Preferential intercalation of isomeric but-2-enedioate anions into the layered double hydroxides

Intercalation of cis-but-2-enedioate anion or trans-but-2-enedioate anion into the layered double hydroxide (LDH), [Mg_0.66Al_0.34(OH)₂]Cl_0.34·0.43H₂O was carried out by the method of ion-exchange procedures. Selective reaction was observed in competitive experiments involving an equal concentration pairs of acids. The trans-but-2-enedioate anion is preferentially intercalated into the Mg-Al-LDH. The obtained intercalation compounds were characterized by X-ray diffraction, infrared and thermogravimetry techniques. The charge density on the oxygens of each of the carboxylate groups for both anions was investigated utilizing ab initio (HF/6-31G) method by G98w. From the X-ray diffraction data, the guest size and the charge density of the oxygen of the guest, the orientation of both anions between the layers was determined and the preferential intercalation mechanism was studied. These results indicate the possibility of a molecular recognition ability of LDHs.     

Fluorination of perovskite-related phases of composition La1−xSrxFe1−yCoyO3−δ

We report here on the fluorination of the perovskite-related phases La_1−xSr_xFe_1−yCo_yO_3−δ. The introduction of fluorine in place of oxygen is achieved through a low-temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction data show that in all cases the fluorination leads to an expansion in the unit cell, which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron and/or cobalt. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The Mössbauer spectra show complex magnetically split hyperfine patterns for the fluorinated samples, reflecting the interactions between Fe³⁺ ions, which are not possible in oxides containing Fe⁴⁺.     

Effect of Bi-doping and Mg-excess on the thermoelectric properties of Mg2Si materials

In this work, Bi-doped magnesium silicide compounds were prepared by applying a combination of both, short-time ball milling and heating treatment. The effect of Mg excess was also studied, aiming towards further improvement in thermoelectric properties. The structural modifications of all materials were followed by Powder X-ray diffraction and Scanning Electron Microscopy. Highly dense pellets of Mg₂Si_1−xBi_x (0≤x≤0.035) and Mg_2+δSi_0.975Bi_0.025 (δ=0.04, 0.06 and 0.12) were fabricated via hot pressing and studied in terms of Seebeck coefficient, electrical and thermal conductivities and free carrier concentration. Their thermoelectric performance, at high temperature range, is presented and the maximum value of the dimensionless-figure-of-merit (ZT) is found to be 0.68 at 810 K, for Mg₂Si_0.97Bi_0.03.     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.     

Effect of aluminium doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xAl_xO_2−δ compounds were prepared by the chemical precipitation method using cerium chloride and aluminium chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of aluminium doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Al-doped CeO₂ samples were found to decrease with Al concentration and it increases from 6 to 20 nm as annealing temperature increases to 900 °C.     

Effect of iron doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xFe_xO_2−δ compounds were prepared by chemical precipitation method using cerium chloride and iron chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of iron doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Fe-doped CeO₂ samples were found to decrease with iron concentration and it increases from 9 to 26 nm as annealing temperature increases to 900 °C.     

Fluorination of perovskite-related SrFeO3−δ

We report here on the fluorination of the perovskite-related phase SrFeO_3−δ through a low temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction shows that fluorination leads to an expansion in the unit cell which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron giving SrFeO₂F. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The latter show complex magnetically split hyperfine patterns for the fluorinated sample, reflecting the interactions between Fe³⁺ ions which are not possible in oxides containing Fe⁴⁺.     

Synthesis, structural and physical properties of new Terbium containing Tb-Hg-Sr-Ca-Cu-O superconducting system

Samples with various nominal compositions in the Tb-Hg-Sr-Ca-Cu-O system were prepared and studied by EDX, powder X-ray diffraction including the Rietveld refinement, electrical resistivity, magnetic susceptibility and thermoelectric power measurements. EDX and powder X-ray diffraction studies showed that Tb is required for the stabilization of the 1212, (Hg_1−yTb_y)Sr₂TbCu₂O_6+δ; y≈0.5 phase. Electrical resistivity and magnetic susceptibility measurements indicated that substitution of Tb by Ca is necessary to induce superconductivity in the 1212, (Hg_0.5Tb_0.5)Sr₂(Tb_1−xCa_x)Cu₂O_6+δ samples. The Rietveld refinements of the X-ray data of two samples with x=0.0 and 0.5 were carried out on the basis of tetragonal symmetry (space group P4/mmm) and the results indicated that the phase with x=0.5 has less puckered Cu-O planes than the Ca-free (Hg_0.5Tb_0.5)Sr₂TbCu₂O_6+δ phase. Syperconductivity is observed only for samples with x>0.2 and T_c increases with increasing Ca content, x. The results of thermoelectric power measurements suggest that the samples with x<0.8 are located in the underdoped region and the x=0.8 sample is optimally doped and exhibits the highest T_c of 88 K.     

