Raman scattering study of the geometrically frustrated pyrochlore Y2Ru2O7

Geometrically frustrated pyrochlore Y₂Ru₂O₇, which shows a spin-glass-like transition at T_G ∼ 80 K, were investigated by temperature-dependent Raman scattering. Three discernable phonons appear around 315, 410, and 510 cm⁻¹ without any abrupt change in the number of Raman active modes within the temperature range of 10–300 K. Fitting each phonon with Lorentz oscillators, we analyzed the effects of temperature on the phonon frequencies and the linewidths. The temperature-dependence of the mode near 510 cm⁻¹ shows abnormal behavior below T_G, while the other two phonons follow the usual thermal effect of lattice vibration. This behavior can be understood in terms of spin–phonon coupling. Considering the atomic modulations of each phonon mode, it is conjectured that the 510 cm⁻¹ phonon mode is isotropically coupled to the spin degree of freedom, while the other modes are not.     

Molecular nanoclusters as magnetic refrigerants: The case of Fe14 with very large spin ground-state

We report on the magnetic and thermal properties of the Fe₁₄ molecular nanocluster. We find a huge magnetocaloric response in the temperature range below 10 K. This is to large extent caused by its very large spin ground-state combined with an excess of entropy arising from the admixture of low-lying excited S states. We also show that the high degree of symmetry of the Fe₁₄ cluster core, resulting in a very small cluster magnetic anisotropy, enables the occurrence of long-range antiferromagnetic order below T_N = 1.87 K.     

Ferromagnetic resonance in β-p-NPNN at radio-frequency region

We performed the low-temperature and radio-frequency (rf) ESR of spherically molded single crystals of β-p-NPNN in order to re-investigate the ferromagnetic resonance (FMR). The symmetric FMR signals were clearly observed below 0.8 K for the first time. The resonance field for H//c is markedly shifted about 80 G higher than the EPR resonance field while the anisotropy of the resonance field is very weak when the magnetic field is applied within the ab-plane. These results clearly indicate the spin-hard axis is parallel to the c-axis. We estimate the magnetic anisotropy H_K = 61 G using the FMR relation with uniaxial anisotropy.     

Significant improved electrochemical performance of Li(Ni1/3Co1/3Mn1/3)O2 cathode on volumetric energy density and cycling stability at high rate

Li(Ni_1/3Co_1/3Mn_1/3)O₂ microspheres with a tap density of 2.41 g cm⁻³ have been synthesized for applications in high power and high energy systems, using a simple rheological phase reaction route. Cyclic voltammograms (CV) showed no shift of anodic and cathodic peaks centred at 3.81, 3.69 V for the Ni²⁺/Ni⁴⁺ couple after first cycle. The results of power pulse area specific impedance (ASI) and differential scanning calorimetry (DSC) tests showed lower power impedance and increased thermal stability of the electrode at high rate. These merits mentioned above provided significant improved capacity and rate performance for Li(Ni_1/3Co_1/3Mn_1/3)O₂ microspheres, which 159, 147 mAh g⁻¹ discharge capacity was delivered after 100 cycles between 2.5–4.6 V vs. Li at a different discharge rate of 2.5 C (500 mA g⁻¹), 5 C and a constant 0.5 C charge rate, respectively.     

Doping effect of nonmagnetic impurity on the spin-Peierls-like transition in the quasi-one-dimensional (quasi-1D) spin system of 1-(4′-nitrobenzyl)pyridinium bis(maleonitriledithiolato)nickelate

A nonmagnetic compound, [NO₂BzPy][Cu(mnt)₂] (mnt^2? = maleonitriledithiolate; NO₂BzPy⁺ = 1-(4′-nitrobenzyl)pyridinium), is isostructural with [NO₂BzPy][Ni(mnt)₂], which is a quasi-1D spin system and exhibits a spin-Peierls-like transition with J = 192 K in the gapless state and spin energy gap = 738 K in the dimerization state, respectively. Further, five nonmagnetic impurity doped compounds [NO₂BzPy][Cu_xNi_1?x(mnt)₂] (x = 0.04–0.74) were prepared, and their crystal structures as well as magnetic properties were investigated. The nonmagnetic doping causes the suppression of the spin transition with an average rate of 139(13) K/percentage of dopant concentration, and the transition collapse is estimated at around x > 0.5.     

