Phase Transitions in the BaCe1–x Nd x O3–δ System (x = 0–0.15)

Phase transitions in BaCe_{1 – x} Nd _x O_3– (x = 0–0.15) are studied on ceramic samples by dilatometry (at 370–1100 K) and by measuring electroconductivity (at 750–1220 K). Ion transport numbers are measured by an EMF method at 850–1240 K. All measurements are done in dry air (₂ 40 Pa). By treating the obtained temperature dependences of linear expansion with a difference method (difference between first-degree polynomial approximating the dependence and experimental points), the position and the sort of the phase transitions are determined. High-temperature phase transitions are confirmed by measuring the electroconductivity and ion transport numbers. Second-order phase transitions in pure BaCeO₃ are discovered at 480 ± 10, 530 ± 10, 900 ± 10, 1030 ± 20, and 1170 ± 10 K and a first-order transition, at 665 ±10 K. Phase transitions at 900 and 1030 K are discovered for the first time ever.     

Hydrogen content in proton-conducting perovskites CaZr1 − x Sc x O3 − x /2 (x = 0.0–0.2)

The hydrogen content in CaZr_{1 ? x} Sc _x O_{3 ? x/2} (x = 0.00–0.20) and BaZr_0.9Y_0.1O_3-α (for comparison) was studied by powder nuclear microanalysis. The samples were saturated with heavy water (D₂O) vapors at 350 and 400°C in air. The chemical expansion of the CaZr_0.95Sc_0.05O_3-α and BaZr_0.95Y_0.05O_3-α samples at 700°C was measured at different water vapor pressures. A model was suggested to explain the lowered hydrogen content in oxides based on CaZrO₃.     

Synthesis and crystal structure of Mo6−x Nb x I11 (x = 1–1.5)

A solid solution Mo_{6 ? x} Nb _x I₁₁ (x = 1.1–1.5) containing cluster cores {Mo₅NbI₈} is obtained by the high-temperature reaction of molybdenum, niobium, and iodine (550°C, 70 h, quartz ampule). According to the X-ray diffraction data, heating at 800°C in a molybdenum container results in the decomposition of the solution to Mo₆I₁₂ and Nb₆I₁₁. According to the X-ray structure analysis data, the compounds are isostructural to the high-spin modification Nb₆I₁₁ (space group Pccn). The presence of Nb atoms in the structure changes the structural type from the layered (Mo₆I₁₂) to framework structure, noticeably increases the metalmetal distances (2.661–2.716 Å, 2.695 Å) Mo₆ octahedron with the retention of the distance from the metal (M) to the μ₃-“capped” I atoms, and strongly elongates the M₆-I-M₆ bridges almost to the value observed in Nb₆I₁₁.     

Determination of the stability constants of Pb–(DIPSO)x–(OH)y and Pb–(AMPSO)x–(OH)y systems

In this work, complexation between lead ion and the ligands 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) and N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid (AMPSO), which are commercial pH buffers, is presented. Both ligands form complexes with lead in their pH buffer range (between pH 6.5 and 8.5 for DIPSO and between pH 8.0 and 9.0 for AMPSO). The final models and the overall stability constants, which are reported here, were determined by direct current polarography and glass electrode potentiometry [only for the Pb–(DIPSO)_x–(OH)_y system] at 25.0 °C and 0.1 M KNO₃ ionic strength. For the Pb–(DIPSO)_x–(OH)_y system, the proposed final model contains PbL, PbL₂, PbL₂(OH), and PbL₂(OH)₂ with stability constants, as log β, of 3.4 ± 0.1, 6.35 ± 0.15, 12.8 ± 0.2, and 18.0 ± 0.3, respectively. For the Pb–(AMPSO)_x–(OH)_y system, the species observed are PbL, PbL(OH), and PbL(OH)₂ with stability constants, as log β, of 2.9 ± 0.5, 9.4 ± 0.1, and 14.5 ± 0.2, respectively. For AMPSO, the possible adsorption of the ligand at the mercury electrode surface was evaluated by alternating current polarography through calculation of the capacitance of the double layer.     

