Crystallization and electrochemical performance of La0.6Sr0.4Co0.2Fe0.8O3 − δ-Ce0.8Gd0.2O1.9 thin film cathodes processed by single solution spray pyrolysis

La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ-Ce_0.8Gd_0.2O_1.9 (LSCF-CGO) thin films obtained by spray pyrolysis of a single precursor solution were investigated by XRD, TEM and impedance spectroscopy at annealing temperatures ranging from 500 to 900 °C. Films annealed at 600 °C contained a mixture of amorphous regions and crystalline regions composed of fine crystallites (< 5 nm). Annealing above 600 °C increased the ratio of crystalline to amorphous material, led to the segregation of the films into distinct LSCF and CGO phases, and promoted grain growth. The electrical behavior of the films depended on annealing temperature. At testing temperatures of 400 °C and below, the polarization resistance of films with lower annealing temperatures was larger than the polarization resistance of films with higher annealing temperatures. However, at testing temperatures of 500 °C and above the polarization resistance of films with lower annealing temperatures was equal to or lower than the polarization resistance of films with higher annealing temperatures. This was reflected by the activation energy that decreased with increasing annealing temperature. The varying electrical behavior may be related to microstructural changes that caused bulk diffusion to be the rate-limiting step in films with lower annealing temperatures and oxygen dissociation to be the rate-limiting step in films with higher annealing temperatures.     

Neutron Spectroscopy of the Atomic Dynamics of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> at Fluorite–Pyrochlore Structural Transformations

The formation of the structural and dynamic properties of La₂Zr₂O₇ in the process of crystallization at the isothermal annealing of initially amorphous precursors obtained by the coprecipitation of corresponding salts has been studied by neutron spectroscopy. The existence of vibrational states characteristic of hydrogen, which is in one or another of the possible chemical states and is incorporated into a solid matrix, has been detected in the spectra of amorphous and fluorite phases. The DFT calculation of the phonon density of states has been performed to analyze the energy structure of experimental phonon spectra for various phases of the La₂Zr₂O₇ compound. The amount of hydrogen in the fluorite phase has been estimated.     

Anomalous structural states of rare-earth oxides in solid-phase synthesis during continuous heating

The kinetics of solid-phase synthesis of complex rare-earth (RE) oxides from an amorphous precursor state during continuous heating is studied by X-ray diffraction. Using the synthesis of RE borates (ReBO₃) as an example, it is shown that a continuous heating of such precursors stimulates the formation of new phases which are unknown in an equilibrium state of one rare-earth element but known for other rare-earth borates. It is established that the regime of continuous heating during solid-phase synthesis of garnets (Re ₃ M ₅O₁₂) exhibiting the only crystal phase state for all rare-earth atoms results in an accelerated growth of crystallites and that the higher the heating rate the higher the growth rate.     

Mössbauer and magnetic studies of the phase state of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/La<Subscript>0.9</Subscript>Ca<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> composites

The formation of an intermediate phase in SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composites was demonstrated for the first time using only Mössbauer spectroscopy. The SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composite was prepared by the two-stage (sol–gel and hydrothermal) synthesis with varying initial conditions. The X-ray diffraction studies showed that the composite consisted of two phases: well-formed structures of manganite La_0.9Ca_0.1MnO₃ and hexagonal ferrite SrFe₁₂O₁₉. It was found that nanocrystalline La_0.9Ca_0.1MnO₃ particles with size d ? 150 nm formed in the composites at the surface of plate-like SrFe₁₂O₁₉ crystallites. The Mössbauer studies showed that the composite contained additional (intermediate) phase La_0.9Ca_0.1Mn(Fe)O₃ that formed at the interface between SrFe12O19 and La_0.9Ca_0.1MnO₃ phases. The intermediate phase concentration increased with the molar content of La_0.9Ca_0.1MnO₃; in this case, the fraction of the surface of SrFe₁₂O₁₉ crystallites coated with La_0.9Ca_0.1MnO₃ increased, which led to the increase in the total area of the interface surface and the intermediate phase concentration.     

