Tartarate precursors to CoxZn1−xGa2O4 spinel oxides

The Co_xZn_1−xGa₂O₄ spinels (x = 0.25, 0.5, 0.75, 1) were synthesized by a wet chemistry method–the precursor route via tartarate decomposition. The complex precursors have been characterized by IR, UV–Vis, magnetic measurements and luminescence spectroscopy. XRD, SEM, IR, UV–Vis and luminescence spectroscopy were used for the structural and morphological investigation of Co_xZn_1−xGa₂O₄ spinels; magnetic susceptibilities of the spinel were also measured. The X-ray diffraction patterns confirmed the formation of gallate spinel phase, Co_xZn_1−xGa₂O₄.     

Mössbauer and magnetic studies of Mg1+2xSbxFe2−3xO4 spinel ferrites

Spinel-related Mg_1+2xSb_xFe_2−3xO₄ samples (x = 0.0, 0.05, 0.10, 0.15, 0.20, and 0.30) prepared using the conventional double sintering technique were investigated using ⁵⁷Fe Mössbauer spectroscopy and magnetic measurements. Mössbauer spectra favor a cationic distribution of the form (Mg_δFe_1−δ)^A[Mg_1+2x−δSb_xFe_1+δ−3x]^BO₄ among the tetrahedral-A and octahedral-B sites of the spinel structure. The cation distribution parameter (δ) was found to vary with the Sb⁵⁺ concentration (x). The Mössbauer hyperfine magnetic fields at both sites and the Curie temperatures of the ferrites decrease as x increases. This was attributed to gradual weakening in the magnetic exchange interaction as more Fe³⁺ ions are substituted by diamagnetic Sb⁵⁺ and Mg²⁺ ones. The sample with x = 0.30 exhibits short range magnetic order due to cationic clustering and/or superparamagnetism. The magnetization of all samples was found to be temperature-dependent implying that δ depends on temperature in addition to x. At low temperatures the substituted ferrites (x ≠ 0.0) unexpectedly exhibit higher magnetization values relative to that of the pure ferrite MgFe₂O₄. This behavior, while at variance with the Néel's model for ferrimagnetism, is explicable in terms of the spin canting mechanism proposed in the Yafet–Kittel model.     

Decomposition of (Ti2xFe1−xSb1−x)O4 solid solutions below 1673 K

The pseudo-binary TiO₂-FeSbO₄ system was investigated by means of thermogravimetric analysis below 1673 K in O₂. Rutile-type solid solutions were synthesised at 1373 K in O₂ by means of a solid state reaction between the two pure end members TiO₂ (rutile) and FeSbO₄ mixed in stoichiometric amounts. Thermal stability of the (Ti_2xFe_1−xSb_1−x)O₄ solid solution increases with rutile content; equimolar (Ti_1.00Fe_0.50Sb_0.50)O₄ solid solutions decompose at about 1673 K forming a TiO₂-enriched solid solution and FeSbO₄, that subsequently decomposes into Fe₂O₃ (hematite) and a volatile Sb oxide, probably Sb₄O₆. For compositions characterised by higher Ti content the decomposition temperature is higher than 1673 K.     

Preparation of La1−xSrxMnO3 nanoparticles by sonication-assisted coprecipitation

La_1−xSr_xMnO₃ (x=0.3) (LSM) nanoparticles were prepared by a sonication-assisted coprecipitation method. The coprecipitation reaction is carried out with ultrasound radiation. Lower sintering temperatures are required for the sonication-assisted product. Fully crystallized LSM with an average particle size 24 nm is obtained after the as-prepared mixture is annealed at 900 °C for 2 h. Magnetic properties indicate that the transition temperature from the paramagnetic to ferromagnetic state of the sample is quite sharp and occurs at 366 K for samples annealed for 2 h at 900 and 1100 °C.     

