Magnetic susceptibility of vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12

Vanadium garnets NaPb₂Co₂V₃O₁₂ and NaPb₂Ni₂V₃O₁₂ have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group with the lattice constant of 12.742 Å (NaPb₂Co₂V₃O₁₂) and 12.666 Å (NaPb₂Ni₂V₃O₁₂), respectively. The magnetic susceptibility of NaPb₂Ni₂V₃O₁₂ shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μ_eff of NaPb₂Ni₂V₃O₁₂ is 3.14 μ_B due to Ni²⁺ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb₂Co₂V₃O₁₂, the simple Curie-Weiss law cannot be applicable. The ground state is the spin doublet and the first excited state is spin quartet , according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb₂Co₂V₃O₁₂.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Room temperature stable structures and phase transition of Na2Al2B2O7

By Rietveld refinement of the X-ray diffraction (XRD) data of powdered Na₂Al₂B₂O₇ samples aged for over 3 months, we found that Na₂Al₂B₂O₇ at room temperature is a mixture of two phases with space group and P6₃/m, respectively. The structures of the two phases can be refined with identical cell parameters of a=4.80760(11) Å, c=15.2684(5) Å and are composed by [Al₂B₂O₇]²⁻_∝ double layers stacking alternatively with Na⁺ ions along the c-direction, but differ at in-plane bond orientations of the BO₃/AlO₄ groups within the double layers: in P6₃/m phase B-O₁/Al-O₁ bonds of the two layers are perfectly aligned, whereas in phase they are twisted by 46.4/41.6° around c-axis against each other. It is also found that a freshly prepared sample contains only the phase, but part of the phase will transfer to P6₃/m phase slowly at room temperature and the transition can be reversed by heating the aged sample above 220 °C.     

Synthesis and laser patterning of Bi-doped Y3Fe5O12 crystals in germanosilicate glasses

New germanosilicate glasses giving the crystallization of yttrium iron garnet Y₃Fe₅O₁₂ (YIG) and Bi-doped YIG, 23Na₂O-xBi₂O₃-(12−x)Y₂O₃-25Fe₂O₃-20SiO₂-20GeO₂ (mol%), are developed, and the laser-induced crystallization technique is applied to the glasses to pattern YIG and Bi-doped YIG crystals on the glass surface. It is clarified from the Mössbauer effect measurements that iron ions in the glasses are present mainly as Fe³⁺. It is suggested from the X-ray diffraction analyses and magnetization measurements that Si⁴⁺ ions are incorporated into YIG crystals formed in the crystallization of glasses. The irradiations (laser power: 32-60 mW and laser scanning speed: 7 μm/s) of continuous wave Yb:YVO₄ fiber laser (wavelength: 1080 nm) are found to induce YIG and Bi-doped YIG crystals, indicating that Fe³⁺ ions in the glasses act as suitable transition metal ions for the laser-induced crystallization. It is suggested that YIG and Bi-doped YIG crystals in the laser irradiated part might orient. The present study will be a first step for the patterning of magnetic crystals containing iron ions in glasses.     

Preparation, characterization and electrochemical property of Sn-Fe-Mo-Al2O3 multi-phase composites

A novel technique has been developed to synthesize Sn-Fe-Mo-Al₂O₃, while nanoscale dispersion of a highly active tin phase was finely distributed in a stable inert multi-phase. The precursor was prepared by co-precipitation method with SnCl₄, FeCl₃, AlCl₃ and (NH₄)₆Mo₇O₂₄ as the raw materials. Sn-Fe-Mo-Al₂O₃ mixture was produced by reducing the precursor with H₂. The product was characterized by X-ray diffraction (XRD), ICP and scanning electron microscopy (SEM). The performance of the electrode was investigated. The Sn-Fe-Mo-Al₂O₃ electrode was found to have an initial charge capacity of over 461 mAh/g, and a reversible volumetric capacity of 2090 mAh/cm³, which is two times larger than that of graphite electrode (800 mAh/cm³). The coulomb efficiency in the first cycle was over 55%, but its cyclability was not improved significantly. In order to enhance the cycle performance, we investigated the anode after heat treated at 270 °C for 12 h. Under the same condition, the first charge-discharge characteristics were almost equivalent to the as-coated anode, and the retention capacity ratio after 20 cycles was improved from 41.1% to 86.5%. The heat-treated Sn-Fe-Mo-Al₂O₃ electrode exhibited better cycle life. The electrochemical reaction of the Sn-Fe-Mo-Al₂O₃ electrode with Li may obey the alloying-dealloying mechanism of Li_xSn(x?4.4) formation in the other tin-based electrodes.     