Effects of dilute columnar defects on the vortex matter in Bi2Sr2CaCu2O8+δ near the disorder-driven phase transition

Magneto-optical measurements are employed to characterize the disorder-driven vortex phase transition and the metastable disordered vortex state in heavy-ion irradiated and pristine regions of the same Bi₂Sr₂CaCu₂O_8+δ crystal. We find that dilute columnar defects, while hardly affect the disorder-driven phase transition line, significantly increase the lifetime of the metastable state created in the vicinity of this line. Study of flux injection from the pristine region into the irradiated region suggests that in presence of columnar defects a metastable disordered state may be created in the bulk, in addition to being injected from the edges.     

Europium-doped nanocrystalline Y2O3−La2O3 solid solutions with bixbyite structure

Nanocrystals of Y₂O₃ and La₂O₃ solid solutions were synthesized with a cubic bixbyite structure containing La₂O₃ content of up to 50 mol%. This is comparatively higher than that in bulk materials of the same structure, where La₂O₃ content of only 20 mol% can be obtained. A set of europium-doped (Y_1−xLa_x)₂O₃ (x=0.1, 0.2, 0.3, 0.4) cubic bixbyite solid solutions with crystallites of approximately 10 nm in size was prepared using the polymer complex solution method. Structural analysis was performed using X-ray diffraction measurements, Rietveld full profile refinement, and from Eu³⁺ luminescence emission. The energy levels of the Eu³⁺ ion, second order crystal field parameters, and crystal field strength were obtained for all compositions of solid solutions. We show that the crystal field parameters linearly depend on unit cell parameter and that these dependencies may be considered as part of an overall dependence for the entire sesquioxide family.     

Combined strain and high-pressure oxygen treatment effects and phase separation in La0.67Ca0.33MnO3 thin films

Evidence for phase separation in high-pressure oxygen-treated La_0.67Ca_0.33MnO₃/(100)MgO thin films is presented. This phase-separated state is not observed for the x=0.33 classical doping level either for bulk material or for as-prepared thin films. It originates from the combined influence of two factors: large strains due to the film-substrate lattice mismatch and high-pressure (100 bar) annealing in oxygen. These factors weaken double exchange, drastically enhance low-field magnetoresistance, and lead to hysteresis phenomena. The experimental results are interpreted in terms of phase separation of the film volume into the ferromagnetic conductive matrix with inclusions of spin-glass-like clusters.     

Structural and optical properties of La and Gd substituted Bi4 − x M x V2O11 − δ (0.1 ≤ x ≤ 0.3)

Bi_4???x M _x V₂O_11???δ (M?=?La, Gd; 0.1?≤?x?≤?0.3) is synthesised by a solid state reaction method to study the effect of La³⁺ and Gd³⁺ substitution for Bi on the structural and optical properties. The as-prepared samples are characterised by X-ray diffraction, Fourier transform infrared analysis, UV–visible spectroscopy, scanning electron microscopy and energy-dispersive spectroscopy. The refinement results confirmed that even substituted samples exhibit monoclinic structure with space group C2/m. The parameters like band gap energy; Urbach energy has been calculated from the UV–visible spectra. It has been observed that even substitution at the bismuth site by isovalent cations decreases the energy band gap. The lowest observed band gap is 1.86 eV for Bi_3.9La_0.1V₂O_11???δ . The grain size and defects were observed to increase with increasing substitution along with the amount of secondary phase.     

Thermodynamic stability of solid and fluid phases in the Si3B3N7 system

We investigate the thermodynamic properties of the ceramic material Si₃B₃N₇ which has so far only been synthesized as an amorphous compound. Using Monte Carlo simulations, we investigate the stability of both solid and fluid phases of Si₃B₃N₇, in order to gain insights into the proper synthetic conditions needed to generate the stable amorphous and crystalline phases of this compound. We study the ternary liquid–gas region of the phase diagram at temperatures above the theoretical glass transition in this system, and construct an approximate ‘metastable’ phase diagram of Si₃B₃N₇. In addition we study the stability of the amorphous and crystalline phases in the solid state against the decomposition into the binary phases h-BN and β-Si₃N₄ as a function of the size of the crystallites involved, and the stability of the melt against evolution of nitrogen as a function of nitrogen pressure.     