Ab initio study of the magnetic behavior of four dithiadiazolyl radical compounds

We have studied, by means of ab initio calculations, the magnetic interaction mechanisms in four radical crystals, X–C₆F₄–CNSSN (X = O₂N, α-NC, β-NC, Br), which has allowed us to explain their different magnetic behaviour (ferromagnetism, antiferromagnetism, paramagnetism, spin frustration, etc.). First, we have identified the magnetic exchange pathways considering those with distances between two atoms of different dithiadiazolyl rings shorter than 7 Å and those with an intermolecular distance between an atom of the heterocyclic ring and an atom in a neighbouring radical shorter than 4 Å. Second, the calculations have been carried out in the framework of the DFT Broken Symmetry. Following this procedure we have determined the magnitude and the sign of the relevant coupling constants for the X–C₆F₄–CNSSN (X = O₂N, α-NC, β-NC, Br) radicals. In the cases where the radicals order magnetically, ordering temperatures determined with our ab initio calculations agree very well with the experimental ones. Thus, in the case of the O₂N derivative ferromagnetic ordering is observed below 1.3 K, in very good agreement with an ordering temperature around 1.6 K predicted from our calculated exchange constants and using a mean field approximation.     

Lithium–manganese–nickel-oxide electrodes with integrated layered–spinel structures for lithium batteries

A series of lithium–manganese–nickel-oxide compositions that can be represented in three-component notation, xLi[Mn_1.5Ni_0.5]O₄ · (1 − x){Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂}, in which a spinel component, Li[Mn_1.5Ni_0.5]O₄, and two layered components, Li₂MnO₃ and Li(Mn_0.5Ni_0.5)O₂, are structurally integrated in a highly complex manner, have been evaluated as electrodes in lithium cells for x = 1, 0.75, 0.50, 0.25 and 0. In this series of compounds, which is defined by the Li[Mn_1.5Ni_0.5]O₄–{Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂} tie-line in the Li[Mn_1.5Ni_0.5]O₄–Li₂MnO₃–Li(Mn_0.5Ni_0.5)O₂ phase diagram, the Mn:Ni ratio in the spinel and the combined layered Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂ components is always 3:1. Powder X-ray diffraction patterns of the end members and the electrochemical profiles of cells with these electrodes are consistent with those expected for the spinel Li[Mn_1.5Ni_0.5]O₄ (x = 1) and for ‘composite’ Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂ layered electrode structures (x = 0). Electrodes with intermediate values of x exhibit both spinel and layered character and yield extremely high capacities, reaching more than 250 mA h/g with good cycling stability between 2.0 V and 4.95 V vs. Li° at a current rate of 0.1 mA/cm².     

Syntheses,crystal structures and magnetic properties of coordination polymers Ni(NO2)2 and Ni(4,4′-bipy)(NO2)2

Two new coordination polymer frameworks Ni(NO₂)₂ (1) and Ni(4,4′-bipy)(NO₂)₂ (2) (4,4′-bipy = 4,4′-bipyridine) were synthesized by solvothermal reaction in formamide, and were characterized by elemental analysis, IR spectroscopy, single crystal X-ray diffraction, and magnetic measurement. In compound 1, each Ni²⁺ ion is linked with four neighboring Ni²⁺ ions through μ_1,3-nitrito bridges forming 2D layered structure. In compound 2, each Ni²⁺ ion is bridged with six neighboring Ni²⁺ ions through four μ_1,3-nitrito groups and two 4,4′-bipy ligands forming 3D structure. Magnetic measurements show weak ferromagnetism within framework of the two compounds with T_N = 19 K (1) and 21 K (2).     

Measurement of the local temperature in an ion track using low-dimensional quantum confinement structure

We measured the local temperature in an ion track utilizing fast exciton recombination in (C₃H₇NH₃)₂PbBr₄. We obtained the local temperatures of 1000–1500 K for 1.0 MeV H⁺, 2.0 MeV H⁺ and He⁺, and the local temperature was found to be an increasing function of the linear energy transfer. Furthermore, the measured local temperature increased with increasing ion dose, which is attributed to the diffusion of the phonons in an ion track.     

Quaternary Gd4Ni2Sb1±xSi2±x and Gd4Ni2Bi1±xSi2±x: Crystal structure,homogeneity regions and magnetic behavior

A new quaternary Gd₄Ni₂Sb_1.07(1)Si_1.93(1) phase was synthesized by arc-melting and its structure was determined through single crystal X-ray diffraction techniques. It crystallizes in an orthorhombic unit cell (the Pnma space group) with a = 11.1735(9), b = 4.2054(2) and c = 16.711(1) Å and represents a new structure type. The isostructural Gd₄Ni₂BiSi₂ phase was obtained and characterized using the powder X-ray diffraction techniques: Pnma space group, a = 11.2715(2), b = 4.2046(1) and c = 16.7421(3) Å. By the means of electron microprobe analysis, Sb/Si and Bi/Si solid solutions were proven to exist for corresponding phases, and their general formulas can be given as Gd₄Ni₂Sb_1±xSi_2±x and Gd₄Ni₂Bi_1±xSi_2±x. Gd₄Ni₂Sb_1.07Si_1.93 and Gd₄Ni₂BiSi₂ order ferromagnetically at 93 K and 46 K, respectively.     