Structural deformations in metastable cubic compounds Ni(1 − x)Zn x O (0.60 ≤ x ≤ 0.99)

X-ray diffraction has been used for studying the fine crystal structure of metastable cubic oxide compounds Ni_{(1 ? x)}Zn _x O (0.60 ≤ x ≤ 0.99) obtained from the initial hexagonal phase by quenching of the samples at a high temperature and under external hydrostatic pressure. It has been found that the diffraction patterns of these compounds include a system of diffuse superstructure maxima, whose number and intensity essentially depend on the composition. The origin of this superstructure has been discussed.     

Observation of unfilled electron states in alkali graphite intercalation compounds by secondary electron spectroscopy

The energy position of distinct σ-electron energy bands above the Fermi level has been measured in pure graphite, in a variety of stage 1 alkali intercalation compounds and in several stages of C_xK. Changes of the σ-band gap between occupied and unoccupied states near the Λ-point by a nonuniform shift of the valence- and conduction-bands are small for the heavy alkali graphite intercalation compounds, whereas a change of 1 eV is observed for C₆Li.     

Dielectric,optical and structural properties of Bi4V2−xSrxO11−δ (0.05≤x≤0.20)

Composition Bi₄V_2−xSr_xO_11−δ (0.05≤x≤0.20) is synthesized by melt quench technique followed by heat treatment at 800 °C for 12 h. These compounds are characterised by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, impedance spectroscopy and scanning electron microscopy. X-ray diffraction patterns of all the samples show γ-phase stabilization at room temperature except x=0.05 heat treated sample. The optical band gap of all the samples is observed in semiconducting range. The lowest and the highest optical band gap is 2.39 eV and 2.57 eV for x=0.10 heat treated and x=0.20 quenched samples, respectively. The highest value of dielectric constant is obtained ~10⁷ with very low dielectric loss for x=0.15 and 0.20 samples at ~350 °C and below 10 Hz. The grain size increases with dopant concentration leads to increase the dielectric constant.     

X-ray study of the order—disorder transition in high-stage alkali metal intercalated graphite single crystals

Graphite intercalation compounds display a variety of structural properties because of their composite nature (graphite + intercalate) and their layered arrangement. Alkali metal intercalated graphite compounds undergo order-disorder phase transitions when the temperature is varied in the range 300–10 K. The disordered state shows true two-dimensional character, whereas three-dimensional coupling takes place on ordering. Results of single-crystal X-ray diffractometric and photographic studies of stage-2 KC₂₄ single crystals are presented. The positional and orientational correlations of the modulated liquid phase have been studied from 300 K down to the temperature transition T_u = 123.5° K. At the transition, the hexagonal incommensurate solid structure of the alkali metal is modulated by the graphite potential. This transition is discussed in terms of the relaxed-close packed structure model (Dicenzo, 1982). At low temperature a second transition takes place at T_L ≈ 95 K. It is found to correspond to the breaking of the 2D hexagonal symmetry of the K layer.     

Microwave solid synthesis and electrical properties of Aurivillius-type oxide-ion conductor

A new oxide-ion conductor of Aurivillius-type structure, namely BISRVOX (Bi₂Sr_xV_1−xO_{5.5-(3x/2)-δ}, 0≤x≤0.20), was successively synthesized by the microwave-assisted solid state reaction. 25 min of microwave irradiation was found to be quite sufficient to ensure the completion of reaction. Powder X-ray diffraction and differential thermal analysis showed better structural properties for the microwave-prepared samples compared to those obtained from the conventional solid synthesis route. Interestingly, the highly conducting γ-phase was effectively stabilized for x≥0.10. AC impedance spectroscopy evidenced the superiority of the microwave heating over conventional solid synthesis routes in exhibiting high oxide-ion performance.     

The role of oxygen vacancies in magnetism of CoxTi1−xO2−δ films on Si(001) prepared by sol-gel method

Co_xTi_1−xO_2−δ films have been prepared on Si(001) substrates by sol-gel method. When heat treated in air, Co_xTi_1−xO_2−δ films are non-ferromagnetic at room temperature. However, after further vacuum annealing or hydrogenation, Co_xTi_1−xO_2−δ films show room-temperature ferromagnetism (RTFM). When the vacuum annealed Co_xTi_1−xO_2−δ films are reheated in air, the magnetic moments of the films strongly reduce. After these films are vacuum annealed once again, the magnetic moments are greatly enhanced, confirming the role of vacuum annealing in ferromagnetism of Co_xTi_1−xO_2−δ films. The x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and measurements of magnetization (M) vs temperature (T) fail to detect Co clusters in the vacuum annealed and the hydrogenated Co_xTi_1−xO_2−δ films. Oxygen vacancies are formed in Co_xTi_1−xO_2−δ films after vacuum annealing and hydrogenation, determined by XRD and XPS measurements. These results indicate that oxygen vacancies created by vacuum annealing and hydrogenation play an important role in the generation of RTFM in Co_xTi_1−xO_2−δ films.     

