Metal-insulator transitions in reduced molybdenum oxides Sm4Mo18O32 and Nd4Mo18O32

Synthesis, structure and electronic properties are reported on single crystals of Sm₄Mo₁₈O₃₂ and Nd₄Mo₁₈O₃₂. The triclinic crystal structure has three distinctly different Mo metal atom clusters (Mo₂, Mo₄ and Mo₆), which extend in complex chains. The temperature dependence of the electrical resistivity displays a broad minimum around 150-200 K with a pronounced increase in the 30-50 K range. However, unlike the related monoclinic reduced Mo oxides, A₄Mo₁₈O₃₂ (A = Y, Gd-Yb), there is no true metal-insulator transition in the Sm and Nd analogs. We discuss these observations in terms of correlations among the Mo clusters.     

Raman scattering characterization of bismuth based mixed oxides with Bi2O3 related structures

Orthorhombic Bi₁₇Ln₇O₃₆ (=Bi_0.708Ln_0.292O_1.5) and triclinic BiLnO₃ (=Bi_0.5Ln_0.5O_1.5) with Ln = Tm, Yb and Lu, as well as a cubic δ-type Bi_0.7Yb_0.3O_1.5, have been characterized at room temperature by Raman scattering spectrometry. The Raman spectrum deconvolution of the δ-Bi₂O₃ type cubic form allowed us to establish a correlation between the different phases of these mixed metallic oxides for the lattice modes as well as for the internal species. Using the empirical relation from Hardcastle, the BiO bond lengths have been estimated from the stretching frequency value observed at 635, 597 or 639 cm⁻¹ for cubic, orthorhombic or triclinic forms, respectively. A comparison with the X-ray results is discussed. The peritectoïdic transformation BiTmO₃ → δ + Tm₂O₃ has also been characterized by Raman spectroscopy.     

Synthesis and characterisation of the garnet-related Li ion conductor, Li5Nd3Sb2O12

In this paper the synthesis, conductivity, and structure of the garnet-related Li ion conductor, Li₅Nd₃Sb₂O₁₂, are reported. As for the related Li₅La₃M₂O₁₂ (M = Nb, Ta) materials, this phase shows high Li ion conductivity, with a conductivity at 300 °C of 9.2 × 10⁻³ S cm⁻¹. Structural studies using neutron diffraction indicate a cubic unit cell, space group Ia-3d, with Li located in two partially occupied sites. One of the sites is the traditional garnet structure tetrahedral site, while the other Li site is considerably more distorted. Although the latter is nominally a six coordinate site, a close inspection suggests that the coordination could be described as distorted tetrahedral, with the remaining two bonds being significantly longer (≈2.6 Å).     

Synthesis and characterization of the colossal magnetoresistance manganite La1.2(Sr1.4Ca0.4)Mn2O7 by citrate gel

Single-phase La_1.2(Sr_1.4Ca_0.4)Mn₂O₇ has been synthesized from the aqueous solution of metal nitrates and citric acid by the sol-gel technique. Small particle size with high homogeneity of the powder was obtained. The valence of Mn is determined to be 3.45±0.05 by chemical titration. The MR ratio [ρ−ρ(H)]/ρ(H), is 115% (102 K, 1.5 T) for the composition prepared by the citrate route, and is nearly three times larger than that of the sample prepared by solid state reaction.     

Photoluminescence properties of V-doped La0.67Ca0.33Mn1−xVxO3

The optical spectra have been investigated for a prototypical double exchange ferromagnetic La_0.67Ca_0.33MnO₃ with Mn substituted by V above the paramagnetic-magnetic transition temperature T_C. The excitation spectra under the probe wavelength of λ_em = 473 nm for all samples exhibit two activation bands around 360 and 294 nm, involving an electron transfer from oxygen 2p states to the Mn d states in MnO₆ octahedra. The photoluminescence spectra at λ_ex = 290 nm have the similar spectral features for all samples. The photoluminescence spectral peaks located at 400, 473, 534, 670, 738 and 770 nm, and the corresponding photon energy is in a broad range of 3.1-1.6 eV, indicating that the PL bands could have the different origin: the self-trapped excitons localized on MnO₆ octahedra; the interband transition between the O 2p and Mn 3d bands; the transition between the 3d electron states of Mn ions. So, it can be clearly seen that the electronic behavior above T_C is very complicated. Our results suggest that the charge transfer from O 2p to Mn 3d has the important effects on the electronic structure, and it not only contributes to the optical transition but is helpful and even important to understand the electric, magnetic and thermal properties etc. due to the strong correlation among charge, spin and lattice in perovskite manganites.     