Study of Nd2–x Ce x CuO4?±?δ (x?=?0.1–0.25) as cathode material for intermediate-temperature solid oxide fuel cells

The solid solubility limit of Ce in Nd_2–x Ce _x CuO_4 ± δ , prepared by sol–gel process, is established up to x = 0.2. The transition from negative temperature coefficient to positive temperature coefficient, within the solid solubility region, is observed at 620 °C. The area-specific-resistance (ASR) is optimized for electrochemical cell sintered at 800 °C. ASR enhances with increase in sintering temperature of cell. ASR value of 0.93 ohm cm² at 700 °C, determined by electrochemical impedance spectroscopy is comparable against that by voltage versus current (V–I) characteristics at 0.98 ohm cm² at the same temperature. Electrochemical performance and ASR of Nd_1.8Ce_0.2CuO_4 ± δ is improved when prepared by sol–gel route over solid-state reaction, which is attributed to uniform size and shape of nanocrystalline grains.     

Characteristics of nanosized LiNi x Fe1−x PO4/C (x = 0.00–0.20) composite material prepared via sol–gel-assisted carbothermal reduction method

Pure LiFePO₄ and LiNi _x Fe_1?x PO₄/C (x?=?0.00–0.20) nanocomposite cathode materials have been synthesized by cheap and convenient sol–gel-assisted carbothermal reduction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy, and inductively coupled plasma have been used to study the phase, morphology, and chemical composition of un-doped and Ni-doped materials. XRD patterns display the slight shrinkage in crystal lattice of LiFePO₄ after Ni²⁺ doping. The SEM images have revealed that Ni-doped particles are not agglomerated and the particle sizes are practically homogeneously distributed. The particle size is found between 50 and 100 nm for LiNi_0.20Fe_0.80PO₄/C sample. The discharge capacity at 0.2 C rate has increased up to 155 mAh g^?1 for the LiNi_0.05Fe_0.95PO₄/C sample and good capacity retention of 99.1 % over 100 cycles, while that of the unsubstituted LiFePO₄/C and pure LiFePO₄ has showed only 122 and 89 mAh g^?1, respectively. Doping with Ni has a noticeable effect on improving its electrical conductivity. However, serious electrochemical declension will occur when its doping density is beyond 0.05 mol LiNi_0.20Fe_0.80PO₄/C electrode shows only 118 mAh g^?1, which is less than un-doped LiFePO₄/C sample at 0.2 C. The cycling voltammogram demonstrates that Ni-doped LiNi_0.05Fe_0.95PO₄/C electrode has more stable lattice structure, enhanced conductivity, and diffusion coefficient of Li⁺ ions, in which Ni²⁺ is regarded to act as a column in crystal lattice structure to prevent the collapse during cycling process.     

Sol–gel synthesized high anisotropy magnetic nanoparticles of NiCr x Fe2−x O4

The nanoparticles of NiCr _x Fe_2?x O₄ were synthesized through sol–gel reactions involving nitrates of Ni, Cr and Fe in an aqueous medium containing citric acid. The cubic spinel structure in single phase with nanometric crystallite size of ~5 nm, the spherical morphology and magnetic relaxations were examined through XRD, TEM and Mössbauer techniques. The abnormal occurrence of finite remanance (M _r ) and coercivity (H _c ) resulted in the room temperature dc magnetization measurements for the small particles authenticate the ferrimagnetic regime, as proposed by the room temperature Mössbauer results of the samples, with a proximate superparamagnetic regime still at lower particle volumes. This could be attributed to the antiferromagnetic spin interactions of chromium ions at octahedral sites and subsequently the over-occupancy of the rest of the cations at tetrahedral sites. In justification to this, the magnetocrystalline anisotropy constant, K, is estimated to have value relatively high of the order of 10⁷ erg/cm³ at room temperature for all studied concentrations.     

Formation of (C x H2x+1)4NBr·nH2O (x = 1–3) hydrate

The phase diagram of the binary system tetramethylammonium bromide-water was studied by the differential thermal analysis. In the stable region two phases, ice and the salt itself, were detected, and in the metastable region, three tetramethylammonium bromide hydrates (bromide-water, 1 : 4, mp 68.8°C, 1 : 5, mp 36.0°C, 1 : 7.5, mp ?19.5°C) were found. Formation of (C _x H_2x+1)₄NBr·nH₂O (x = 1–3, n = 4, 5, 7.5) hydrates was revealed.     

