Peculiarities of the magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

Manganites of the Sm1?xSr_xMnO₃ system (x=0.33, 0.4, and 0.45) possess giant negative values of the magnetoresistance Δρ/ρ and the volume magnetostriction ω near the Curie temperature T_C. In the compound with x=0.33, the isotherms of Δρ/ρ, ω, and magnetization σ exhibit smooth variation and do not reach saturation up to maximum magnetic field strengths (120 kOe) studied (according to the neutron diffraction data, this substance comprises a ferromagnetic (FM) matrix with distributed clusters of a layered antiferromagnetic (AFM) structure of the A type). In the compounds with x=0.4 and 0.45 containing, besides the FM matrix and A-type AFM phase, a charge-ordered AFM phase of the CE type (thermally stable to higher temperatures as compared to the A-type AFM and the FM phases), the same isotherms measured at T ≥ T_C show a jumplike increase in the interval of field strengths between H_c1 and H_c2 and then reach saturation. In the interval H_c1 > H > H_c2, the σ, ω, and Δρ/ρ values exhibit a metastable behavior. At temperatures above T_C, the anisotropic magnetostriction changes sign, which is indicative of rearrangements in the crystal structure. The giant values of ω and Δρ/ρ observed at T ≥ T_C for all compounds, together with excess (relative to the linear) thermal expansion and a maximum on the ρ(T) curve, are explained by the phenomenon of electron phase separation caused by a strong s-d exchange. The giant values of magnetoresistance and volume magnetostriction (with ω reaching ～10^?3) are attributed to an increase in the volume of the FM phase induced by the applied magnetic field. In the compound with x=0.33, this increase proceeds smoothly as the FM phase grows through the FM layers in the A-type AFM phase. In the compounds with x=0.4 and 0.45, the FM phase volume increases at the expense of the charge-ordered CE-type AFM structure (in which spins of the neighboring manganese ions possess an AFM order). The jumps observed on the σ(H) curves, whereby the magnetization σ reaches ～70% of the value at T=1.5 K, are indicative of a threshold character of the charge-ordered phase transition to the FM state. Thus, the giant values of ω and Δρ/ρ are inherent in the FM state, appearing as a result of the magnetic-field-induced transition of the charge-ordered phase to the FM state, rather than being caused by melting of this phase.     

High stability of the goldalloy fullerenes:A density functional theory investigation of M₁₂@Au₂₀（M=Na,Al,Ag,Sc,Y,La,Lu,and Au） clusters

Discovering highly stable metal fullerenes such as the celebrated C 60 is interesting in cluster science as they have potential applications as building blocks in new nanostructures.We here investigated the structural and electronic properties of the fullerenes M 12 @Au 20(M=Na,Al,Ag,Sc,Y,La,Lu,and Au),using a first-principles investigation with the density functional theory.It is found that these compound clusters possess a similar cage structure to the icosahedral Au 32 fullerene.La 12 @Au 20 is found to be particularly stable among these clusters.The binding energy of La 12 @Au 20 is 3.43 eV per atom,1.05 eV larger than that in Au 32.The highest occupied molecular orbital-lowest unoccupied molecular orbital(HOMO-LUMO) gap of La 12 @Au 20 is only 0.31 eV,suggesting that it should be relatively chemically reactive.     

Investigation on the effects of addition of SiO<Subscript>2</Subscript> nanoparticles on ionic conductivity,FTIR, and thermal properties of nanocomposite PMMA–LiCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>

Composite polymer electrolyte systems composed of poly(methyl methacrylate) (PMMA) as the host polymer, lithium trifluoromethanesulphonate (also known as lithium triflate; LiCF₃SO₃) as dopant salt, and a variety of different concentrations of nano-sized fumed silica (SiO₂) as inorganic filler were studied. The effect upon addition of SiO₂ on the ionic conductivity of the composite polymer electrolytes was investigated, and it was proven that the ionic conductivity had been enhanced. In addition, the interfacial stability also showed improvement. Maximum conductivity was obtained upon addition of 2 wt.% SiO₂. The complexation of PMMA and LiCF₃SO₃ was verified through Fourier transform infrared studies. The thermal stability of the polymer electrolytes was also found to improve after dispersion of inorganic filler. This was proven in the thermogravimetric studies.     