Synthesis and electrochemical performances of spinel LiCr0.1Ni0.4Mn1.5O4 compound

The spinel compound LiCr_0.1Ni_0.4Mn_1.5O₄ was synthesized by a solid reaction method and a sol-gel method using citric acid as chelating agent. The pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ was obtained by the wet method. The electrochemical performances of the pure phase sample were measured at different current rates. There were three voltage plateaus at about 4.9, 4.7 and 4.0 V in the charge-discharge curves, which were attributed to the oxidation/reduction of chromium, nickel and manganese respectively. In the range of 3.5-5.0 V, its first discharge capacity was 143, 118 and 111 mAh/g corresponding to current densities of 1.0, 4.0 and 5.0 mA/cm², respectively. After 50 cycles, the capacity retention remained well at the current densities of 1.0, 4.0 and 5.0 mA/cm². The electrochemical performances of pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ at 55 °C was also measured, and the results were discussed.     

Controlled synthesis of Mn3O4 and MnCO3 in a solvothermal system

Controlled synthesis of Mn₃O₄ nanocrystals and MnCO₃ aggregates was achieved by a facile solvothermal method using different divalent manganese source in the solvent of N,N-dimethylformamide (DMF) with/without the introduction of poly(vinylpyrrolidone) (PVP). PVP was used as a co-reducing reagent in the controlled formation of MnCO₃ crystal. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED), Fourier transform infrared (FTIR) spectra, Raman spectrum and magnetic measurement. Higher process temperature and longer solvothermal time were favorable for the formation of MnCO₃ single phase using MnCl₂ as the manganese source. Mn₃O₄ nanocrystals were prepared at a relatively lower temperature. MnCO₃ aggregates consisted by small nanoparticles have a certain orientation, showing that the nanocrystals formed earlier through oriented aggregation. The size of Mn₃O₄ nanocrystals was 22.5 ± 7.3 nm and 7.3 ± 1.4 nm prepared using MnCl₂ and Mn(CH₃COO)₂, respectively, at 160 °C for 24 h. Raman spectra showed size-dependent characteristics. Smaller Mn₃O₄ nanoparticle resulted in a red-shift in Raman spectra. Magnetic property of the prepared Mn₃O₄ nanoparticle was influenced by the size distribution and crystallinity.     

Complete oxidation of acetaldehyde on Pt/CeO2-ZrO2-Bi2O3 catalysts

Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts for catalytic combustion of acetaldehyde, which is one of volatile organic compounds (VOCs), were prepared by a wet impregnation method in the presence of polyvinylpyrrolidone K25 (PVP). The addition of PVP in the preparation process was effective to enhance the specific surface area and the Pt²⁺ ratio on the surface. Additionally, the pore volume and size of the catalysts were modified by the PVP addition. The Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts are specific for the total acetaldehyde oxidation and CO and any acetaldehyde-derivative compounds were not observed as by-products. The catalytic activity of the Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts was significantly promoted by the PVP addition and the total oxidation temperature decreased. By the optimization of the amount of platinum, the complete oxidation of acetaldehyde was realized at a temperature as low as 140 °C on a 10 wt%Pt/CeO₂-ZrO₂-Bi₂O₃ catalyst.     

