramagnetic behavior.

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Graphical abstract A series of RE₂O₂SO₄ microspheres were prepared using a coordination polymer precursor method.

     

Size effect in nanocrystalline manganites La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">A</Emphasis><Subscript>x</Subscript>MnO<Subscript>3</Subscript> (<Emphasis Type="Italic">A</Emphasis>=Ag,Sr)

Nanocrystalline samples of the manganites La_0.9Ag_0.1MnO₃, La_0.7Ag_0.3MnO₃, and La_0.7Sr_0.3MnO₃ were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.     

Enhancement of antiferromagnetic interaction and transition temperature in M<Subscript>3</Subscript>TeO<Subscript>6</Subscript> systems (M = Mn,Co, Ni,Cu)

Several M₃TeO₆ (M = Mn, Co, Ni, Cu) oxides order antiferromagnetically at low temperatures (?60 K), while displaying interesting dielectric properties at high temperatures (ferroelectricity below 1000 K in M = Ni case). We have investigated and analyzed the structural and magnetic properties of Mn-doped Co₃TeO₆ and Ni₃TeO₆, which order antiferromagnetically at temperatures higher than their undoped counterparts.     

Synthesis and Luminescent Properties of M<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>: Eu (M=Sr,Ba) Nanophosphors

A solution combustion route for the synthesis of Eu³⁺-activated M₂V₂O₇ (M = Sr, Ba) and their luminescent properties have been investigated. Structure and luminescent characteristics of Sr₂V₂O₇:Eu³⁺ and Ba₂V₂O₇:Eu³⁺ nanophosphors have been studied by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, fluorescence spectrometry and Fourier transform infra-red spectroscopy. The incorporation of Eu³⁺ activator in these nanoparticles has been checked by luminescence characteristics. These nanoparticles have displayed red color under a UV source which is due to characteristics transition of Eu³⁺ from ⁵D₀ → ⁷F₂ at 613 nm in both Sr₂V₂O₇:Eu³⁺ and Ba₂V₂O₇:Eu³⁺ nanophosphors. In addition, the optimal Eu³⁺ - doped contents of Sr_2(1-x)Eu_2xV₂O₇ and Ba_2(1-x)Eu_2xV₂O₇ nanophosphors for both were 4 mol%.     

Superconductivity and spectroscopy of heterofullerides Rb<Subscript>2</Subscript>MC<Subscript>60</Subscript>, K<Subscript>2</Subscript>MC<Subscript>60</Subscript>, and KM<Subscript>2</Subscript>C<Subscript>60</Subscript> (M = Mg,Be)

A series of new heterofullerides with compositions Rb₂MC₆₀, K₂MC₆₀, and KM₂C₆₀ (M = Mg, Be) have been synthesized. Measurements of the temperature dependences of the magnetic susceptibility in the temperature interval from 4.2 to 300 K reveal a superconducting transition in heterofullerides K₂MgC₆₀, KMg₂C₆₀, K₂BeC₆₀, and Rb₂BeC₆₀ at temperatures T _c = 13–22 K. The electron states with uncompensated spin are studied by the electron paramagnetic resonance technique. The contributions of conduction electrons and localized electrons to the paramagnetic susceptibility are extracted.     

The structural properties of RbMnX<Subscript>3</Subscript> (X = F,Cl, Br) halides

The results of nonempirical calculation of energies of three polytypes (cubic, two-layer hexagonal, and six-layer hexagonal) are given for RbMnX₃ (X = F, Cl, Br) crystals. The calculation is performed using an ionic crystal model with regard for the deformability and the dipole and quadrupole polarizabilities of ions. The behavior of these crystals under the action of hydrostatic pressure is studied. It is demonstrated that, at normal pressure, the RbMnCl₃ and RbMnBr₃ crystals have a six-layer hexagonal structure. At pressures above 11 kbar, RbMnCl₃ passes to a phase with a cubic structure; RbMnBr₃ at pressures above 90 kbar passes to a phase with a two-layer hexagonal structure. The RbMnF₃ crystal under normal conditions has a cubic structure and experiences no phase transformations under the effect of pressure. The obtained results are in satisfactory agreement with the known experimental data.     

