Ultra‐small,Uniform, and Single bcc‐Phased FexCo1‐x/Graphitic Shell Nanocrystals for T1 Magnetic Resonance Imaging Contrast Agents

We have synthesized ultra‐small and uniform Fe_xCo_1‐x/graphitic carbon shell (Fe_xCo_1‐x/GC) nanocrystals (x=0.13, 0.36, 0.42, 0.50, 0.56, and 0.62, respectively) with average diameters of <4 nm by thermal decomposition of metal precursors in approximately 60 nm MCM‐41 and methane CVD. The composition of the Fe_xCo_1‐x/GC nanocrystals can be tuned by changing the Fe:Co ratios of the metal precursors. The Fe_xCo_1‐x/GC nanocrystals show superparamagnetic properties at room temperature. The Fe_0.50Co_0.50/GC, Fe_0.56Co_0.44/GC, and Fe_0.62Co_0.38/GC nanocrystals have a single bcc FeCo structure, whereas the Fe_0.13Co_0.87/GC, Fe_0.36Co_0.64/GC, and Fe_0.42Co_0.58/GC nanocrystals have a mixed structure of bcc FeCo and fcc Co. The single bcc‐phased Fe_xCo_1‐x/GC nanocrystals functionalized with phospholipid–poly(ethylene glycol) (PL–PEG) in phosphate buffered saline (PBS) are demonstrated to be excellent T₁ MRI contrast agents.     

Investigations on Chemical Vapour Transport of Intermetallic Phases in the System Co/Fe/Si

The ternary phases existing on the quasi binary section CoSi/FeSi and CoSi₂/β‐FeSi₂ have been investigated by solid state reactions and chemical transport. The solid solution serie Co_xFe_1‐xSi can be described as a regular solution. The transport behaviour calculated is in good agreement with the experiments. The phases have been characterized by X‐ray powder diffraction, EDX and ICP‐OES. The temperature dependence of the resistivity has been measured from 20 K up to room temperature on single crystals.     

Chemischer Transport und einige Eigenschaften von Kobalteisensulfid CoyFe1−ySx

Chemical Transport and Some Physical Properties of Cobalt Iron Sulphide Co_yFe_1?yS_x The chemical transport behaviour of the ternary phase Co_yFe_1?yS_x is explicable on the base of a thermodynamic model. Theory and experiments show that using Gel₂ as transport agent the phase Co_yFe_1?yS_x with low contents of sulphur and cobalt (x ≈ 1, y < 0.4) will be transported under deminishing the content of Co and under enrichment of Fe and S whereas by use of HI(NH₄I) as transport agent the transport occurs under enrichment of Co and deminishing the Fe and S contents, respectively. The substitution by Co influences on unit-cell dimensions, on the temperature and heat of the phase transition (2C → 1C) as well as on the resistivity jump and hysteresis in connection with this phase transition.     

Electrical properties in the system (La1−xCax)CoO3 (0.1 ≦ x ≦ 0.5)

The electrical resistivity of (La_1?xCa_x)CoO₃ (0.1 ≦ x ≦ 0.5) was measured in the temperature range from 80 to 300K. Cobaltite with x ≦ 0.15 is a semiconductor, but the specimen with chemical composition 0.2 ≦ x ≦ 0.5 is metallic. The change of temperature dependence of electrical resistivity has a break point around T_c. The value of the logarithm of the specific electrical resistivity (log ?) at 300K has a minimum at x = 0.4, and this result is explained by the Zener double-exchange mechanism.     

Analysis of thermo-magnetic fluctuations above the glass-transition temperature in Bi1.6Pb0.5Sr2−xEuxCa1.1Cu2.1O8+δ (0.000 ≤ x ≤ 0.180) system

The resistivity of Bi_1.6Pb_0.5Sr_2−xEu_xCa_1.1Cu_2.1O_8+δ (0.000 ≤ x ≤ 0.180) superconductor has been measured as a function of temperature and magnetic field. The resistivity shows a glassy behavior even at higher temperatures and magnetic fields for the Eu-doped samples as compared with the Eu free sample. The values of glass-transition temperature [T_g], magnetic field dependent activation energy [U₀(B)] and the temperature and magnetic field dependent activation energy [U₀(B,T)] are found to be maximum for optimal doping levels (x = 0.135) which shows that the flux lines are effectively pinned in this sample. Also for temperatures below the superconducting transition temperature (T_C), a scaling of measured resistivity curves in magnetic field (B = 0.4 and 0.8 T) is obtained and this scaling is quite useful for better understanding of the behavior of the flux vortices in high temperature superconductors.     