Phase relations of the Li2O-Ta2O5-B2O3 and Li2O-WO3-B2O3 systems and promising nonlinear optical compounds in K2O-Ta2O5-B2O3 system

The subsolidus phase equilibria of the Li₂O-Ta₂O₅-B₂O₃, K₂O-Ta₂O₅-B₂O₃ and Li₂O-WO₃-B₂O₃ systems have been investigated mainly by means of the powder X-ray diffraction method. Two ternary compounds, KTaB₂O₆ and K₃Ta₃B₂O₁₂ were confirmed in the system K₂O-Ta₂O₅-B₂O₃. Crystal structure of compound KTaB₂O₆ has been refined from X-ray powder diffraction data using the Rietveld method. The compound crystallizes in the orthorhombic, space group Pmn2₁ (No. 31), with lattice parameters a = 7.3253(4) Å, b = 3.8402(2) Å, c = 9.3040(5) Å, z = 2 and D_calc = 4.283 g/cm³. The powder second harmonic generation (SHG) coefficients of KTaB₂O₆ and K₃Ta₃B₂O₁₂ were five times and two times as large as that of KH₂PO₄ (KDP), respectively.     

Fluorine-doped LiNi0.5Mn1.5O4 for 5 V cathode materials of lithium-ion battery

Fluorine-doped 5 V cathode materials LiNi_0.5Mn_1.5O_4−xF_x (0.05 ≤ x ≤ 0.2) have been prepared by sol-gel and post-annealing treatment method. The results from X-ray diffraction and scanning electron microscopy (SEM) indicate that the spinel structure changes little after fluorine doping, but the particle size varies with fluorine doping and the preparation conditions. The electrochemical measurements show that stable cycling performance can be obtained when the fluorine amount x is higher than 0.1, but the specific capacity is decreased and 4 V plateau capacity resulting from a conversion of Mn⁴⁺/Mn³⁺ remains. Moreover, influence of the particle size on the reversible capacity of the electrode, especially on the kinetic property, has been examined.     

Formation of NiFe2O4 nanoparticles by mechanochemical reaction

Preparation of nanosized NiFe₂O₄ particles by mechanochemical reaction(NiO+α-Fe₂O₃) and subsequent thermal treatment was investigated using X-ray diffraction (XRD). Thermal treatment of the as-milled powder at 700 °C for 1 h led to the formation of NiFe₂O₄ nanoparticles with an average crystal size of about 23 nm. Effect of thermal treatment temperature on the crystal size of the nanoparticles was studied. The mechanism of nanoparticles growth was primarily discussed. The activation energy of NiFe₂O₄ nanoparticle formation during calcination was calculated to be 16.6 kJ/mol.     

Structure and lithium ion conductivity of garnet-like Li5La3Sb2O12 and Li6SrLa2Sb2O12

Oxides with the nominal chemical compositions Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂ were prepared by solid-state reaction. The structures were refined by the Rietveld method using powder X-ray diffraction data. The synthesis of Li₅La₃Sb₂O₁₂ resulted in the well known garnet-related structure plus 5 wt.% of La₂LiSbO₆ in the bulk. In contrast to that, Li₆SrLa₂Sb₂O₁₂ could be synthesised in single garnet-related type phase. Lithium ion conductivities of Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂ were studied by the ac impedance method. The grain-boundary contribution to the total (bulk + grain-boundary) resistance is very small and about 5 and 3% for Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂, respectively, at 24 °C and decreases further with increase in temperature. Among the investigated compounds, Li₅La₃Sb₂O₁₂ exhibits the highest total (bulk + grain-boundary) and bulk ionic conductivity of 7.8 × 10⁻⁶ and 8.2 × 10⁻⁶ S cm⁻¹, respectively, at 24 °C. The structural data indicate that the coupled substitution Li + Sr ⇒ La leads to a closure of the bottle neck like O-O distances of the shared edges of neighbouring Li octahedra and therefore reduces the mobility of Li ions in Li₆SrLa₂Sb₂O₁₂. Scanning electron microscope (SEM) images of the Li₆SrLa₂Sb₂O₁₂ compound revealed well crystallised large homogeneous grains (∼4.8 μm) and the grains were in good contact with the neighbouring grain, which leads to a smaller grain-boundary contribution to the total resistance.     