Structure,stability, and thermomechanical properties of Ca-substituted Pr<Subscript>2</Subscript>NiO<Subscript>4 + δ</Subscript>

Ca-substituted layered nickelates with a general Pr_2–x Ca _x NiO_{4 + δ} composition (x = 0–0.7, Δx = 0.1) were prepared in the present work and their structural and physic-chemical properties were investigated in order to select the most optimal materials, which can be used as cathodes for solid oxide fuel cells. With an increase in Ca content in Pr_2–x Ca _x NiO_{4 + δ} the following tendencies were observed: (i) a decrease in the concentration of nonstoichiometric oxygen (δ), (ii) a decrease in the unit cell parameters and volume, (iii) stabilization of the tetragonal structure, (iv) a decrease of the thermal expansion coefficients, and (v) enchancement of thermodynamic stability and compatibility with selected oxygen- and proton-conducting electrolytes. The Pr_1.9Ca_0.1NiO_{4 + δ} material, having highest δ value, departs from the general “properties–composition” dependences ascertained. This indicates that oxygen non-stoichiometry has determining influence on the functional properties of layered nickelates.     

Ab initio study of structural,electronic, and elastic properties of M<Subscript>2</Subscript>SbP (M = Ti,Zr, and Hf)

This study predicts the structural behaviour of selected M₂SbP compounds with the same structure as MAX phases. Zero pressure results of the lattice parameters, equilibrium volume, and the internal parameter Z_M are calculated with an error less than 3%. Band structure, total and partial density of states were calculated and show the metallic character of these phases. Moreover we observed strong hybridising states; M d–P p, and M d–Sb p. The pressure dependence of the volume, and the lattice parameters were studied. The stiffness of M–P, and M–Sb bonds was discussed in term of relative length change under hydrostatic pressure. Hf₂SbP present the highest bulk modulus and the unidirectional elastic modulus C₃₃ is slightly greater than C₁₁.     

Study of ground states of <Superscript>3</Superscript>H, <Superscript>3,4,6</Superscript>He, <Superscript>6</Superscript>Li,and <Superscript>9</Superscript>Be nuclei by Feynman’s continual integrals method

The ground-state energies and the squared moduli of the ground-state wave functions are calculated for the ³H, ^3,4,6He, ⁶Li, and ⁹Be nuclei by Feynman’s continual (path) integrals method. The results are in satisfactory agreement with experimental data.     

Detailed Investigations of Neutron–Neutron Angular Correlations in Slow-Neutron-Induced Fission of <Superscript>233</Superscript>U, <Superscript>235</Superscript>U,and <Superscript>239</Superscript>Pu

An experiment devoted to studying neutron–neutron angular correlations in the slow-neutroninduced fission of ²³³U, ²³⁵U and ²³⁹Pu nuclei was performed. The experimentally determined angular dependence of the number of neutron–neutron coincidences was compared with the results of a Monte Carlo simulation for various values of the neutron-detection threshold in the range between 490 and 2080 keV. It was found that the experimental angular distributions in question can be described well under the assumption that 2% to 5% of all prompt fission neutrons are emitted isotropically in the laboratory frame. Probably, such neutrons can be interpreted as so-called scission neutrons directly associated with the nuclear-rupture instant. Energy distributions of this component were also obtained from the present analysis.     

Effect of mechanical activation on the phase composition,structure, and polymorphism of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O6 + δ oxide

The mechanical activation of YBa₂Cu₃O_6+δ powders is studied comprehensively using thermal analysis, scanning electron microscopy, high-temperature x-ray diffraction, and x-ray photoelectron spectroscopy. It is shown that the size of the coherent scattering regions is substantially reduced by intensive grinding (from 83 to 17 nm) and the material transforms to a nanostructured state. An increase in the reactivity of the material is found, which shows up as an enhanced accumulation of intercalated water in its structure. At the same time, no significant increase in the volume of impurity phases in the powder or in the influence of impurities on the crystal lattice parameters was observed. High-temperature x-ray studies of the lattice parameters of the YBa₂Cu₃O_6+δ phase reveal a jump (at temperatures of 430–630°C) and a decrease (above 630°C) in the parameter c when the powder is mechanically activated. A significant reduction in the temperature of the transition from the orthorhombic to the tetragonal structural modification is observed. These effects are explained by a change in the charge states of oxygen ions belonging to the base plane of the YBa₂Cu₃O_6+δ oxide during mechanical activation.     