Temperature-dependent Raman spectra of Bi2Sn2O7 ceramics

The Bi₂Sn₂O₇ pyrochlore is known to undergo a sequence of structural phase transitions with an increase in temperature. Raman spectroscopy was employed in the investigation of the temperature dependence of the active phonons in the Raman spectrum. We observed 19 broad modes at room temperature, reflecting the low symmetry of the α-phase of Bi₂Sn₂O₇. The modes were discussed in relation to the Raman spectra of other pyrochlore-based oxides. The temperature dependence of the phonons evidences the α → β structural phase transition observed near 127 °C.     

Effect of counter cationic geometry on stacking structures and magnetic properties of [Ni(mnt)2]− complexes

The crystal structures and magnetic properties of two new ion-pair complexes, [N-NH₂Py][Ni(mnt)₂] (1) and [N-NH₂Ql][Ni(mnt)₂] (2) (N-NH₂Py⁺ = 1-aminopyridinium, N-NH₂Ql⁺ = 1-aminoquinolinium; mnt^2? = maleonitriledithiolate) have been investigated. The differences of the molecular topology and size of the counter cation result in distinct anionic and cationic stacking patterns in the crystals, namely, in the crystal of 1 the anions (A) and cations (C) alternate to stack into a mixed column in the fashion of …AACCAACC…, and the neighboring columns are connected together via intermolecular H-bonding interactions as well as van der Waals forces; while, in the crystal of 2, the anions and cations alternate to form a mixed columnar stack in the manner of …ACAC…, and the adjacent columns are held together via weak van der Waals forces. Investigations of variable-temperature magnetic susceptibility indicated that 1 is a spin gap system but its magnetic behavior does not follow the Bleaney–Bowers spin dimer model; 2 shows a magnetic feature of linear decrease of χ_mT value upon cooling, and this kind of magnetic behavior is probably related to the presence of temperature independent paramagnetism.     

Determination of the activities in the (Ni + Al) alloys in the temperature range 870 K to 920 K by a solid-state galvanic cell using a CaF2 electrolyte

The activities of aluminium and nickel are constituents in the (Ni + Al) superalloys in the concentration range 50 mol% to 70 mol% of Al at the temperatures (870, 900, and 920) K were determined with the help of solid-state galvanic cells involving calcium fluoride as an electrolyte. The partial molar Gibbs free energies of aluminium and nickel in the alloys, and the Gibbs free energy of mixing were calculated from the e.m.f. data. The values of the standard Gibbs free energies of formation of the intermetallic compounds NiAl, Ni₂Al₃, and NiAl₃ are also given.     

Synthesis,crystal structure,spectroscopy and magnetism of a trinuclear nickel(II) complex with 3,5-pyrazoledicarboxylic acid as bridge ligands

A new complex of [Ni₃(dcp)₂(H₂O)₁₀] (1) (H₃dcp = 3,5-pyrazoledicarboxylic acid) has been synthesized from H₃dcp and Ni(NO₃)₂·6H₂O by hydrothermal reaction. Complex 1 has the discrete trinuclear structure. Three Ni(II) ions are bridged by two dcp³⁻ ligands, with 10 coordinated water molecules as terminal ligands. The molecules of [Ni₃(dcp)₂(H₂O)₁₀] extend into three-dimensional supramolecular architectures by intermolecular O–H···O hydrogen bonds as well as π-π stacking interactions. Magnetic susceptibility measurement shows that a weak antiferromagnetic interaction is operative between nickel(II) ions and an excellent simulation of the experimental data gives D = 5.27 cm⁻¹, J = −2.19 cm⁻¹ and g = 2.05.     

Synthesis of nickel selenide thin films for high performance all-solid-state asymmetric supercapacitors

As a significant semiconductor, nickel selenide shows enormous potential and extensive application prospects in the field of sensor, photocatalysis and supercapacitor. In this paper, nickel selenide (Ni₃Se₂, NiSe) thin films were successfully fabricated on stainless-steel sheet using a facile, effective electrodeposition technique. The morphologies, microstructures and chemical compositions of the thin films are characterized systematically. Electrochemical tests exhibit that the Ni₃Se₂ and NiSe possess high specific capacitance of 581.1 F/g and 1644.7 F/g, respectively. A flexible, all-solid-state asymmetric supercapacitor is assembled by utilizing NiSe film as positive electrode and activated carbon as negative electrode. The solid device delivers a high areal capacitance of 27.0 mF/cm² at the current density of 0.7 mA/cm². The maximum volumetric energy density and power density of the NiSe//AC asymmetric SCs can achieve 0.26 mWh/cm³ and 33.35 mW/cm³, respectively. The device shows robust cycling stability with 84.6% capacitance retention after 10,000 cycles, outstanding flexibility and satisfactory mechanical stability. Moreover, two devices in series can light up a red light-emitting diode, which displayed great potential applications for energy storage.     

