Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.     

Structural and transport properties of stoichiometric Mn-doped magnetite: Fe3−xMnxO4

The polycrystalline samples of Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50) were prepared by a solid-state route reaction method. X-ray diffraction pattern shows that Mn²⁺ doped magnetites are in single phase and possess cubic inverse spinel structure. The resistivity measurements (10 < T < 300 K) for x = 0.0 and 0.01 confirms the first order phase transition at the Verwey transition T_V = 123 K and 117 K, respectively. No first order phase transition was evidenced for Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50). Small polaron model has been used to fit the semiconducting resistivity behavior and the activation energy ?_a, for samples x = 0.10 and 0.50 is about 72.41 meV and 77.39 meV, respectively. The Raman spectra of Fe_3−xMn_xO₄ at room temperature reveal five phonons modes for Fe_3−xMn_xO₄ (0.01 ≤ x ≤ 0.50) as expected for the magnetite (Fe₃O₄). Increased Mn²⁺ doping at Fe site leads to a gradual changes in phonon modes. The Raman active mode for Fe_3−xMn_xO₄ (x = 0.50) at ≅641.5 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅669.7 cm⁻¹, inferring that Mn⁺² ions are located mostly on the octahedral sites. The laser power is fixed to 5 mW causes the bands to broaden and to undergo a small shift to lower wave numbers as well as increase in the full width half maxima for A_1g phonon mode with the enhancement of Mn²⁺ doping. Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Mn⁺² ions are located mostly on the octahedral sites of the Fe_3−xMn_xO₄ spinel structure.     

R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb and Lu) and Lu3Mn3−3xCu2xVxO9: New noncentrosymmetric oxides related to YMnO3

We report formation of new noncentrosymmetric oxides of the formula, R₃Mn_1.5CuV_0.5O₉ for R = Y, Ho, Er, Tm, Yb and Lu, possessing the hexagonal RMnO₃ (space group P6₃cm) structure. These oxides could be regarded as the x = 0.5 members of a general series R₃Mn_3−3xCu_2xV_xO₉. Investigation of the Lu-Mn-Cu-V-O system reveals the existence of isostructural solid solution series, Lu₃Mn_3−3xCu_2xV_xO₉ for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn³⁺, Cu²⁺ and V⁵⁺ atoms that preserve the noncentrosymmetric RMnO₃ structure.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Synthesis of spherical nanostructured LiMxMn2−xO4 (M = Ni, Co, and Ti; 0 ≤ x ≤ 0.2) via a single-step ultrasonic spray pyrolysis method and their high rate charge-discharge performances

LiM_xMn_2−xO₄ (M = Ni²⁺, Co³⁺, and Ti⁴⁺; 0 ≤ x ≤ 0.2) spinels were prepared via a single-step ultrasonic spray pyrolysis method. Comparative studies on powder properties and high rate charge-discharge electrochemical performances (from 1 to 15 C) were performed. XRD identified that pure spinel phase was obtained and M was successfully substituted for Mn in spinel lattice. SEM and TEM studies confirmed that powders had a feature of ‘spherical nanostructural’, that is, powders consisted of spherical secondary particles with the size of about 1 μm, which were developed from close-packed primary particles with several tens of nanometers. Substitutions enhanced density of second particles to different extents, depending on M and its content. Charge-discharge tests showed that as-prepared LiMn₂O₄ could deliver excellent rate performance (around 100 mAh/g at 10 C). Ni substitution contributed to improving electrochemical performances. In the voltage range of 4.95-3.5 V, the materials showed much better electrochemical performances than LiMn₂O₄ in terms of capacity, cycleability and rate capability.     