The Reconstructive Phase Transformation β‐Co2P → α‐Co2P and the Structure of the High‐Temperature Phosphide β‐Co2P

Metallographical and differential thermoanalytical (DTA) investigatitons indicate that the well known phosphide Co₂P (Pearson code oP12, space group Pnma, Co₂Si type) is not stable up to the melting point, T = 1659 K; it is therefore designated as the low‐temperature phase α‐Co₂P. In the temperature range from 1428 to 1659 K, another, high‐temperature phase, designated as β‐Co₂P, exists. X‐ray powder diffraction investigation of liquid quenched alloys in the composition range x_P = 0.25 to 0.335, with x_P as the mole fraction, show that the high‐temperature phase β‐Co₂P is isotypic with Fe₂P (hP9, P 6 2m). For the ideal composition Co₂P, the unit cell parameters are: a = 5.742(2) Å, c = 3.457(5) Å, c/a = 0.621. Among the binary transition metal‐containing phosphides and arsenides isotypic with Fe₂P, β‐Co₂P is the only known high‐temperature phase and it shows (i) the highest axial ratio c/a and (ii) the “smallest” distortion of the hcp substructure formed by the transition metals atoms in the Fe₂P structure type.     

Electronic, Magnetic, and Magnetoresistance Properties of the n=2 Ruddlesden-Popper Phases Sr3Fe2−xCoxO7−δ (0.25≤x≤1.75)

A series of oxygen-deficient n=2 Ruddlesden-Popper phases, Sr₃Fe_2−xCo_xO_7−δ (0.25≤x≤1.75), were prepared by solid-state reactions. Temperature-dependent susceptibility and field-dependent magnetization data indicate that for x≥0.25 the dominant magnetic interactions are ferromagnetic. The onset of strong ferromagnetic interactions is evident at ∼200 K, and a transition to a cluster-glass state is observed for all compositions below ∼45 K. The temperature variation of resistivity for all the compounds shows variable-range hopping behavior with two different localization energy scales: one for T<40 K and another for T>80 K. Large negative magnetoresistance (the largest MR ∼−65% for x=0.25) is observed for all phases. The magnetic susceptibility, Mössbauer and X-ray absorption near-edge spectroscopy data indicate that the formal oxidation state of Fe is close to 4+. The key role of d delocalization in the Sr₃Fe_2−xCo_xO_7−δ system is compared to the Sr₃Fe_2−xMn_xO_7−δ series, where d localization dominates the properties.     

Unexpected Synergy between Magnetic Iron Chains and Stacked B6 Rings in Nb6Fe1−xIr6+xB8

The synergistic combination of experiment and density functional theory has led to the discovery of the first ferromagnetic material, Nb₆Fe_1?xIr_6+xB₈, containing in its crystal structure iron chains embedded in stacked B₆ rings. The strong ferromagnetic Fe–Fe interactions found in the iron chains induce an unexpected strengthening of the B–B interactions in the B₆ rings. Beside these strong B–B interactions, strong interlayer metal–boron bonds (Ir–B and Nb–B) ensure the overall structural stability of this phase, while the magnetic Fe–Fe interactions are mainly responsible for the observed ferromagnetic ordering below T_C=350 K.     