Enhanced reduction properties of mesostructured Ce0.5Zr0.5O2 solid solutions

Mesostructured Ce_0.5Zr_0.5O₂ solid solutions (M-CZ) were hydrothermally synthesized using Gemini surfactant as the template. X-ray diffraction (XRD), small-angle X-ray diffraction (SAXRD), N₂ adsorption–desorption isotherms and high-resolution transmission electronic microscopy (HRTEM) were adopted to characterize the samples. The product had a surface area of 123.5 m² g⁻¹ with maximum oxygen storage capacity (OSC) of 0.58 mol O₂/mol Ce. Oxygen anions in the M-CZ can be repeatedly released and resumed during the redox recycles. Reduction of Ce⁴⁺ to Ce³⁺ or lower valence and Zr⁴⁺ to Zr³⁺ were fulfilled with an obvious color change during the temperature programmed reduction (TPR) process, while the crystal structure of the product remained unchanged even after severe reduction. The mesostructure of the product can improve the reductive ability of Ce⁴⁺ and Zr⁴⁺ cations, which was beneficial to the enhancement of OSC.     

Synthesis and characterization of pyrochlore-type solid solutions Y2−xLaxRu2O7

New pyrochlore-type solid solutions Y_2−xLa_xRu₂O₇ have been synthesized for x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 as single-phase materials. The lattice parameters for these solid solutions are linear with x and range from 1.0140 nm (x=0.0) to 1.0257 nm (x=0.6). All the samples show semiconducting behavior with an activation energy of about 0.1 eV. They show the spin-glass-like magnetic transition at low temperatures. Its transition temperature T_G decreases from 77 K for Y₂Ru₂O₇ with increasing the La³⁺ content.     

Structural and microstructural evolution due to increasing Co substitution in Ni1−xCoxFe2O4: An X-ray diffraction study using the Rietveld method

X-ray diffraction studies employing the Rietveld analysis is reported on the influence of increasing Co substitution on the structural and microstructural evolution in AB₂O₄ type spinel ferrites: Ni_1−xCo_xFe₂O₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). The specimens were synthesized by the organic precursor method. Structure refinement reveals that the pure NiFe₂O₄ is not an exact inverse spinel and about 11% of Ni²⁺ ions occupy the tetrahedral (A) sites. Increasing Co concentrations had the effect of increasing the ratio of B[Fe3+]/A[Fe3+]${[\text{F}{\text{e}}^{3+}]}^{\text{B}}/{[\text{F}{\text{e}}^{3+}]}^{\text{A}}$ and gradual expansion of the ferrite unit cell. The microstructure refinement estimates that the particle size values are in the order of ∼nm, ranging from 31 to 61 nm, which gradually increase with increasing Co doping accompanied by almost negligible lattice micro strains (∼10⁻⁴). The corresponding particle size distribution for each specimen was obtained from the X-ray diffraction data from the basic assumption that the spherical nanoparticles follow the log-normal distribution. The size distribution for the pure NiFe₂O₄ was also estimated from transmission electron microscope and agreed well with that obtained by the diffraction data analysis.     

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.     

Studies on LiNi0.7Al0.3−xCoxO2 solid solutions as alternative cathode materials for lithium batteries

A series of phase-pure Co- and Al-substituted lithium nickel oxide solid solutions of the composition LiNi_0.7Al_0.3−xCo_xO₂ with x=0.0, 0.1, 0.15, 0.2, and 0.3, has been synthesized by adopting urea-assisted combustion (UAC) route. The structure and the physico-electrochemical features of the doped materials have been evaluated through PXRD, FTIR, SEM, CV, and charge/discharge studies. The stabilization of Ni in the +3 state and the existence of enhanced 2D-layered structure without any cation mixing have been substantiated from XRD. The results of the XRD and FTIR studies have established the complete mixing of Al and Co with Ni, especially at the various levels and the combinations of the dopants attempted in the present study. The enhanced electrochemical performance of LiNi_0.7Al_0.3−xCo_xO₂ may be attributed to the “synergetic effect” resulting from the presence of both Al³⁺ and Co³⁺ dopants in the LiNiO₂ matrix. From CV studies, it was understood that the addition of 10% Co is effective in suppressing the phase transformation during Li⁺ intercalation process that leads to better electrochemical properties. The effect and the extent of substitution of Ni with Al and Co on the structural and electrochemical performance of LiNi_0.7Al_0.3−xCo_xO₂ are discussed elaborately in this communication.     