Multifunctional Zn0.99−xMn0.01CuxS nanowires: Structure, luminescence and magnetism

The wurtzite-type Zn_0.99−xMn_0.01Cu_xS (x = 0, 0.003, 0.01) nanowires were prepared by a simple hydrothermal method at 180 °C. The structure and morphology of the samples were characterized by X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), field emission scanning electron micrograph (FESEM) and X-ray photoelectron spectrum (XPS). The results showed that both the Mn²⁺ and Cu²⁺ ions substituted for the Zn²⁺ sites in the host ZnS. The ethylenediamine-mediated template was observed, which was used to explain the growth mechanism of the nanowires. The color-tunable emission can be obtained by adjusting the concentrations of Mn²⁺ and Cu²⁺ ions. The ferromagnetism was observed around room temperature.     

Structural,transport and spectroscopic properties of Ti substituted magnetite: Fe3−xTixO4

The effect of Ti⁴⁺ ion on the formation of magnetite, which were prepared by solid-state route reaction method, were studied by resistivity, Raman and ⁵⁷Fe Mössbauer spectrometry. Resistivity measured in the range of 10 < T < 300 K for Ti⁴⁺ magnetite Fe_3−xTi_xO₄ exhibit first order phase transformations at the Verwey transition T_v for Fe₃O₄, Fe_2.98Ti_0.02O₄ and Fe_2.97Ti_0.03O₄ at 123 K, 121 K and 118 K, respectively. No first order phase transition was observed for Fe_2.9Ti_0.1O₄ and small polaron model retraces the semiconducting resistivity behavior with activation energy of about 72 meV. The changes in Raman spectra as a function of doping show that the changes are gradual for samples with higher Ti doping. The Raman active mode for Fe_2.9Ti_0.1O₄ at ≅634.4 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅670 cm⁻¹, inferring that Mn²⁺ ions are located mostly on the octahedral sites. ⁵⁷Fe Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Ti⁴⁺ ions are located mostly on the octahedral sites of the Fe_3−xTi_xO₄ spinel structure.     

Magnetic properties of Cu substituted Co-ferrite

Copper substituted Co-Cu ferrites Co_1 − xCu_xFe₂O₄ (0 ≤ x≤0.5) have been studied with Mössbauer spectroscopy, x-ray diffraction, and vibrating sample magnetometer (VSM). The Co-Cu ferrite toroidal core samples were sintered at 860-940 °C for 2 h and the initial permeability, quality factor, density and shrinkage were also measured. The crystal structure was found to be an inverse cubic spinel with the lattice constant a₀ = 8.390 Å and a₀ = 8.386 Å for Co-ferrite and Cu²⁺ substituted Co-ferrite, respectively, by Rietveld profile analysis using the FULLPROF program. Hyperfine field was decreased with increasing Cu²⁺ concentration. The saturation magnetization (Ms) of the Co-Cu ferrite annealed at 900 °C decreased drastically and the coercivity, Hc, dropped dramatically from about 1419 to 455 Oe as copper concentration x decreased from 0.0 to 0.5. This shows that Ms, Hc can be controlled using Cu content, and initial permeability and quality factor Q is nearly constant in Cu²⁺ substituted Co-ferrite. The toroidal core data showed that the density and shrinkage of Co_1 − xCu_xFe₂O₄ (0≤x ≤ 0.5) ferrites increased with increasing quantity of Cu ions.     

Growth and characterisation of CaCu2Ox thin films by pulsed injection MOCVD

Oxides with the structure MCu₂O₂ (M = Ca, Ba, Mg and Sr) are promising materials for the development of new p-type transparent conducting oxide thin films. This paper reports preliminary results on the growth and characterisation of CaCu₂O_x thin films by pulsed injection MOCVD. By using as precursors calcium and copper tetramethylheptanedionate dissolved in meta-xylene, mixed calcium-copper films have been grown in the temperature range from 450 °C to 550 °C. At these temperatures, deposited films exhibited a high mirror reflection effect, good adherence and were reasonably uniform with the cationic composition of the films being easily controlled by adjusting the copper-calcium ratio in the precursor solution. In CaCu₂O₂, copper is in the Cu¹⁺ oxidation state and depending on the oxygen partial pressure used, the films either contained CaCu₂O₃ or a mixture of CaO, CuO and Cu₂O. Optimisation of annealing conditions increased the presence of Cu¹⁺ in the film. Films had a maximum transmittance of 50% in the visible range and were highly resistive. Appropriate annealing conditions reduced the resistivity of the films.     