Synthesis and electrochemical performances of Li4Ti4.95Al0.05O12/C as anode material for lithium-ion batteries

Spinel compounds Li₄Ti_5−xAl_xO₁₂/C (x=0, 0.05) were synthesized via solid state reaction in an Ar atmosphere, and the electrochemical properties were investigated by means of electronic conductivity, cyclic voltammetry, and charge-discharge tests at different discharge voltage ranges (0-2.5 V and 1-2.5 V). The results indicated that Al³⁺ doping of the compound did not affect the spinel structure but considerably improved the initial capacity and cycling performance, implying the spinel structure of Li₄Ti₅O₁₂ was more stable when Ti⁴⁺ was substituted by Al³⁺, and Al³⁺ doping was beneficial to the reversible intercalation and deintercalation of Li⁺. Al³⁺ doping improved the reversible capacity and cycling performance effectively especially when it was discharged to 0 V.     

Preparation and characterization of NiFe2O4/NiO composite film via a single-source precursor route

NiFe₂O₄/NiO nanocomposite thin films have been successfully prepared through a facile route using nickel iron layered double hydroxide (NiFe-LDH) as a single-source precursor. This synthetic approach mainly involves the formation of NiFe-LDH film by casting the slurry of NiFe-LDH precursor on the α-Al₂O₃ substrate, followed by high-temperature calcination. The composition, microstructure and properties of the films were characterized in detail by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and vibrating sample magnetometer (VSM). The results indicate that NiFe₂O₄/NiO composite film was composed of granules with diameter less than 100 nm, and the thickness of the film was in the range 1-2 μm. The magnetization of the film can be tuned by alternating the Ni/Fe molar ratio of LDH precursor. In addition, the method developed should be easily extended to fabricate other MFe₂O₄/MO composite film systems with specific applications just by an appropriate combination of divalent/trivalent composition in the precursor of LDHs.     

Synthesis of high tap density Li3V2(PO4)3 cathode materials using mixed lithium precursors

High tap density Li₃V₂(PO₄)₃ cathode materials were synthesized using mixed LiF and LiNO₃ as lithium precursors, LiNO₃ was used as the sintering agent. Rietveld refinement results show that no impurities phases are detected in products. Particle size distribution and tap density measurement results show that particle size and tap density of products can be increased by the addition of LiNO₃. Electrochemical characterization results show that electrochemical performance of products is declined with the increase in contents of LiNO₃ in the lithium precursors. Only a small amount of LiNO₃ added in the lithium precursors (mole ratio of LiNO₃ to LiF is 1:9) can increase the tap density and also retain the good performance of products. Scanning electron microscopy (SEM) images indicate that the samples prepared by mixed lithium precursors present particles agglomerate, and the particle size increased with increase in contents of LiNO₃. Large amount of LiNO₃ added in the lithium precursors induces the particles to become spheric and smooth, which worsens the performance. The particles obtained with the mole ratio of LiNO₃ to LiF in 1:9 show a flake-like shape with a high specific surface area, which leads to good electrochemical performance.     

Synthesis,photophysical properties,and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles

Phase formation and photophysical properties of bismuth doped sodium tantalum oxide (perovskite, defect pyrochlore) nanoparticles prepared by a hydrothermal method were studied in detail. It was revealed that the synthesis conditions like NaOH concentration and bismuth precursor (NaBiO₃·2H₂O) markedly affect the crystal structure of sodium tantalum oxide. At low NaOH concentration and high bismuth precursor (NaBiO₃·2H₂O) content, Bi doped Na₂Ta₂O₆ (defect pyrochlore) phase was predominantly formed, while at higher NaOH concentration, Bi doped NaTaO₃ (perovskite) phase was formed. It was observed that the defect pyrochlore (Bi doped Na₂Ta₂O₆) phase was formed and stabilized by the presence of dopant precursor (NaBiO₃·2H₂O). The chemical analysis of the samples confirmed the doping of Bi³⁺ cations in both phases. Doping of bismuth enabled visible light absorption up to 500 nm in perovskite and defect pyrochlore type sodium tantalum oxide. Bi doped NaTaO₃ samples showed better performance for the photocatalytic degradation of rhodamine B than that of Bi doped Na₂Ta₂O₆, under visible light irritation (λ>420 nm). The present results shed light on phase formation of sodium tantalate and these results are useful in understanding properties of NaTaO₃ based compounds, synthesized by the hydrothermal method.     

Photocatalytic degradation efficacy of Bi4Ti3O12 micro-scale platelets over methylene blue under visible light

〈001〉 textured Bi₄Ti₃O₁₂ platelets with micro scale size were synthesized by a facile molten salt method. The photocatalytic activities of the as-prepared samples were measured with the photodegradation of methylene blue at room temperature under visible light irradiation. The Bi₄Ti₃O₁₂ with the aspect ratio of 35 exhibited good absorption in the visible light region and the photodegradation against methylene blue was higher than that of anatase TiO₂ reference, showing that the high degree of preferred {001} facets on the plate surface benefits the electronic transmission. In addition, the layer-pervoskite structure facilitates the mobility of the photogenerated carriers and hampers their recombination. The above results indicated that the large specific surface area of the as-prepared samples could attribute to the presence of a number of oxygen vacancies and then lead to the good photo-electric property. This work proposed an alternative way to tailor the structure of micro-sized platelets to get excellent properties comparable to the nano materials.     