X-ray photoelectron spectroscopy characterization of the layered intercalated compound K2xMn1 − xPS3

Polycrystalline powders of the layered MnPS₃ compound have been intercalated with K⁺ ions by ion-exchange to yield the K_2xMn_1 − xPS₃ intercalate. X-ray photoelectron spectroscopy has been applied to learn about the electronic structure of this compound. In particular, we have studied the XPS spectra of the Mn 2p and 3p, P and S 2p, K 2p and 3p core levels and of the valence band region. The binding energies for various core levels of the elements present in this compound and their observed chemical shifts are analyzed. The data give evidence for the lack of non-equivalent atoms of K, Mn, P and S. Shake-up satellites are present at the Mn 2p and 3p core levels. The occurrence of such lines allows us to hypothesize that K_2xMn_1 − xPS₃ is a large-gap insulating Mn compound. Confirmation that only an ion transfer accompanies the intercalation process is given from both the strong observed similarity with the corresponding XPS spectra in MnPS₃ and the observed binding energy positions of the K 2p and 3p levels. As regards the valence band XPS spectrum, the observed analogies with the corresponding XPS spectra of the pure compound and of other K compounds have allowed us to single out two regions and their probable contributors.     

Structural, sorption and thermodynamic correlations from a layered barium phenylarsonate/n-alkyldiamine system

The barium phenylarsonate compound, Ba(HO₃AsC₆H₅)₂·2H₂O, has the ability to intercalate n-alkyldiamine molecules, H₂N(CH₂)_nNH₂ (n=2-5). The amount intercalated (n_f) from a batchwise procedure and the variation of the original basal distance (d) of 1245 ppm determined through X-ray diffractions, gave linear correlations as a function of the number of carbon atoms in the aliphatic chain (n_c): n_f=(2.66±0.06)−(0.13±0.02)n_c and d=(2168±65)+(114±14)n_c. The intercalation process was calorimetrically followed to give exothermic enthalpy and negative Gibbs energy, reflecting spontaneous intercalation reactions at the solid/liquid interface. The displacement of solvent molecules bonded to amine and of those on the matrix during the intercalation increases the disorder to result in positive entropy, giving a favorable set of thermodynamic data for this system.     

Structural, electrical and magnetic characterizations of Ni-Cu-Zn ferrite synthesized by citrate precursor method

Fine powders of NiCuZn ferrite with composition Ni_(0.7−x)Cu_xZn_0.3Fe₂O₄ (where x=0, 0.2, 0.4 and 0.6) were prepared by the citrate precursor method. X-ray diffraction measurements confirm the formation of single-phase cubic spinel structure. The grain size was estimated by SEM micrograph which increases with Cu content. Dielectric constant (?) and loss tangent (tan δ) were measured as a function of frequency. The ? and tan δ show a decreasing trend with increase of frequency for all the samples. The DC resistivity was measured as a function of temperature. The temperature-dependent DC resistivity measurements show that the room-temperature DC resistivity of NiCuZn ferrite with x=0.2 is of the order of 10⁹ Ω cm. The AC conductivity (σ_AC) was studied as a function of frequency. The hysteresis data indicate that the maximum saturation magnetization of 38.66 emu/g is obtained for the composition with x=0.2.     

Electrical conductivity and chemical bond of graphite intercalation compound with fluorine and metal fluoride

Graphite intercalation compounds with fluorine and metal fluoride (MgF₂ or CuF₂) were prepared from petroleum coke and pyrolytic graphite. With progress in the intercalation reaction, the first stage compound with identity period 9.4 Å changed to another structure of identity period 10.7 Å. It was found from ESCA measurements that the chemical interaction between intercalated fluorine and carbon was similar to the covalent bond around the surface and slightly ionic in the bulk. The maximum electrical conductivities in the direction of the ab-axis were (1.9–2.0) × 10⁵ (ωcm)^-1, which were 10–13 times that of the original pyrolytic graphite.     

Structure,superconductivity, characterization and Fe Mössbauer studies on high temperature superconductor Y1Ba2(Cu3−x Fe x )O7−δ

Different samples of Y₁Ba₂(Cu_3?x Fe _x )O_7?δ compounds have been synthesized, the resultant bulk superconductors were found to show diamagnetic behaviour and they have been investigated from the point of view of superconductivity through the study of X-ray diffraction, A.C. magnetic susceptibility, electrical conductivity and Mössbauer effect.     

Absorption and emission properties of Nd in lithium cesium mixed alkali borate glasses

Lithium cesium mixed alkali borate glasses of the composition 67B₂O₃·xLi₂O·(32−x)Cs₂O (where x=8, 12, 16, 20 and 24) containing 1 mol% Nd₂O₃ were prepared by melt quenching. The absorption spectra of Nd³⁺ were studied from the experimental oscillator strengths and the Judd-Ofelt intensity parameters were obtained. The intensity parameters are used to determine the radiative decay rates (emission probabilities of transitions) (A_T), branching ratios (β) and integrated absorption cross-sections (Σ) of the Nd³⁺ transitions from the excited state J manifolds to the lower lying J′ manifolds. Radiative lifetimes (τ_R) are estimated for certain excited states of Nd³⁺ in these mixed alkali borate glasses. Luminescence spectra were measured and the emission cross-sections (σ_p) were evaluated for the three emission transitions. The variation of luminescence intensity with x was recorded for the three transitions at different excitation power to see the effect of mixed alkalies in these borate glasses.