Conductance studies of NaCl,KCl, NaBr,KBr, NaI,Bu<Subscript>4</Subscript>NI,and NaBPh<Subscript>4</Subscript> in water + 2-propoxyethanol mixtures at 298.15 K

The molar conductances of sodium chloride (NaCl), potassium chloride (KCl), sodium bromide (NaBr), sodium iodide (NaI), tetrabutylammonium iodide (Bu₄NI), and sodium tetraphenylborate (NaBPh₄) in water?+?2-propoxyethanol binary mixtures at T?=?298.15 K were determined. Conductance values were analyzed using a low-concentration chemical model (lcCM). The ionic conductivities of Na⁺, K⁺, Bu₄N⁺, Cl^?, I^?, Br^?, and BPh₄ ^? ions were determined using Fuoss–Hirsch’s assumption. The dependencies of the values of limiting molar conductance (Λ _o), limiting ionic conductance of the individual anions (λ _o ^± ), and Walden products (Λ _o???η and λ _o ^± ???η) as a function of the composition of water?+?2-propoxyethanol binary mixtures were discussed.

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Graphical abstract Limiting molar conductances Λ _o [S·cm²·mol^?1] of NaCl, NaBr, NaI, KCl, NaBPh₄, and NBu₄I as a function of the mole fraction, x ₂, in water (1)?+?2-propoxyethanol (2) mixtures at 298.15 K: empty diamonds NaCl, empty squares NaBr, empty triangles NaI, filled triangles KCl, filled squares NaBPh₄, and filled diamonds NBu₄I

     

Emission of light nuclei (<Emphasis Type="Italic">p,d, t</Emphasis>, α) in the process of annihilation of slow antiprotons in nuclear emulsion

The annihilation of slow (∼7 MeV) antiprotons in nuclear emulsion has been studied. The yields and energy spectra of p, d, t, and α particles in the evaporation region have been measured. The shape of the spectra of p, d, and t is in agreement with the Maxwell distribution and the excitation energy of a nucleus is consistent with a theoretical estimate for evaporation from the equilibrium state. The probability of the absorption of antiprotons inside the nucleus estimated from the multiplicity of h particles is ɛ = (2.0 ± 0.6) × 10⁻². The relative d/p yield coincides with a similar ratio appearing in the capture of slow π⁻ mesons by nuclei in the nuclear emulsion. The yields of t and α particles in the process of the annihilation of antiprotons are much higher than those in a similar process for pions. To identify g particles (0.29 < β < 0.70), energy losses dE/dx on ionization and multiple scattering have been measured. In this velocity region, the yields of p, d, t, and pions have been observed. The ratios (n _d /n _p ) _g , (n _d /n _p ) _b , and n _d /n _p measured in the capture of π⁻ mesons are almost the same. In this velocity range (g particles), α particles have not been observed.     

Simulation of the structural,electronic, and magnetic properties of Fe<Subscript>3</Subscript>C<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>x</Subscript> borocementites

The structural, electronic, and magnetic properties and the enthalpy of formation of iron borocementites Fe₃C_1?x B_x (x= 0, 0.25, 0.50, 0.75, 1.00) are analyzed using ab initio calculations in the framework of the electron density functional theory. It is found that the unit cell parameter a of the orthorhombic lattice increases linearly and the parameters b and c decrease as the boron concentration increases. The density of states at the Fermi level changes only slightly, and the main variations in the band structure occur in the region of the bottom of the valence bands. The magnetic moment of the iron atoms and the total magnetization and stability of the Fe₃C_1?x B_x phases increase linearly with an increase in the boron concentration.     

Co-substitution effect on the structural,electrical, and electrochemical properties of nanostructured Co<Subscript>x</Subscript>Ni<Subscript>1-x</Subscript>MnP<Subscript>2</Subscript>O<Subscript>7</Subscript> (x = 0, 0.25, 0.5, and 0.75)

Cobalt-nickel-manganese pyrophosphate nanostructures with formula Co_xNi_1-xMnP₂O₇ were prepared via the hydrothermal method at 150 °C, with further calcinations at 500 °C. A structural analysis of Co_xNi_1-xMnP₂O₇ samples was carried out using X-ray diffraction (XRD). The effect of Co substitution on the structural, electrical, and electrochemical properties of Co_xNi_1-xMnP₂O₇ is reported. The electrochemical results show that the specific capacity increases from 59 to 205 mAh/g with increasing Co content. This study demonstrates the Co substitution effect on the mixed electrical conductivity. The temperature dependence of the dc electrical conductivity, for both pure and Co²⁺-doped samples, obeys the Arrhenius law. The frequency dependence of ac conductivity for the materials exhibited a Jonscher’s universal power law. The plots of pre-exponent (n) versus temperature suggested that the conduction mechanism can be described using correlated barrier hopping model. The improved electrical conductivity and electrochemical proprieties of Co_xNi_1-xMnP₂O₇ nanomaterials could be ascribed to the synergistic effect of nickel and cobalt ions. The best results have been obtained for the composition x(Co) = 0.75, where the electrical conductivity is maximum, and the Co_0.75Ni_0.25MnP₂O₇ demonstrates the highest specific capacity, implying their promising potential applications in the energy storage.     