Combinatorial screening of ternary Pt–Ni–Cr catalysts for methanol electro-oxidation

Methanol electro-oxidation activity of ternary Pt–Ni–Cr system was studied by using a combinatorial screening method. A Pt–Ni–Cr thin-film library was prepared by sputtering and quickly characterized by a multichannel multielectrode analyzer. Among the 63 different composition thin-film catalysts, Pt₂₈Ni₃₆Cr₃₆ showed the highest methanol electro-oxidation activity and good stability. This new composition was also studied in its powder form by synthesizing and characterizing Pt₂₈Ni₃₆Cr₃₆/C catalyst. In chronoamperometry testing, the Pt₂₈Ni₃₆Cr₃₆/C catalyst exhibited “decay-free” behavior during 600 s operation by keeping its current density up to 97.1% of its peak current density, while the current densities of Pt/C and Pt₅₀Ru₅₀/C catalysts decreased to 14.0% and 60.3% of their peak current densities, respectively. At 600 s operation, current density of the Pt₂₈Ni₃₆Cr₃₆/C catalyst was 23.8 A g_{noble metal}⁻¹, while that of those of the Pt/C and Pt₅₀Ru₅₀/C catalysts were 2.74 and 18.8 A g_{noble metal}⁻¹, respectively.     

In search for mixed transition metal/lanthanide single-molecule magnets: Synthetic routes to NiII/TbIII and NiII/DyIII clusters featuring a 2-pyridyl oximate ligand

Employing the mononuclear complex [Ni{(py)C(Me)NO}₂{(py)C(Me)NOH}] (1) as ‘ligand’ [(py)C(Me)NOH = methyl 2-pyridyl ketone oxime], the use of the ‘metal complexes as ligands’ approach has led to the synthesis of the mixed Ni^II/Ln^III complexes [NiTb{(py)C(Me)NO}₂(NO₃)₃{(py)C(Me)NOH}] (2), [Ni₂Ln₂{(py)C(Me)NO}₆(NO₃)₄] (Ln = Dy, 3; Ln = Tb, 4) and [Ni₂Tb{(py)C(Me)NO}₆](NO₃) (5). The structures of 2, 3, and 5, and the magnetic properties of 2 and 5 are briefly discussed.     

Anomalous capacity and cycling stability of xLi2MnO3 · (1 − x)LiMO2 electrodes (M = Mn,Ni, Co) in lithium batteries at 50 °C

Transition metal oxides with composite xLi₂MnO₃ ·  (1 − x)LiMO₂ rocksalt structures (M = Mn, Ni, Co) are of interest as a new generation of cathode materials for high energy density lithium-ion batteries. After electrochemical activation to 4.6 or 4.8 V (vs. Li⁰) at 50 °C, xLi₂MnO₃ · (1 − x)LiMn_0.33Ni_0.33Co_0.33O₂ (x = 0.5, 0.7) electrodes deliver initial discharge capacities (>300 mAh/g) at a low current rate (0.05 mA/cm²) that exceed the theoretical values for lithiation back to the rocksalt stoichiometry (240–260 mAh/g), at least during the early charge/discharge cycles of the cells. Attention is drawn to previous reports of similar, but unaccounted and unexplained anomalous behavior of these types of electrode materials. Possible reasons for this anomalous capacity are suggested. Indications are that electrodes in which M = Mn, Ni and Co do not cycle with the same stability at 50 °C as those without cobalt.     

Influence of the external electric field distribution on α-Ni(OH)2 electrochemical-synthesis from a NiCl2 solution

Electrolytic systems of a symmetric, an asymmetric and a two-compartment were established in the present work to investigate the effect of the external electric field distribution on α-Ni(OH)₂ electrochemical-synthesis from a NiCl₂ solution. Results demonstrate the sample particle size increased in the order of symmetric, two-compartment and asymmetric systems, with a sharpened diffraction peak of the (1 0 1), (0 1 5) and (1 1 0) plane, and a broadened diffraction peak of (0 0 3) plane. However, the reversibility of the redox reactions and the energy transferred in the redox reactions in the electrodes assembled by the samples from the three electrolytic systems increased based on the contrary order. In terms of the electrolysis process, the energy consumption per unit mass increased in the order of symmetric, two-compartment and asymmetric systems. The catholyte pH for both symmetric and asymmetric systems were more stable than that for the two-compartment system. The external electric filed distribution affected the transportation of Ni²⁺ from the anolyte to the catholyte. At the end of electrolysis, the Ni²⁺ concentration in the anolyte of two-compartment system was obviously higher than that of symmetric and asymmetric systems.