Experimental evidence of a two-phase magnetic state in thin epitaxial films of Re<Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> (Re = La,Pr, Nd,Gd)

Thin epitaxial films of Re_0.6Ba_0.4MnO₃ (Re = La, Pr, Nd, Gd) on (001)-oriented single crystal SrTiO₃ and ZrO₂(Y₂O₃) substrates have been prepared and studied. All films possess a cubic perovskite structure, except for the film with Re = La, which exhibited a rhombohedral distortion of the perovskite lattice. The results show evidence for the presence of two magnetic phases, ferromagnetic (FM) and antiferromagnetic (AFM), in the films studied: (i) the magnetization isotherm M(H) appears as a superposition of a linear component (characteristic of antiferromagnets) and a small spontaneous magnetization component; (ii) the magnetic moment per formula unit is significantly reduced as compared to the value expected for the complete FM or ferrimagnetic ordering; (iii) there is a difference between magnetizations of the samples cooled with and without an applied magnetic field, which is preserved in the entire range of magnetic fields studied (50 kOe); (iv) the temperature dependence of the magnetization M(T) in strong magnetic fields is close to linear (for the composition with Re = Gd, M(T) is described by a Langevin function for superparamagnets with a cluster moment of 2μ_B); and (v) the magnetization hysteresis loops of the field-cooled samples are shifted along the field axis. The exchange integral (characterizing the Mn-O-Mn coupling via the FM-AFM phase boundary) estimated from the latter shift is | J|=10^?6 eV. This value is two orders of magnitude lower than the negative exchange integral between the FM layers in ReMnO₃, which makes the presence of a transition layer at the FM-AFM phase boundary unlikely. The temperature dependences of electrical resistance and magnetoresistance exhibit maxima at the Curie temperature (T_C), where the magnetoresistance reaches a colossal value. This behavior indicates that the two-phase magnetic state is caused by a strong s-d exchange.     

Electronic structure of titanium dichalcogenides TiX<Subscript>2</Subscript> (X = S,Se, Te)

The electronic structure and the chemical bond in titanium dichalcogenides TiX₂ (X = S, Se, Te), which are promising electrode materials for lithium batteries, are studied experimentally and theoretically. It is found that the X-ray photoelectron spectra of the valence bands and the core levels of titanium and its X-ray L _{2, 3} absorption spectra demonstrate a change in the ionic and covalent components of the chemical bond in these compounds. The densities of states in these compounds are calculated by the full-potential augmented-plane-wave method, and multiplet calculations of the X-ray L _{2, 3} absorption spectra of titanium are performed. It is shown that, in the row TiS₂-TiSe₂-TiTe₂, the covalence increases, the ionicity of the chemical bond decreases, and the effect of the crystal field of a ligand is weakened.     

Synthesis and characterization of mesoporous and hollow-mesoporous M<Subscript>x</Subscript>Fe<Subscript>3-x</Subscript>O<Subscript>4</Subscript> (M=Mg,Mn, Fe,Co, Ni,Cu, Zn) microspheres for microwave-triggered controllable drug delivery

Spinel ferrites can be used in magnetic targeting and microwave heating and can therefore be used for targeted and controllable drug delivery. We used the cetyltrimethylammonium bromide-assisted solvothermal method to synthesize a series of spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) with a mesoporous or hollow-mesoporous structure suitable for direct drug loading and the particle diameters ranging from 200 to 350 nm. We investigated the effects of M²⁺ cation on the morphology and properties of these products by analyzing their transmission electron microscopy images, mesoporous properties, magnetic properties, and microwave responses. We chose hollow-mesoporous M_xFe_3-xO₄ (M=Fe, Co, Zn) nanoparticles, which had better overall properties, for the drug VP16 (etoposide) loading and microwave-controlled release. The Co_xFe_3-xO₄ and Fe₃O₄ particles trapped 61.5 and 64.8%, respectively, of the VP16, which were higher than that (60.4%) of Zn_xFe_3-xO₄. Controllable drug release by these simple magnetic nanocarriers can be achieved by microwave irradiation, and VP16-loaded Co_xFe_3-xO₄ released the most VP16 molecules (more than 50% after 1 h and 69.1% after 6 h) under microwave irradiation. Our results confirm the favorable drug loading and microwave-controlled delivery by these ferrites, and lay a theoretical foundation to promote clinical application of the targeted controllable drug delivery system.

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Graphical abstract In the present study, we prepared mesoporous or hollow-mesoporous spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) by CTAB-assisted solvothermal method and solved the problem of Cu and Ni impurities in Cu_xFe_3-xO₄ and Ni_xFe_3-xO₄ products by means of magnetic separation and additional redox reactions, respectively. We investigated the effects of the M²⁺ cation on the morphology, mesoporous properties, magnetic properties, and microwave responses of these ferrites. Then, the drug loading and microwave-controlled drug release of hollow-mesoporous M_xFe_3-xO₄ (M?=?Fe, Co, Zn) nanoparticles with better overall properties were also studied. Co_xFe_3-xO₄ has the best overall performances for microwave-controlled drug release.