Low temperature synthesis of nanocrystalline Li4Mn5O12 by a hydrothermal method

A mild hydrothermal method using Li-birnessite (Li_xMnO₂·nH₂O) ultrafine fiber as the precursor has been adopted to prepare Li₄Mn₅O₁₂, which is of interest as an electrode material for 3 V rechargeable lithium ion batteries. X-ray diffraction data reveal that the obtained powders have a pure spinel structure with a lattice constant of 8.135 Å. The scanning electron microscopy image of the obtained powders shows the particles are cubic-shaped whose average size is about 40-50 nm. The results from inductively coupled plasma-atomic emission spectroscope and wet chemical analysis indicate that a Li/Mn ratio of 0.796, and an average valence of 3.96 of Mn ion have been achieved in the as-prepared products. The thermogravimetric and differential thermal analysis data also agree with the previous reports on Li₄Mn₅O₁₂, suggesting that near stoichiometry of Li₄Mn₅O₁₂ has been synthesized by this procedure at the rather low temperature 110°C.     

The effect of the concentration of citric acid and pH values on the preparation of MgAl2O4 ultrafine powder by citrate sol-gel process

Ultrafine MgAl₂O₄ was synthesized by citrate sol-gel process. A model was presented to evaluate the concentration of species in a citric solution for preparing MgAl₂O₄ ultrafine powder. The evaluated concentration of species can provide valuable information and help in selecting the optimal condition for preparation of MgAl₂O₄ powder by citrate sol-gel process. The influence of molar ratio of cations, citric acid and pH on the formation of MgAl₂O₄ was studied. The spinel precursor gel and the ultrafine MgAl₂O₄ spinel were characterized by X-ray diffraction (XRD), differential thermal analysis, thermogravimetric (TG-DTA) and scanning electron microscope (SEM). The results show that the MgAl₂O₄ spinel phase begins to form at 600 °C, and most of MgAl₂O₄ crystals are spherical with a crystal size about 30-50 nm.     

Topotactic route for new layered perovskite oxides containing fluorine: Ln1.2Sr1.8Mn2O7F2 (Ln = Pr, Nd, Sm, Eu, and Gd)

A series of fluorine intercalated Ruddlesden-Popper layered perovskites, Ln_1.2Sr_1.8Mn₂O₇F₂ (Ln = La, Pr, Nd, Sm, Eu, and Gd) were synthesized from the parent oxides, Ln_1.2Sr_1.8Mn₂O₇, with the organic reagent, poly(vinylidene fluoride), as the fluorine source. Oxidative intercalation readily occurred by decomposition of the organic fluoride immediately adjacent to the parents.     

Ce K-edge EXAFS study of nanocrystalline CeO2

Nanocrystalline CeO₂ samples were prepared via precipitation from aqueous cerium(IV) sulfate solution under a controlled pH. After being subjected to calcination at various temperatures to yield different particle sizes, the local structure around Ce atom was investigated by using EXAFS spectroscopy at Ce K-edge. The first shell CeO distance was found to be slightly shorter with small CeO₂ particles, indicating a contraction of the lattice. There exists a significant growth in coordination number of the second cell CeCe when the particle size became larger. In terms of Debye-Waller factor (σ²), the first three coordination shells showed a consistent trend, i.e. the degree of disorder increases with reduced particle size. Moreover, σ² of the second shell was found to be smaller than that of the first shell for all samples.     

Optimization of reaction conditions in selective oxidation of styrene over fine crystallite spinel-type CaFe2O4 complex oxide catalyst

The CaFe₂O₄ spinel-type catalyst was synthesized by citrate gel method and well characterized by thermogravimetric analysis, atomic absorption spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction and transmission electron microscopy. The crystallization temperature of the spinel particle prepared by citrate gel method was 600 °C which was lower than that of ferrite prepared by other methods. CaFe₂O₄ catalysts prepared by citrate gel method show better activity for styrene oxidation in the presence of dilute H₂O₂ (30%) as an oxidizing agent. In this reaction the oxidative cleavage of carbon-carbon double bond of styrene takes place selectively with 38 ± 2 mol% conversion. The major product of the reaction is benzaldehyde up to 91 ± 2 mol% and minor product phenyl acetaldehyde up to 9 ± 2 mol%, respectively. The products obtained in the styrene oxidation reaction were analyzed by gas chromatography and mass spectroscopy. The influence of the catalyst, reaction time, temperature, amount of catalyst, styrene/H₂O₂ molar ratio and solvents on the conversion and product distribution were studied.     