Hydrostatic pressure effect on the transformations of amorphous La3Si

Measurements of the relative resistance versus temperature at different hydrostatic pressures up to 30 kbar for amorphous La₃Si have been carried out. The results show that there is a metastable phase only at low pressures and that pressure decreases the crystallization temperature of the stable state (or final phase) from both the amorphous and meta-stable phases. If the pressure is sufficiently high, the transformation from the amorphous to a stable state might occur at room temperature.     

Anomalous pressure-induced phase transformation in nano-crystalline erbium sesquioxide (Er2O3): partial amorphization under compression

Nanophase materials have novel physical and chemical properties, differing from bulk materials. It is of exceptional interest to investigate the size effect on structural stability in nanocrystals. Here, we investigated pressure-induced phase transitions in nanosized Er₂O₃ using angle-dispersive synchrotron X-ray diffraction up to 40.6 GPa. Nano-Er₂O₃ has enhanced transition pressure and higher bulk modulus (K₀) than its bulk counterparts. Amorphous Er₂O₃ nanoclusters with traces of monoclinic phase are obtained upon compression. This is the first time that partial amorphous structure under compression was observed in nano-Er₂O₃, indicating a kinetic trapping of partial amorphous Er₂O₃ on pressurizing.     

Correlation of conductivity and angle integrated valence band photoemission characteristics in single crystal iron perovskites for 300 K < T < 800 K: Comparison of surface and bulk sensitive methods

A single crystal monolith of La_0.9Sr_0.1FeO₃ and thin pulsed laser deposited film of La_0.8Sr_0.2Fe_0.8Ni_0.2O₃ were subject to angle integrated valence band photoemission spectroscopy in ultra high vacuum and conductivity experiments in ambient air at temperatures from 300 K to 800 K. Except for several sputtering and annealing cycles, the specimens were not prepared in situ. Peculiar changes in the temperature dependent, bulk representative conductivity profile as a result of reversible phase transitions, and irreversible chemical changes are semi-quantitatively reflected by the intensity variation in the more surface representative valence band spectra near the Fermi energy. X-ray photoelectron diffraction images reflect the symmetry as expected from bulk iron perovskites. The correlation of spectral details in the valence band photoemission spectra (VB PES) and details of the conductivity during temperature variation suggest that valuable information on electronic structure and transport properties of complex materials may be obtained without in situ preparation.     

Elastic properties of a La<Subscript>0.5</Subscript>Pr<Subscript>0.2</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound and internal friction in the ferromagnetic La_0.5Pr_0.2Ca_0.3MnO₃ single crystal with magnetic first-order phase transition were studied. It was found that the sound velocity decreases by ≈20% in transition from the ferromagnetic to paramagnetic state. In the paramagnetic region, the extended temperature hysteresis of the sound velocity and the internal friction was observed. It was shown that La_0.5Pr_0.2Ca_0.3MnO₃ has two paramagnetic phases with different sound velocities.     

Co掺杂纳米ZnO微结构的正电子湮没研究

利用正电子湮没技术研究了10 at.% Co掺杂的Co₃O₄/ZnO纳米复合物中退火对缺陷的影响. 利用X射线衍射(XRD)测量了Co₃O₄/ZnO纳米复合物的结构和晶粒尺寸. 随着退火温度升高,Co₃O₄相逐步消失,ZnO晶粒尺寸也有显著增加. 经过1000 ℃以上退火后,Co₃O₄相完全消失,并出现了CoO的岩盐结构. 正电子湮没寿命测量显示出Co₃O₄ /ZnO纳米复合物中存在大量的Zn空位和空位团. 这些空位缺陷可能存在于纳米复合物的界面区域. 当退火温度达到700 ℃后Zn空位开始恢复,空位团也开始收缩. 900 ℃以上退火后,所有空位缺陷基本消失,正电子寿命接近ZnO完整晶格中的体态寿命值. 符合多普勒展宽谱测量也显示Co₃O₄ /ZnO纳米复合物经过900 ℃以上退火后电子动量分布与单晶ZnO基本一致,表明界面缺陷经过退火后得到消除. 关键词： ZnO 界面缺陷 正电子湮没     

Microstructure and mechanical properties of a partially crystallized La-based bulk metallic glass