On the influence of the milling time in the Curie temperature of Fe56Co7Ni7Zr10B20 investigated by magnetic thermogravimetric analysis

Magnetic thermogravimetric analysis (TGM) was used to investigate the influence of the milling time (t _mill) in the Curie temperature (T _C) of nanocrystalline powders and of a melt-spun amorphous ribbon with composition Fe₅₆Co₇Ni₇Zr₁₀B₂₀. The TGM analysis was carried in a continuous flow of 99.99% pure argon from room temperature up to 1250?K. A magnetic field of 100?Oe was applied throughout the measurements. Nanopowders of Fe₅₆Co₇Ni₇Zr₁₀B₂₀ were produced by mechanical alloying the samples in an argon atmosphere for milling times ranging from 1 to 100?h. The samples were characterized by X-ray diffraction and by scanning electron microscopy. The average particle size decreased from 45.4?nm for a powder milled for 1?h to 5?nm after being milled for 100?h. Moreover, T _C (=1126.4?±?4.4?K) was found to be nearly independent of t _mill while for the melt-spun amorphous ribbon it was found to be substantially smaller (T _C?=?482?K). This is a clear indication that T _C is quite sensitive to the degree of amorphosity present in the sample. The activation energy associated to the crystallization process was estimated from DSC data by using the Kissinger??s method to be 193?kJ/mol.     

Temperature and concentration dependent electrical resistivity of Ho1−xTixCo2–hydrogen system

The structure and electrical resistivity of Ho_1−xTi_xCo₂–hydrides (x = 0.1–0.6) have been determined through the powder X-ray diffraction (XRD) and temperature dependence of electrical resistivity (ρ(T)) at different hydrogen concentrations. The variations in the lattice parameters in different phase regions and the lattice expansion with respect to the hydrogen concentration have been studied. The temperature dependent electrical resistivity of hydrides has been discussed based on the conduction electron scattering and spin fluctuation scattering mechanisms. The changes in ρ(T) upon Ti substitution and increasing concentration of hydrogen have been discussed and the results have been correlated with their structural properties.     

Bisphenol-A polycarbonate–diluent interactions

The effect of low-molecular-weight miscible additives on the sub-T_g (β) relaxation process in bisphenol-A polycarbonate (BPAPC) was studied using high-resolution carbon-13 solid-state NMR. The trend of the spin-lattice relaxation times T₁ at 50 MHz suggests that strong intermolecular interactions occur upon mixing when BPAPC is physically stiffened by the antiplasticizing diluent, diphenylphthalate. The values of ¹³C T₁ at 15 MHz in d-chloroform solutions for similar BPAPC-diluent mixtures suggest that diluent effects on the megahertz mobility of the polymer occur exclusively in the solid state. These results are explained using equilibrium thermodynamics, in the Ehrenfest sense, at the second-order glass transition temperature T_g. Theory predicts that the temperature dependence of the Flory–Huggins interaction parameter ?χ/?T changes abruptly as the polymer-diluent blends are cooled below T_g from the molten state. The difference between ?χ/?T in the liquid and glassy states is the major factor which determines the diluent concentration dependence of T_g. A method is developed to estimate the relative magnitudes of χ for polymerdiluent blends in the glassy state.     

A Hybrid Assembly of MXene with NH2−Si Nanoparticles Boosting Lithium Storage Performance

A facile hybrid assembly between Ti₃C₂T_x MXene nanosheets and (3‐aminopropyl) triethoxylsilane‐modified Si nanoparticles (NH₂?Si NPs) was developed to construct multilayer stacking of Ti₃C₂T_x nanosheets with NH₂?Si NPs assembling together (NH₂?Si/Ti₃C₂T_x). NH₂?Si/Ti₃C₂T_x exhibits a significantly enhanced lithium storage performance compared to pristine Si, which is attributed to the robust crosslinking architecture and considerably improved electrical conductivity as well as shorter Li⁺ diffusion pathways. The optimized NH₂?Si/Ti₃C₂T_x anode with Ti₃C₂T_x: NH₂?Si mass ratio of 4 : 1 displays an enhanced capacity (864 mAh g^?1 at 0.1 C) with robust capacity retention, which is significantly higher than those of NH₂?Si NPs and Ti₃C₂T_x anodes. Furthermore, this work demonstrates the important effect of the MXene‐based electrode architecture on the electrochemical performance and can guide future work on designing high‐performance Si/MXene hybrids for energy storage applications.     