NiMn2−xFexO4 prepared by a reverse micelles method as conversion anode materials for Li-ion batteries

Synthesis by reverse micelles was used to produce NiMn_2−xFe_xO₄ with nanometric particle sizes for their use as conversion anode materials for lithium ion batteries. The hydroxide precursor was characterized by infrared spectroscopy and the decomposition was followed by thermal analysis. Cation distribution in the spinel structure of pristine samples was evaluated by Mössbauer spectroscopy evidencing that octahedral Fe³⁺ is substituted by Mn³⁺ ions in NiMnFeO₄. Capacity values of 750 mA h g⁻¹ were retained for 50 cycles for NiMnFeO₄ and NiFe₂O₄, respectively. A good kinetic response was observed in NiMnFeO₄ at 2C.     

Pechini sol-gel deposition and luminescent properties of SrLa1−xRExGa3O7 (RE = Eu, Tb) phosphor films

SrLa_1−xRE_xGa₃O₇ (RE = Eu³⁺, Tb³⁺) phosphor films were deposited on quartz glass substrates by a simple Pechini sol-gel method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy, field-emission scanning electron microscopy, photoluminescence spectra, and lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 700 °C and crystallized fully at 900 °C. The results of FT-IR spectra were in agreement with those of XRD. Uniform and crack-free films annealed at 900 °C were obtained with average grain size of 80 nm, root mean square roughness of 46 nm and thickness of 130 nm. The RE ions showed their characteristic emission in crystalline SrLa_1−xRE_xGa₃O₇ films, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Tb³⁺⁵D₄-⁷F_J (J = 6, 5, 4, 3) emissions, respectively. The optimum concentrations (x) of Eu³⁺ and Tb³⁺ were determined to be 50, and 80 mol% in SrLa_1−xRE_xGa₃O₇ films, respectively.     

Low loss Sr1−xCaxLa4Ti5O17 microwave dielectric ceramics

Microwave dielectric ceramics in the Sr_1−xCa_xLa₄Ti₅O₁₇ (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca²⁺ for Sr²⁺ which was associated with a decrease in the dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) but an increase in quality factor, Qf_o. Optimum properties were achieved for Sr_0.4Ca_0.6La₄Ti₅O₁₇ which exhibited, ?_r ∼ 53.7, Qf_o ∼ 11,532 GHz and τ_f ∼ −1.4 ppm/°C.     

Soft chemistry synthesis and crystal structure of MgxCu3−xV2O6(OH)4·2H2O

Mg_xCu_3−xV₂O₆(OH)₄·2H₂O (x ∼ 1), with similar crystal structure as volborthite Cu₃V₂O₇(OH)₂·2H₂O, was successfully prepared by a soft chemistry technique. The method consists of mixing magnesium nitrate and copper nitrate with a boiling solution of vanadium oxide (obtained by reacting V₂O₅ with few mL of 30 vol.% H₂O₂ followed by addition of distilled water). When ammonium hydroxide NH₄OH 10% was added (pH 7.8), a green yellowish precipitate was obtained. Using X-ray powder diffraction data, its crystal structure has been determined by Rietveld refinement. Compared to volborthite, the vanadium coordination changes from tetrahedral VO₄ to trigonal bipyramidal VO₅, and magnesium replaces copper, preferably, in the less distorted octahedron. At 300 °C, the phase formed is similar to the high pressure (HP) monoclinic Cu₃V₂O₈ phase. However at higher temperature, 600 °C, the phase obtained is different from known Cu₃V₂O₈ phases.     