Multicanonical sampling of the space of states of ℋ(2, <Emphasis Type="Italic">n</Emphasis>)-vector models

Problems of temperature behavior of specific heat are solved by the entropy simulation method for Ising models on a simple square lattice and a square spin ice (SSI) lattice with nearest neighbor interaction, models of hexagonal lattices with short-range (SR) dipole interaction, as well as with long-range (LR) dipole interaction and free boundary conditions, and models of spin quasilattices with finite interaction radius. It is established that systems of a finite number of Ising spins with LR dipole interaction can have unusual thermodynamic properties characterized by several specific-heat peaks in the absence of an external magnetic field. For a parallel multicanonical sampling method, optimal schemes are found empirically for partitioning the space of states into energy bands for Ising and SSI models, methods of concatenation and renormalization of histograms are discussed, and a flatness criterion of histograms is proposed. It is established that there is no phase transition in a model with nearest neighbor interaction on a hexagonal lattice, while the temperature behavior of specific heat exhibits singularity in the same model, in case of LR interaction. A spin quasilattice is found that exhibits a nonzero value of residual entropy.     

Structural and magnetic properties of Fe<Subscript>m</Subscript>Y<Subscript>n</Subscript> (m + n = 7, Y = Ru,Rh, Pd,and Pt) nanoalloys

The structural and magnetic properties of the small binary clusters Fe _m Y _n (with m + n = 7, Y = Ru, Rh, Pd, Pt) were studied through extensive ab initio calculations, by means of the fully unconstrained version of the density-functional method, as implemented in the SIESTA code, within the generalized gradient approximation. The lowest energy state geometries, the chemical ordering, and the electronic and the magnetic structures were calculated. We found that the lowest energy geometrical structures for the pure Ru, Rh, Pd, Pt, and Fe heptamers, are a cube without an apex, a triangular prism capped on a square face, a decahedron, a side capped double square, and a decahedron, respectively. Starting from these geometries of the pure element heptamers, we followed the changes in the geometric structure as a function of the chemical composition. We analyzed all the different chemical arrangements, which depend on the particular geometry, and magnetic moment orientations, in the whole range of compositions. In general, there are important modifications to the magnetic moment of the Y atoms as soon as one of them is substituted by an Fe atom in the cluster. In contrast, under the same circumstances, the Fe magnetic moment takes values larger than 3 μ _B and keeps almost this value, insensitive to the structure, composition and chemical order of the system.     

Magnetic Properties of Hydrides of RNi1 – xSix Compounds (R = Dy,Gd, x = 0.05, 0.02)

Physics of the Solid State - The magnetic properties of intermetallic compounds GdNi0.98Si0.02 and DyNi0.95Si0.05 and hydrides based on them have been studied. It is found that a partial...     

Controlled flame synthesis of αFe<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles: effect of flame configuration,flame temperature,and additive loading

Superparamagnetic iron oxide nanoparticles are used in diverse applications, including optical magnetic recording, catalysts, gas sensors, targeted drug delivery, magnetic resonance imaging, and hyperthermic malignant cell therapy. Combustion synthesis of nanoparticles has significant advantages, including improved nanoparticle property control and commercial production rate capability with minimal post-processing. In the current study, superparamagnetic iron oxide nanoparticles were produced by flame synthesis using a coflow flame. The effect of flame configuration (diffusion and inverse diffusion), flame temperature, and additive loading on the final iron oxide nanoparticle morphology, elemental composition, and particle size were analyzed by transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy. The synthesized nanoparticles were primarily composed of two well known forms of iron oxide, namely hematite αFe₂O₃ and magnetite Fe₃O₄. We found that the synthesized nanoparticles were smaller (6–12 nm) for an inverse diffusion flame as compared to a diffusion flame configuration (50–60 nm) when CH₄, O₂, Ar, and N₂ gas flow rates were kept constant. In order to investigate the effect of flame temperature, CH₄, O₂, Ar gas flow rates were kept constant, and N₂ gas was added as a coolant to the system. TEM analysis of iron oxide nanoparticles synthesized using an inverse diffusion flame configuration with N₂ cooling demonstrated that particles no larger than 50–60 nm in diameter can be grown, indicating that nanoparticles did not coalesce in the cooler flame. Raman spectroscopy showed that these nanoparticles were primarily magnetite, as opposed to the primarily hematite nanoparticles produced in the hot flame configuration. In order to understand the effect of additive loading on iron oxide nanoparticle morphology, an Ar stream carrying titanium-tetra-isopropoxide (TTIP) was flowed through the outer annulus along with the CH₄ in the inverse diffusion flame configuration. When particles were synthesized in the presence of the TTIP additive, larger monodispersed individual particles (50–90 nm) were synthesized as observed by TEM. In this article, we show that iron oxide nanoparticles of varied morphology, composition, and size can be synthesized and controlled by varying flame configuration, flame temperature, and additive loading.     