Infrared studies on the behavior in oxygen of cobalt-substituted magnetites: Comparison with zinc-substituted magnetites

Infrared spectroscopic measurements were carried out on the cobalt-substituted magnetites (Fe³⁺)_A(Co²⁺_xFe²⁺_1?xFe³⁺)_BO^2?₄, pretreated in oxygen, to investigate as a function of temperature the defect phases γ and their transformation to hematite. It has been found that the defect spinels for which x < 0.30 show a partial vacancy ordering on octahedral sites. Referring to the disappearance of the 720-cm^?1 absorption band of the defect phases γ or the appearance of the 470-cm^?1 absorption band of αFe₂O₃, we show that the transition temperature γ → α increases with cobalt substitution. By comparison with zinc-substituted magnetites, the divalent cation distribution is shown to be important to vacancy ordering and to setting the temperature of hematite precipitation.     

Transition‐Metal‐Mediated Cleavage of a SiSi Double Bond

Reaction of carbene‐stabilized disilicon ( 1 ) with Fe(CO)₅ gives the 1:1 adduct L:Si?Si[Fe(CO)₄]:L (L:=C{N(2,6‐Prⁱ₂C₆H₃)CH}₂) ( 2 ) at room temperature. At raised temperature, however, 2 may react with another equivalent of Fe(CO)₅ to give L:Si[μ‐Fe₂(CO)₆](μ‐CO)Si:L ( 3 ) through insertion of both CO and Fe₂(CO)₆ into the Si₂ core, which represents the first experimental realization of transition metal‐carbonyl‐mediated cleavage of a Si?Si double bond. The structures and bonding of both 2 and 3 have been investigated by spectroscopic, crystallographic, and computational methods.     

Debye–Einstein model and anomalies of heat capacity temperature dependences of solid solutions at low temperatures

An approach is proposed for analysing the deviations of the heat capacity C_p(T) of solid solutions from the Kopp–Neumann rule (KNR) ΔC(T)?=?C_p(T)???C_KNR(T). Temperature dependences of the heat capacity C_p(T) of selected compositions of systems (InP)_x (InAs)_1?x and (GaAs)_x (InAs)_1?x at temperatures of 5–300 K are analysed in the Debye–Einstein approximation. It was established that in the case of substitution of atoms in the cation subsystem (Ga³⁺???In³⁺) with the same subsystem of anions (As^3?), the positive values of ΔC(T) at T?<?100 K are due to the appearance of the low-frequency Einstein mode, whereas the negative values of ΔC(T) at T?>?100 K are the result of a decrease in the fraction of the Debye contribution without changing the upper limit of the oscillation frequency. In the case of substitution in the cation subsystem (P^3????As^3?) with the invariant cation subsystem (In³⁺) to the low-temperature positive contribution of the additional low-frequency Einstein mode, a positive part is added from the modified Debye mode having the characteristic temperature θ_D below the additive value θ_DKNR. The adequacy of this model is confirmed by Raman scattering data.

     

Thermal fluctuation and magnetization of Ni-Zn ferrite nanoparticles by particle size

Summary Ni_1-xZn_xFe₂O₄ (0≤x≤1) mixed ferrite nanoparticles encapsulated with amorphous-SiO₂ were prepared by a wet chemical method. Particle sizes were controlled to range from 2.6 to 33.7 nm by heat treatment, and the particle size dependence of saturation magnetization M_s was investigated for the x=0.5 region. The M_s value decreased abruptly for particle sizes below about 6 nm. From the temperature dependence of the magnetization under field-cooled and zero-field-cooled conditions, blocking temperatures T_b were observed to be between 28 and 245 K depending on the particle size. At the blocking temperature, the superparamagnetic spins in the particle are supposed to be blocked against the thermal fluctuation energy. A smaller particle volume causes a lower blocking temperature; so an extremely small particle would be strongly affected by thermal fluctuation.     

Endohedrally Filled [Ni@Sn9]4− and [Co@Sn9]5− Clusters in the Neat Solids Na12Ni1−xSn17 and K13−xCo1−xSn17: Crystal Structure and 119Sn Solid‐State NMR Spectroscopy