Synthesis and crystal structure of [3,5-(CH3)2C6H3NH3]4P4O12·3H2O

Chemical preparation and crystal structure are given for a new cyclotetraphosphate: [3,5-(CH₃)₂C₆H₃NH₃]₄P₄O₁₂·3H₂O. This compound is triclinic P with the following unit-cell parameters: a=8.298(3), b=8.299(3), c=17.242(7)Å, α=97.13(3), β=102.72(3), γ=64.55(3)°, Z=1 and V=1045.2(8)ų. The crystal structure has been solved and refined to R=0.040 using 6086 independent reflections. The atomic arrangement can be described as layers organization. Layers built by P₄O₁₂ ring anions, ammonium groups and water molecules parallel to the plan (001), between which the organic groups are located. Characterization by X-ray diffraction, IR absorption, and thermal analysis are described.     

High temperature structural studies of SrRhO3

High resolution X-ray powder diffraction studies have shown SrRhO₃ to transform from an orthorhombic Pnma structure at room temperature through an intermediate Imma phase to a tetragonal I4/mcm structure near 800 °C. The orthorhombic Imma phase exists over a very limited temperature range, of less than 20°. The diffraction data suggests the Pnma to Imma transition is continuous and demonstrates that the Imma-I4/mcm transition is first order.     

Effect of chromium substitution in Ca4Mn3O10

Ca₄Mn_3−xCr_xO₁₀ compounds were synthesized in order to investigate the role of an isoelectronic substitution in the layered manganite. Induced structural changes are mainly described as a distortion of the two types of octahedra in the n=3 RP structure. The results indicate that Cr³⁺ is not the only significant valence state for chromium ions. Electrical and magnetic characterization allow to conclude that chromium does not favour the double exchange mechanism in these compounds.     

Recent development and application of Li4Ti5O12 as anode material of lithium ion battery

Lithium-ion batteries with both high power and high energy density are one of the promising power sources for electric devices, especially for electric vehicles (EV) and other portable electric devices. One of the challenges is to improve the safety and electrochemical performance of lithium ion batteries anode materials. Li₄Ti₅O₁₂ has been accepted as a novel anode material of power lithium ion battery instead of carbon because it can release lithium ions repeatedly for recharging and quickly for high current. However, Li₄Ti₅O₁₂ has an insulating character due to the electronic structure characterized by empty Ti 3d-states, and this might result in the insufficient applications of LTO at high current discharge rate before any materials modifications. This review focuses first on the present status of Li₄Ti₅O₁₂ including the synthesized method, doping, surface modification, application and theoretical calculation, then on its near future development.     

Synthesis of ZnFe2O4 nanocrystallites by mechanochemical reaction

Mechanochemical reaction of ZnO and α-Fe₂O₃ in a planetary mill formed an amorphous precursor, which was subsequently heated to successfully produce zinc ferrite (ZnFe₂O₄) nanocrystallites. The amorphous precursor and nanocrystallites were characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Calcination of the precursor powder at 600 °C led to the formation of ZnFe₂O₄ nanocrystallites of about 22 nm in crystal size, and most of particle was about 10-50 nm in diameter. Effect of calcination temperature on the crystal size of the nanoparticles was investigated. The mechanism of nanocrystallite growth was primarily investigated. The activation energy of ZnFe₂O₄ nanocrystallite formation during thermal treatment was calculated to be 18.5 kJ/mol.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Facile synthesis and electrochemical properties of hierarchical MnO2 submicrospheres and LiMn2O4 microspheres

Hierarchical MnO₂ submicrospheres have been successfully synthesized by a wet chemical method. The as-prepared products were characterized by means of XRD, SEM, FTIR, TG, and TEM. With the as-prepared MnO₂ submicrospheres as precursors, LiMn₂O₄ microspheres were conveniently prepared by a simple solid-state reaction between MnO₂ and LiOH at a temperature as low as 600 °C. Electrochemical properties of the as-prepared MnO₂ submicrospheres and LiMn₂O₄ microspheres as cathode materials in lithium ion cells were investigated by galvanostatic charge/discharge tests.     

Synthesis and enhanced photocatalytic activity of NaNbO3 prepared by hydrothermal and polymerized complex methods

We prepared NaNbO₃ by several methods, namely solid-state reaction (SSR), hydrothermal (HT) and polymerized complex (PC) methods, and investigated the relationships between the photocatalytic activity and the particle size and morphology. The photocatalytic activity was evaluated by H₂ evolution from an aqueous methanol solution and pure water splitting in the presence of the Pt(0.5 wt%)/NaNbO₃ and RuO₂(1.25 wt%)/NaNbO₃, respectively. It is found that the sample prepared by PC with smallest particles exhibits the highest photocatalytic activity in both reactions. Moreover, the HT sample with the cubic and rectangular shape also shows the enhanced photocatalytic activity for H₂ evolution from an aqueous methanol solution in comparison with that of the sample prepared by SSR.