First-principles study of mechanical stability and thermal properties of MNNi₃ （M=Zn,Mg,Al） under pressure

The mechanical stability,elastic,and thermodynamic properties of the anti-perovskite superconductors MNNi 3(M=Zn,Mg,Al) are investigated by means of the first-principles calculations.The calculated structural parameters and elastic properties of MNNi 3 are in good agreement with the experimental and the other theoretical results.From the elastic constants under high pressure,we predict that ZnNNi 3,MgNNi 3,and AlNNi 3 are not stable at the pressures above 61.2 GPa,113.3 GPa,and 122.4 GPa,respectively.By employing the Debye model,the thermodynamic properties,such as the heat capacity and the thermal expansion coefficient,under pressures and at finite temperatures are also obtained successfully.     

Magnetic,electrical, magnetoelectrical,and magnetoelastic properties of La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3 − <Emphasis Type="Italic">y</Emphasis></Subscript> manganites

This paper reports on a study of the influence of oxygen deficiency on the magnetization, paramagnetic susceptibility, electrical resistivity, magnetoresistance, and volume magnetostriction of the La_0.9Sr_0.1MnO_{3 − y} manganite with y = 0.03, 0.10, and 0.15. The magnetization M(T) behaves in a complex way with temperature; for T < 80 K, it only weakly depends on T, and at 80 ≤ T ≤ 300 K, the M(T) curve shows a falloff. Within the interval 240 K ≤ T ≤ 300 K, the long-range magnetic order breaks up into superparamagnetic clusters. For T < 80 K, the magnetic moment per formula unit is about one-fourth that which should be expected for complete ferromagnetic alignment of Mn ion moments. Although the composition with y = 0.03, in which part of acceptor centers is compensated by donors (oxygen vacancies), the negative magnetoresistance Δρ/ρ and volume magnetostriction ω are observed to pass through maxima near the Curie point, their values are one to two orders of magnitude smaller than those for the y = 0 composition. In compositions with y = 0.10 and 0.15 with electronic doping, the values of Δρ/ρ and ω are smaller by one to two orders of magnitude than those observed for the y = 0.03 composition. They do not display giant magnetoresistance and volume magnetostriction effects, which evidences the absence of ferrons near unionized oxygen vacancies. This allows the conclusion that the part played by both compensated and uncompensated doubly charged donors consists in forming dangling Mn-O-Mn bonds, which lead to a decrease in the Curie temperature with increasing y and to the formation above it of superparamagnetic clusters of the nonferron type.     

A study of pre-equilibrium emission of neutrons in <Superscript>93</Superscript>Nb(α, xn) reactions

With a view to study the pre-equilibrium emission mechanism in α-induced reactions the excitation functions for ⁹³Nb(α, n)^96m Tc, ⁹³Nb(α, n)⁹⁶Tc, ⁹³Nb(α, 2n)^95m Tc, ⁹³Nb(α, 2n)^95g Tc and ⁹³Nb(α, 3n)⁹⁴Tc reactions have been measured in the energy range threshold to ≈ 10MeV/nucleon using the activation technique. The measured excitation functions have also been compared with theoretical predictions based on the semi-classical code, which takes into account compound nucleus as well as pre-equilibrium emission. The analysis of the data indicates significant contribution from pre-equilibrium emission at these energies particularly in the high-energy tail portion of EFs. The effect of the variation of the parameters used in the code has been studied. The isomeric cross-section ratios have also been measured. It has been observed that the pre-equilibrium fraction increases rapidly with the increase in α-particle bombarding energy.     

Impact of growth temperature on the crystal habits,forms and structures of VO<Subscript>2</Subscript> nanocrystals

We investigated the impact of the process temperature on the habits, forms and crystal structure of VO₂ nanocrystals grown by a vapor-transport method on (0001) quartz substrates. Four distinct growth regimes were discerned: orthorhombic nanowires, sheets, hemispheres, and nanowires with a monoclinic structure. The nanostructures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). I/V characterization of individual nanowires was enabled by Ti/Au contact formation via electron beam lithography and lift-off techniques. The expected metal–insulator transition (MIT) was found in monoclinic VO₂ nanowires.