High-pressure densities and P–T–x–y diagrams for carbon dioxide+linalool and carbon dioxide+limonene

Liquid and vapor densities for carbon dioxide+linalool, and carbon dioxide+limonene were measured by using a system consisting of two vibrating tube densimeters. The P–T–x–y diagrams and saturated liquid and vapor densities for these two binary mixtures were determined at 313, 323 and 333 K, respectively, as well as at pressures up to 11 MPa. The density of the saturated CO₂ phase increased with increasing pressure. At higher pressure, the density of the liquid phase decreased with increasing pressure, corresponding to an increasing amount of carbon dioxide.     

Nature of conductivity of Perovskites La1 ? x Sr x ScO3 ? α (x = 0.01–0.15) under oxidative and reducing conditions

Transport numbers of ions and protons are measured on ceramic samples of La_{1 ? x} Sr _x ScO_{3 ? ??} (x = 0.01?C0.15); partial conductivities (hole, proton, and oxygen-ion) are determined in the temperature range of 500?C900°C at pH₂O = 0.04?C2.35 kPa and pO₂ from air to 10^?15 Pa.     

Forms of oxygen in La1 − x Ca x MnO3 + δ (x = 0–1) perovskites and their reactivities in oxidation reactions

The effects of substitution in the cationic sublattice and of the synthesis procedure on the reactivity of different forms of oxygen in La_{1 ? x} Ca _x MnO_{3 + δ} perovskites synthesized by mechanochemical and ceramic processing was studied by temperature-programmed reduction (TPR) with hydrogen. As the calcium content of the perovskite is raised, the maxima of the TPR peaks shift to lower temperatures and the extent of reduction of the perovskite increase, implying an increase in the reactivity of the system. Conversely, raising the calcination temperature or extending the calcination time shifts the maxima of the peaks to higher temperatures and diminishes the extent of reduction of the sample. TPR data for the intermediate-composition samples can be explained in terms of the dependence of microstructure on the synthesis procedure (near-surface calcium segregation in the mechanochemically synthesized samples and the microheterogeneous structure of the ceramic samples). The reduction process Mn⁴⁺ → Mn²⁺ takes place in the low- and medium-temperature regions. According to the literature, the bulk reduction process Mn³⁺ → Mn²⁺ occurs at high temperatures. The activity of the system in CO oxidation is correlated with the amount of the most reactive surface oxygen, which is eliminated in hydrogen TPR runs below 250–300°C.     

Sol–gel synthesis and catalytic property of Ce1−x Ti x O2 nanocomposites supported on attapulgite clay

Ce_1?x Ti _x O₂ nanocomposites supported on attapulgite clay (Ce_1?x Ti _x O₂/ATP) were prepared by a facile sol–gel route. The textural and structural properties of the prepared products were characterized by thermogravimetric-differential scanning calormetry analysis, X-ray diffraction, transmission electron microscopy, energy-dispersive spectrometry, Fourier transform infrared spectroscopy and Nitrogen adsorption-desorption measurements. The catalytic activity of the prepared Ce_1?x Ti _x O₂/ATP catalysts for rhodamine B degradation was investigated. Results indicate that the particle size of Ce_1?x Ti _x O₂ nanoparticles evenly attached onto the surface of ATP is approximately 10 nm. The Ti⁴⁺ doping ratios exhibit considerable impact on the redox ability and catalytic activity of Ce_1?x Ti _x O₂/ATP composites. The introduction of an optimal amount of Ti⁴⁺ contributes to forming structure defects and electronic defects in the oxide lattice, increasing concentration of oxygen vacancies, consequently improving low-temperature redox ability of Ce⁴⁺ and enhancing catalytic activity of the composites. Ce_1?x Ti _x O₂/ATP (x = 0.5) catalyst has the best catalytic degradation efficiency, which can reach as high as 97 % after reaction for 240 min. It is also found that attapulgite clay exhibit a positive synergistic effect to the Ce_1?x Ti _x O₂ nanoparticles.     