Structure analysis on the Ba3Mg(Ta1−xNbx)2O9 ceramics: Coexistence of order and disorder

The Ba₃ZnTa₂O₉ (BZT) and Ba₃MgTa₂O₉ (BMT) ceramics, a family of A₃B²⁺B⁵⁺₂O₉ complex perovskites, are extensively utilized in mobile based technologies due to their intrinsic high unloaded quality factor, high dielectric constant and a low (near-zero) resonant frequency temperature coefficient at microwave frequencies. The preparation conditions as well as size and nature of B cations have a profound effect on the final dielectric properties. In this article, we report the effect of Nb⁵⁺ at the Ta⁵⁺ site on the BMT structure prepared at four synthesis temperatures (1300, 1400, 1500 and 1600 °C). The analysis has been carried out using the Rietveld technique on the X-ray powder diffraction data. Results suggest that both the preparation temperatures and Nb⁵⁺ content have significant effect on the ordering of B cations in the Ba₃Mg(Ta_1−xNb_x)₂O₉ solid solution. A disordered (cubic) structure is preferred by the 1300 °C compounds. The weight percentage of the ordered (trigonal) phase escalates, for a given composition, with increasing calcination temperature. A fully ordered trigonal arrangement exists only for x = 0.0 and 0.2 compounds calcined at 1600 °C, and the rest are biphasic (cubic and trigonal). The increase in the cubic fraction upon Nb⁵⁺ augmentation suggests that the solid solution leans more toward the disordered structural arrangement of B²⁺ and B⁵⁺ cations.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Mössbauer effect and dielectric behavior of NixCu0.8−xZn0.2Fe2O4 compound

The ferrite system Ni_xCu_0.8−xZn_0.2Fe₂O₄ with 0.0 ≤ x ≤ 0.8 was synthesized. XRD measurement confirmed the presence of single-phase spinel structure. The area ratio of Fe³⁺ at the tetrahedral A- and octahedral B-sites was deduced from the spectral analysis of Mössbauer measurements. The results give evidence that Ni²⁺ replaces Cu at B-site in the present unit cell. The dielectric properties ?′, ?″, loss tangent tan(δ) and ac conductivity σ_ac have been studied for the prepared samples in the temperature range (300-600 K) and over the frequency range (10² to 10⁵ Hz). The electrical conductivity results revealed a semiconductor behavior with increasing nickel concentration with a change in the slope at the transition temperature T_c. The variation of the dielectric parameters (?′, ?″ and tan(δ)) with frequency and temperature displayed a strong dependence on nickel concentration. Dielectric anomaly at the transition temperature T_c was pronounced in the relations of ?′ and ?″ with temperature. The determined T_c was found to increase with increasing Ni content. The relation of tan(δ) with frequency at different temperatures showed two relaxation processes where the relaxation time and maximum frequency of the hopping conduction mechanism were determined. The results are explained in the light of cation-anion-cation and cation-cation interactions over the octahedral site in the spinel structure.     

Structural study of (50 − x)Na2O-xCuO-10Bi2O3-40P2O5 glasses

(50−x)Na₂O-xCuO-10Bi₂O₃-40P₂O₅ glasses (0≤x≤25) were prepared by melting at 900-1100°C mixtures of Na₂CO₃, Bi₂O₃, CuO and (NH₄)₂HPO₄. DSC measurements give the variation of glass transition temperature T_g from 318 (x=0) to 378°C (x=25). FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO₃)_∞ chains for 60Na₂O-40P₂O₅ to P₂O₇ groups in the glass containing Bi₂O₃ or both Bi₂O₃ and CuO. When bismuth and copper oxides replace Na₂O, phosphate chains are depolymerized by the incorporation of Bi₂O₃ and CuO through POBi and POCu bonds. P₂O₇ groups are predominant structural units in the richest CuO glass. The variation of T_g also supports these results.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.     

SrSn1−xFexO3−δ (0≤x≤1) perovskites: a novel mixed oxide ion and electronic conductor

The electrical conductivity of SrSn_1−xFe_xO_3−δ increases with the Fe content and reaches a value of ∼10⁻¹ S/cm at 25°C at x=1. Compounds with low Fe content exhibit both ionic and electronic conductivity, while the higher Fe content perovskites are mainly electronic conductors with a conductivity independent of the oxygen partial pressure over a wide range from 0.21 to 10⁻²² atm.