     

Static displacement of a guest atom in filled skutterudites MFe<Subscript>4</Subscript>Sb<Subscript>12</Subscript> (M = La,Ca, Na)

The properties of filled skutterudites MFe₄Sb₁₂ (M = La, Ca, Na) have been analyzed using the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) methods. Two lines have been observed on the ¹³⁹La NMR spectrum of the LaFe₄Sb₁₂ compound and a substructure has been revealed in the ¹²¹Sb and ¹²³Sb lines in the NQR spectra of LaFe₄Sb₁₂ and CaFe₄Sb₁₂. The concept of the partial static displacement of guest atoms (M) in LaFe₄Sb₁₂ and CaFe₄Sb₁₂ has been proposed. The ab initio calculations confirm this assumption as well as give the displacement of a guest atom and indicate the absence of the splitting of the ¹³⁹La NMR line in the LaFe₄Sb₁₂ spectrum.     

Magnetic properties of X<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>/Cu(001) overlayers,X = Fe,Mn, and Ir: ab-initio calculations

We study the effect of a magnetic impurity with spin-half on a single propagating electron in a one-dimensional model system via the tight-binding approach. Due to the spin-dependent interaction, the scattering channel for the flying qubit is split, and its transmission spectrum is obtained. It is found that, the spin orientation of the impurity plays the role as a spin state filter for a flying qubit.     

Magnetic ordering in Dy<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>BaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> for x = 0.0 and 0.1

The crystal and magnetic structures of Dy_1-xCa_xBaCo₂O_5.5 for x = 0.0 and 0.1 have been studied by neutron powder diffraction and the crystal structures of both compounds were found to be best described in space group Pmmm with a a_p × 2a_p × 2a_p unit cells where a_p is the lattice parameter of the cubic perovskite unit cell. The a- and b-axes were found to decrease and increase abruptly between 315 and 350 K as the temperature increases and the unit cell volumes exhibit signs of excess thermal expansion in the temperature range from 260 to 315 K. Dy_0.9Ca_0.1BaCo₂O_5.5 orders antiferromagnetically for T ≤ 305 K into a G-type magnetic structure with a 2a_p × 2a_p × 2a_p magnetic unit cell. DyBaCo₂O_5.5 exhibits two magnetically ordered phases and a G-type magnetic structure was observed at the investigated temperatures 260 and 290 K. A 2a_p × 2a_p × 4a_p magnetic unit cell was needed for indexing of the magnetic reflections observed for T ≤ 230 K. The low temperature magnetic structure of DyBaCo₂O_5.5 is different from the observed magnetic structures of TbBaCo₂O_5.5 and HoBaCo₂O_5.5 despite the proximity of Tb, Dy and Ho in the periodic table. It is a relatively complex antiferromagnetic structure with both pyramidally and octahedrally coordinated Co ions in the intermediate spin state. It contains both ferro- and antiferromagnetic interactions and the magnetic moments are canted in the a, b-plane. The canting angles between the magnetic moments and the b-axis are 6.6 and 50.0° at 20 K for the pyramidally and octahedrally coordinated Co ions, respectively. The high and low temperature magnetic phases were found to coexist at 230 K.     

Synthesis and application of nanostructured MCo<Subscript>2</Subscript>O<Subscript>4</Subscript>(M=Co,Ni) for hybrid Li-air batteries

We report the synthesis of MCo₂O₄ (M=Co, Ni) on Ni-mesh by a simple metal acetate decomposition method. Stability tests of the samples in aqueous acidified LiCl, LiOH and LiTFSI in H₂O/DME showed that Co₃O₄/Ni and Co₃O₄-PVP/Ni are relatively stable in alkaline and neutral environments, with Co₃O₄/Ni being relatively more stable. For NiCo₂O₄/Ni and NiCo₂O₄-PVP/Ni, the low weight percentage change of cobalt in LiTFSI in H₂O/DME suggests that they are mostly stable in this electrolyte. The electrochemical performance of the Li-air cell was evaluated using Li anode and a LAGP ceramic separator with above mentioned electrolytes. Co₃O₄ showed slightly higher catalytic activity for oxygen reduction reaction (ORR) than for oxygen evolution reaction (OER) for the first three cycles. The cell with LiTFSI in H₂O/DME as aqueous catholyte showed that NiCo₂O₄ is a better catalyst for the OER than for the ORR, while the reverse was observed when LiOH was used as the electrolyte.