Structure and magnetism in the oxygen-deficient perovskites Ce1−xSrxCoO3−δ (x ≥ 0.90)

We have examined the structure and phase behaviour of strontium-doped Ce_1−xSr_xCoO_3−δ and found that the perovskite form is stabilised over a relatively narrow solid solution range (x > 0.85). A combination of electron, powder X-ray and neutron diffraction has revealed tetragonal superstructures of the basic perovskite unit; (I4/mmm) 2a_p × 2a_p × 4a_p (x = 0.90) and (P4/mmm) a_p × a_p × 2a_p (x = 0.95). Magnetisation measurements show ferromagnetic behaviour under applied magnetic fields. Low temperature neutron diffraction of Ce_0.10Sr_0.90CoO_2.80 in zero field reveals a magnetic cell of dimension 2a_p × 2a_p × 4a_p with an ordered cobalt moment of 1.7 B.M. at 25 K.     

Sonochemical preparation and characterization of SrZrxTi1−xO3/CdS composites

CdS nanoparticles coating SrZr_xTi_1−xO₃ (x=0.27) microparticles were obtained by sonochemical reaction, and the as-prepared composites products were characterized by TEM, XRD, FT-IR and other techniques and the fabrication mechanism was also discussed. Moreover, the photocatalystic properties of the final products were investigated by degradation of methylene blue. By coating CdS nanoparticles, the absorption of the UV-Vis spectrum of SrZr_xTi_1−xO₃ red-shifted, but the emission fluorescence spectrum blue-shifted. Adjusting the reactant concentration can control the thickness of the CdS layer.     

Electrochemical lithium insertion in Nb8−xW9+xO47 (1≤x≤6)

Electrochemical lithium insertion have been carried out in different compositions within the solid solution Nb_8−xW_9+xO₄₇ (1≤x≤6). Through galvanostatic and potentiostatic techniques, we have detected that lithium insertion proceeds by at least two reduction steps. These processes seem to be related to the molar ratio Nb/W. The maximum lithium content reached during the discharge process leads to a specific capacity of 170 Ah/kg. Nevertheless, due to irreversible structural transformations in the matrix-host, such capacity is lost after the first cycle. A detailed analysis about the insertion kinetic process will be discussed in this work.     

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.     

Effects of La substitution for Nd in (Nd2−xLax)BaZnO5 solid solutions on microwave dielectric properties and crystal structure

The synthesis and microwave dielectric characterization of (Nd_2−xLa_x)BaZnO₅ solid solutions were studied. The samples obtained were single phase over the whole composition range and the lattice parameters of solid solutions were linearly increased with increasing the composition. Moreover, the expansion of the volume in the RO₈ (R=Nd and La) and BaO₁₀ polyhedra and ZnO₄ tetrahedron were recognized when Nd ions were substituted by La. These variations in the lattice parameters and volumes of the polyhedra are basically attributed to the differences in ionic radii between Nd³⁺ and La³⁺ ions. From the evaluation of microwave dielectric properties, it was shown that the appropriate τ_f values ranging from 4.6 to −5.0 ppm/°C are obtained by La substitution for Nd ion, and the highest Q · f value was 17,832 GHz at x=2.     

Compositionally controlled rhombohedral to cubic phase transition in CsTe2−xWxO6 related to pyrochlore structure

CsTe₂O₆ adopts a rhombohedrally distorted pyrochlore related structure due to the 1:3 ordering of Te⁴⁺ and Te⁶⁺ in the octahedral sites respectively. Phases of the type CsTe_2−xW_xO₆ were found to have the cubic pyrochlore structure from x = 0.2 to 0.5. These phases all contain Te⁴⁺ and Te⁶⁺ (mixed with W⁶⁺) and are disordered in octahedral sites of the pyrochlore structure. This mixed valence situation results in strong optical absorption in the visible region of the spectrum but does not produce a measurable electrical conductivity.     

Hydrothermal synthesis of nanocrystalline MxZn1−xFe2O4 (M=Ni, Mn, Co; x=0.40-0.60) powders

A series of nanocrystalline M_xZn_1−xFe₂O₄ (M=Ni, Mn and Co; x=0.40-0.60) powders have been successfully prepared via hydrothermal process and characterized by XRD, TEM and IR techniques. The effects of reaction temperature and the initial pH value of the starting suspension solution on the particulate properties such as the particle size and morphology are discussed. IR spectra indicate that there are no hydroxyl in as-prepared Ni_xZn_1−xFe₂O₄ and Co_xZn_1−xFe₂O₄ powders, while there are obvious hydroxyl adsorption on the IR spectrum of Mn_xZn_1−xFe₂O₄ powder.     