A systematic approach to the formation of endohedrally filled atom clusters by a high‐temperature route instead of the more frequent multistep syntheses in solution is presented. Zintl phases Na₁₂Ni_1?xSn₁₇ and K_13?xCo_1?xSn₁₇, containing endohedrally filled intermetalloid clusters [Ni@Sn₉]^4? or [Co@Sn₉]^5? beside [Sn₄]^4?, are obtained from high‐temperature reactions. The arrangement of [Ni@Sn₉]^4? or [Co@Sn₉]^5? and [Sn₄]^4? clusters, which are present in the ratio 1:2, can be regarded as a hierarchical replacement variant of the hexagonal Laves phase MgZn₂ on the Mg and Zn positions, respectively. The alkali‐metal positions are considered for the first time in the hierarchical relationship, which leads to a comprehensive topological parallel and a better understanding of the composition of these compounds. The positions of the alkali‐metal atoms in the title compounds are related to the known inclusion of hydrogen atoms in the voids of Laves phases. The inclusion of Co atoms in the {Sn₉} cages correlates strongly with the number of K vacancies in K_13?xCo_1?xSn₁₇ and K_5?xCo_1?xSn₉, and consequently, all compounds correspond to diamagnetic valence compounds. Owing to their diamagnetism, K_13?xCo_1?xSn₁₇, and K_5?xCo_1?xSn₉, as well as the d‐block metal free binary compounds K₁₂Sn₁₇ and K₄Sn₉, were characterized for the first time by ¹¹⁹Sn solid‐state NMR spectroscopy.     

Pyridine based cerium(IV) phosphate hybrid fibrous ion exchanger

Both temperatures, T _C, (T _C —Curie temperature) and heat of the phase transition: ferroelectric-paraelectric, ΔH, in the Ba_xSr_1−xTiO₃ materials have been studied by means of the microcalorimetric method. The determined parameters were verified by either temperature dependence of the dielectric permittivity (Curie-Weiss law) or thermodynamic method. The effect of strontium content on T _C has been discussed. It was found that microcalorimetry is useful tool studying phase transition phenomena in ferroelectric perovskites.     

Transport, magnetic, internal friction, and Young's modulus in the Y-doped manganites La0.9−xYxTe0.1MnO3

The resistivity, magnetization, internal friction, and Young's modulus for the polycrystalline samples La_0.9−xY_xTe_0.1MnO₃ (x=0, 0.05, 0.10 and 0.15) have been investigated. All samples have rhombohedral crystallographic structure with the space group . The Curie temperature T_C of the studied samples decreases with increasing Y-doping level. For the samples with x=0,0.05 and 0.10, the temperature dependence of the resistivity ρ(T) exhibits two metal-insulator transitions (MIT) at T_p1 (which is close to its Curie temperature T_C) and T_p2 (which is below T_p1). When the doping level to 0.15, these two MIT temperatures are suppressed and an upturn at low temperatures below T* is observed from the ρ(T) curve. A change of Young's modulus E is observed in the vicinity of T_C accompanied by a broad peak of the internal friction Q⁻¹ for all studied samples. The values of the relative Young's modulus ΔE increase with increasing Y-doping level at the low temperatures. These results are discussed in terms of the local Jahn-Teller (JT) distortion by the substitution of smaller Y³⁺ ions for larger La³⁺ ions and the increased bending of the Mn-O-Mn bond with decreasing the average ionic radius of the A-site element 〈r_A〉 and the tolerance factor t, resulting in the narrowing of the bandwidth, the decrease of the mobility of e_g electrons and the weakening of double-exchange (DE) interaction.     

Magnetic,transport and thermodynamic properties of Ce5Ni2Si3 compound

The magnetization M(T,H), specific heat C_p(T,H), electrical resistivity ρ(T), magnetoresistance MR(T,H), thermal conductivity κ(T) and thermopower S(T) measurements were performed on the antiferromagnetic compound Ce₅Ni₂Si₃ with the Néel temperature T_N = 5.7 K. The estimated effective moment μ_eff is close to the free ion value of Ce in its trivalent state. The negative sign of the paramagnetic Curie temperature θ_p indicate the antiferromagnetic nature of the magnetic ordering. The variation of magnetic resistivity ρ_mag with temperature in Ce₅Ni₂Si₃ can be explained by a competition of the crystal electric field (CEF) splitting, the Kondo effect and the magnetic order. Based on the thermopower and employing a simple single-ion Kondo model the Kondo temperature have been estimated. Magnetocaloric effect is small but shows a sign change, which may be caused by a metamagnetic behavior.