New substitutions and novel derivatives of the Aurivillius phases, Bi5TiNbWO15 and Bi4Ti3O12

We investigated isomorphous substitution of several metal atoms in the Aurivillius structures, Bi₅TiNbWO₁₅ and Bi₄Ti₃O₁₂, in an effort to understand structure-property correlations. Our investigations have led to the synthesis of new derivatives, Bi₄LnTiMWO₁₅ (Ln = La, Pr; M = Nb, Ta), as well as Bi₄PbNb₂WO₁₅ and Bi₃LaPbNb₂WO₁₅, that largely retain the Aurivillius (n = 1) + (n = 2) intergrowth structure of the parent oxide Bi₅TiNbWO₁₅, but characteristically tend toward a centrosymmetric/tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2Ti^IV → M^V + Fe^III in Bi₄Ti₃O₁₂, yields new Aurivillius phases, Bi₄Ti_3−2xNb_xFe_xO₁₂ (x = 0.25, 0.50) and Bi₄Ti_3−2xTa_xFe_xO₁₂ (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi₄Ti₃O₁₂. Two new members of this family, Bi₂Sr₂Nb₂RuO₁₂ and Bi₂SrNaNb₂RuO₁₂ that are analogous to Bi₂Sr₂Nb₂TiO₁₂, possessing tetragonal (I4/mmm) Aurivillius structure have also been synthesized.     

Ion exchange and electrochemical evaluation of the microporous phosphate Li9Fe7(PO4)10

A new lithium iron(III) phosphate, Li₉Fe₇(PO₄)₁₀, has been synthesized and is currently under electrochemical evaluation as an anode material for rechargeable lithium-ion battery applications. The sample was prepared via the ion exchange reaction of Cs₅K₄Fe₇(PO₄)₁₀1 in the 1 M LiNO₃ solution under hydrothermal conditions at 200 °C. The fully Li⁺-exchanged sample Li₉Fe₇(PO₄)₁₀2 cannot yet be synthesized by conventional high-temperature, solid-state methods. The parent compound 1 is a member of the Cs_9−xK_xFe₇(PO₄)₁₀ series that was previously isolated from a high-temperature (750 °C) reaction employing the eutectic CsCl/KCl molten salt. The polycrystalline solid 1 was first prepared in a stoichiometric reaction via conventional solid-state method then followed by ion exchange giving rise to 2. Both compounds adopt three-dimensional structures that consist of orthogonally interconnected channels where electropositive ions reside. It has been demonstrated that the Cs_9−xK_xFe₇(PO₄)₁₀ series possesses versatile ion exchange capabilities with all the monovalent alkali metal and silver cations due to its facile pathways for ion transport. 1 and 2 were subject to electrochemical analysis and preliminary results suggest that the latter can be considered as an anode material. Electrochemical results indicate that Li₉Fe₇(PO₄)₁₀ is reduced below 1 V (vs. Li) to most likely form a Fe(0)/Li₃PO₄ composite material, which can subsequently be cycled reversibly at relatively low potential. An initial capacity of 250 mAh/g was measured, which is equivalent to the insertion of thirteen Li atoms per Li_9+xFe₇(PO₄)₁₀ (x = 13) during the charge/discharge process (Fe²⁺ + 2e → Fe⁰). Furthermore, 2 shows a lower reduction potential (0.9 V), by approximately 200 mV, and much better electrochemical reversibility than iron(III) phosphate, FePO₄, highlighting the value of improving the ionic conductivity of the sample.