Dielectric properties of Ba(Ti1 − xZrx)O3 solid solutions

This paper reports the structural and dielectric properties of Ba(Ti_1 − xZr_x)O₃ (x = 0-0.3) ceramics. Single-phase solid solutions of the samples were determined by X-ray diffraction. Microscopic observation by scanning electron microscope revealed dense, single-phase microstructure with large grains (20-60 μm). The evolution of dielectric behavior from a sharp ferroelectric peak (for x ≤ 0.08) to a round dielectric peak (for 0.15 ≤ x ≤ 0.25) with pinched phase transitions and successively to a ferroelectric relaxor (for x = 0.3) was observed with increasing Zr concentration. Compared with pure BaTiO₃, broaden dielectric peaks with high dielectric constant of 25,000-40,000 and reasonably low loss (tanδ: 0.01-0.06) in the Ba(Ti_1 − xZr_x)O₃ ceramics have been observed, indicating great application potential as a dielectric material.     

Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi₂O_4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K₂S₂O₈/Na₂S₂O₈. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O₂ losses prior to finally transforming into γ-Bi₂O₃. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi₂O_4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi₂O₃. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi₂O_{3.65 ± 0.10}.     

The ionic conductivity and dielectric properties of Ba1−xSnxF2 solid solutions prepared by mechanochemical milling

Solid solutions of Ba_1−xSn_xF₂ fluoride ion conductors, with x = 0.1–0.4, have been synthesized by the mechanochemical milling technique for the first time. All of the prepared materials crystallize in the cubic fluorite-type structure, which indicates that the solid solution can be synthesized in the studied composition range by the mechanochemical milling technique at ambient temperature and pressure. The ionic conduction of the investigated materials has been studied by impedance spectroscopy. The ionic conductivity increased considerably, by up to six orders of magnitude compared to pure un-milled BaF₂, with increasing SnF₂ content. From the analysis of the conductivity spectra of the investigated materials it is found that the concentration of mobile fluoride ions is independent of temperature with almost the same values for the investigated materials. The present results suggest that the enhanced mobility of mobile ions is the origin of the higher ionic conductivity. The dielectric properties and the associated relaxation phenomena of the current materials are also described.     

Polymorphism of Bi1−xLnxO1.5 phases (0<x<0.40): Characterization of a new compound Bi4Ln2O9 (x=0.33; Ln=La, Pr, Nd)

The Bi_1−xLn_xO_1.5 solid solutions (Ln=La, Pr, Nd), of the β₂/β₁/ε (Bi-Sr-O) structural type, have been investigated in their Ln-rich domains. For Ln=La, Pr, and Nd, the upper limits are 0.35, 0.35 and 0.33, respectively. The Bi₄Ln₂O₉ ε phase (x=0.33) appears to be the single definite compound. For Bi₄La₂O₉, Bi₄Pr₂O₉ and Bi₄Nd₂O₉, the ε-type cells are respectively: a=9.484(4) Å, b=3.982(2) Å, c=7.030(3) Å, β=104.75(3)°; a=9.470(5) Å, b=3.945(2) Å, c=6.968(4) Å, β=104.73(3)° and a=9.439(3) Å, b=3.944(2) Å, c=6.923(2) Å, β=105.03(3)°. Upon heating, each monoclinic (ε) compound transforms successively into rhombohedral phases (β₂/β₁) and finally into a cubic fluorite-type phase. For La- and Pr-based compounds, all transitions are reversible; for Nd, depending on the thermal treatment, the reversibility of ε→β₂ can be incomplete. These transformations are characterized using X-ray thermodiffractometry, differential thermal analysis, dilatometry and impedance spectroscopy versus temperature. Examination of Bi₄(Ln, Ln′)₂O₉ samples allows to correlate the evolution of the thermal behavior and of the unit cell parameters, to the lanthanide size. A partial plot of the (Bi₂O₃)_1−x-(La₂O₃)_x phase diagram (0≤x≤0.40) is proposed.