Effect of post-annealing on the band gap of sol–gel prepared nano-crystalline Mg x Zn1−x O (0.0 ≤ x ≤ 0.3) thin films

Polycrystalline Mg _x Zn_1?x O (MZO) thin films on glass substrates were prepared by sol–gel method. All the films retained the hexagonal wurtzite structure of ZnO. The band gap values determined from transmission spectra were found to be smaller than the values obtained from Vegard’s law for the as-deposited MZO films. For the films with x = 0.1, 0.2 and 0.3, the band gap blue-shifted initially and then red-shifted with increase in the annealing temperature. The reason for this anomalous shift in the band gap is attributed to the proper substitution of Mg atoms into the Zn lattice sites after a certain critical annealing temperature.     

Electrical, electrochemical, and thermomechanical properties of perovskite-type (La1?x Sr x )1?y Mn0.5Ti0.5O3?δ (x?=?0.15–0.75, y?=?0–0.05)

Strontium additions in (La_1?x Sr _x )_1?y Mn_0.5Ti_0.5O_3?δ (x?=?0.15–0.75, y?=?0–0.05) having a rhombohedrally distorted perovskite structure under oxidizing conditions lead to the unit cell volume contraction, whilst the total conductivity, thermal and chemical expansion, and steady-state oxygen permeation limited by surface exchange increase with increasing x. The oxygen partial pressure dependencies of the conductivity and Seebeck coefficient studied at 973–1223?K in the p(O₂) range from 10^?19 to 0.5?atm suggest a dominant role of electron hole hopping and relatively stable Mn³⁺ and Ti⁴⁺ states. Due to low oxygen nonstoichiometry essentially constant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti⁴⁺ cations and oxygen anions, the tracer diffusion coefficients measured by the ¹⁸O/¹⁶O isotopic exchange depth profile method with time-of-flight secondary-ion mass spectrometric analysis are lower compared to lanthanum–strontium manganites. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range 9.8–15.0?×?10^?6?K^?1 at 300–1370?K and oxygen pressures from 10^?21 to 0.21?atm. The anodic overpotentials of porous La_0.5Sr_0.5Mn_0.5Ti_0.5O_3?δ electrodes with Ce_0.8Gd_0.2O_2-δ interlayers, applied onto LaGaO₃-based solid electrolyte, are lower compared to (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3?δ when no metallic current-collecting layers are introduced. However, the polarization resistance is still high, ~2 Ω?×?cm² in humidified 10?% H₂–90?% N₂ atmosphere at 1073?K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H₂S traces in H₂-containing gas mixtures did not result in detectable decomposition of the perovskite phases.     

Electronic and Structural Properties of Neutral, Anionic, and Cationic Rh x Cu4−x (x = 0–4) Small Clusters: A DFT Study

In this study, electronic structure, stability, and tendency to exchange electron of neutral, anionic, and cationic Rh _x Cu_4?x (x = 0–4) small clusters were investigated by density functional theory calculations. For neutral small clusters, it was found that the most stable structures of Rh₄, Rh₃Cu and Rh₂Cu₂ have distorted tetrahedral shape while the most stable structures of RhCu₃ and Cu₄ have quasi-planer shape. Adding charges to the clusters, caused shapes of the most stable structures undergo variations. Stabilities of the neutral, anionic, and cationic clusters decrease linearly with increasing the copper content. In addition, calculated chemical harnesses indicated that the small cluster with 75 % copper content has the least chemical hardness. Interestingly, prevailing number of electronegative (Rh) and electropositive (Cu) atoms in the anionic and cationic clusters coincides with high dipole moment in these species that occur at 25 and 75 % copper respectively.     

Comparative study on hydrothermally synthesized LiMn x Fe1–xPO4 (x = 0∼1) cathode at full-cell level

Russian Journal of Applied Chemistry - Olivine structure LiMn x Fe1–xPO4 (x = 0～1) cathode materials were successfully synthesized through hydrothermal method. All obtained samples...     

Synthesis and properties of fuel cell anodes based on (La0.5 + x Sr0.5 − x )1 − y Mn0.5Ti0.5O3 − δ (x = 0–0.25, y = 0–0.03)

Results are presented of studying electrochemical properties of perovskite-like solid solutions (La_{0.5 + x} Sr_{0.5 ? x} )_{1 ? y} Mn_0.5Ti_0.5O_{3 ? δ} (x = 0–0.25, y = 0–0.03) synthesized using the citrate technique and studied as oxide anodic materials for solid oxide fuel cells (SOFC). X-ray diffraction (XRD) analysis is used to establish that the materials are stable in a wide range of oxygen chemical potential, stable in the presence of 5 ppm H₂S in the range of intermediate temperatures, and also chemically compatible with the solid electrolyte of La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O_{3 ? δ} (LSGMC). It is shown that transition to a reducing atmosphere results in a decrease in electron conductivity that produced a significant effect on the electrochemical activity of porous electrodes. Model cells of planar SOFC on a supporting solid-electrolyte membrane (LSGMC) with anodes based on (La_0.6Sr_0.4)_0.97Mn_0.5Ti_0.5O_{3 ? δ} and (La_0.75Sr_0.25)_0.97Mn_0.5Ti_0.5O_{3 ? δ} and a cathode of Sm_0.5Sr_0.5CoO_{3 ? δ} are manufactured and tested using the voltammetry technique.     

Synthesis of sol–gel Na2Zr x Ti6−x O13 (0≤x≤1) materials and their performance in photocatalytic degradation of organic dyes

In this work we present the synthesis and characterization of four compositions of the solid solution with the general formula Na₂Zr _x Ti_6−x O₁₃ (0≤x≤1) which was prepared by a sol-gel method. The main goal of this work is to evaluate the influence of the incorporation of different amounts of zirconium into the binary phase Na₂Ti₆O₁₃ on two properties: the textural surface and E _g values. This titanate phase crystallizes in a monoclinic unit cell and is built into an octahedral TiO₆ framework forming a rectangular tunnel structure. Additionally, we have compared their photocatalytic performances in degradation of organic dyes under visible light. The heat-treated sol-gel samples were characterized by X-ray powder diffraction, N₂ physisorption, thermal analysis, UV and FT-IR spectroscopy. According to the X-ray powder diffraction results, the new ternary tunnel compound Na₂ZrTi₅O₁₃ was obtained as a single phase at 800°C. The cell parameters for the two end-member phases of that solid solution were refined to confirm that Zr ion was incorporated into the structural framework. This ternary compound had an E _g value of about 2.9 eV. The activity of all heat-treated Na₂ZrTi₅O₁₃ samples was tested in the photocatalytic degradation of methylene blue and Rhodamine B under visible light. The Na₂ZrTi₅O₁₃ calcined at 400°C showed the best performance with 95% of photodegradation of methylene blue and a half time t _1/2 of 15 min.     

Electrode performance and X-ray absorption spectroscopy of La0.8Sr0.2Mn1−x Cu x O3 (0 ≤ x ≤ 0.2)–GDC composite cathodes for SOFCs

Electrochemical properties of composite cathodes consisting of La_0.8Sr_0.2Mn_1?x Cu _x O₃ (LSMCu, 0?≤?x?≤?0.2) and Ce_0.8Gd_0.2O_2?x (GDC) were determined by impedance spectroscopy, and conduction mechanism for the composite cathodes was investigated by a near-edge X-ray absorption fine-structure analysis (NEXAFS). LSMCu–GDC cathodes showed lower polarization resistance (R _p) than LSM–GDC up to 750 °C, whereas they exhibited better performance at higher temperature (≥800 °C). The best performance was achieved with the LSMCu10–GDC cathode: 0.27 and 0.08?Ω cm² at 800 °C and 850 °C, respectively. NEXAFS and refinement results confirmed that Cu doping caused the oxidation of Mn³⁺ to Mn⁴⁺ and lattice contraction. This additional Mn⁴⁺ can lead to the formation of oxygen vacancies when Mn⁴⁺ is converted to Mn³⁺ at relatively high temperatures (above 600 °C). This in turn contributes to improved oxygen ion transport in LSM. The LSMCu–GDC composite cathode can thus be considered a suitable potential cathode for SOFC applications.