The evolution of microstructure upon partial crystallization and its influence on the mechanical properties such as hardness, elastic modulus and viscosity in a La 55 Al 25 Cu 10 Ni 5 Co 5 bulk metallic glass alloy are studied. Specimens with various volume fractions of crystalline phases were obtained by annealing the as-cast amorphous alloy above its glass transition temperature and were characterized by transmission electron microscopy. Microscopic examination of the heat-treated samples shows short-range-ordered domains prior to nanocrystallization within the amorphous matrix, followed by the growth and impingement of the crystallites. Whereas the hardness of the annealed samples increases linearly with increasing crystallinity, the elastic modulus and the viscosity both increase abruptly when the crystalline volume fraction is about 40 vol.%, with a only minor variation on either side of this range. The sudden rises in the modulus and viscosity are similar to those in the literature data on the fracture strength of partially crystallized bulk amorphous alloys that shows a steep drop in strength at 30-50 vol.% crystallinity. On the basis of the microscopic observations, it is suggested that the interaction and formation of rigid networks of crystalline phases upon the attainment of a critical second-phase volume fraction may be the possible reason for the sudden change in mechanical properties. Percolation theory is utilized in further substantiating this hypothesis.     

Rearrangement of the Ultrasmooth Surface of La3Ga5SiO14 Crystals at Heating

The morphology and phase composition of the surface of La₃Ga₅SiO₁₄ (langasite) crystals at annealing in a temperature range 1000–1200°C have been studied using electron and atomic force microscopy. It has been shown that trigonal lanthanum oxide (La₂O₃) crystals with sizes to 3–4 μm, as well as a microstructure with sizes to 50 μm with gallium excess, with the approximate composition of 15 mol % La₂O₃, 65 mol % Ga₂O₃, and 20 mol % SiO₂ are formed on the surface of langasite crystals annealed in air at temperatures above 1100°C. Possible reasons for thermal destruction of the compound can be a significant rearrangement of the disordered crystal structure of langasite caused by the interaction with air oxygen and under the intense surface diffusion of atoms of the crystal, as well as the incongruent character of melting of the La₃Ga₅SiO₁₄ compound. The revealed thermal destruction of the surface of langasite crystals should be taken into account when using this material to fabricate piezoelectric elements for operation at high temperatures.

     

Equation of state and high-pressure irreversible amorphization in Y3Fe5O12

The change of crystal structure in yttrium iron garnet Y₃Fe₅O₁₂ was studied at room temperature at high pressures up to ∼55 GPa by the x-ray diffraction technique in diamond anvil cells. At a pressure of about ∼50 GPa, a drastic change in the x-ray diffraction pattern was observed indicating the transition into an amorphouslike state. When the pressure was increased, the bulk modulus of YIG was found to be 193 ± 4 GPa. It was also found that the amorphous state was retained after decompression down to ambient pressure. From the shape of x-ray patterns in the “amorphous” phase, it was concluded that the local atomic structure consists of iron-oxygen FeO₆ octahedral complexes with disordered orientations of local axis and of randomly arranged others ion fragments with the overall Y₃Fe₅O₁₂ composition. For the amorphous phase, it was evaluated that the bulk modulus of FeO₆ octahedral complexes is about 260 GPa. The text was submitted by the authors in English.     

Structural transformations of Fe81B13Si4C2 amorphous alloy induced by heating

Thermal stability and crystallization of the Fe₈₁B₁₂Si₄C₂ alloy were investigated in the temperature range 25-700 °C by the XRD and Mössbauer analysis. It was shown that on heating the as-prepared amorphous Fe₈₁B₁₂Si₄C₂ alloy undergoes thermal stabilization through a series of structural transformations involving the process of stress-relieving (temperature range 200-400 °C), followed by a loss of ferromagnetic properties (Curie temperature at 420 °C) and finally crystallization (temperature range 450-530 °C). The process of crystallization begins by formation of two crystal phases: Fe₃B and subsequently Fe₂B, as well as a solid solution α-Fe(Si). With increase in annealing temperature, the completely crystallized alloy involved only two phases, Fe₂B and solid solution α-Fe(Si).XRD patterns established a difference in phase composition and size of the formed crystallites during crystallization depending on the side (fishy or shiny) of the ribbon. The first nuclei of the phase α-Fe₃Si were found on the shiny side by XRD after heat treatment even at 200 °C but the same phase on the fishy side of ribbon was noticed after heat treatment at 450 °C. The largest difference between the contact and free surface was found for the Fe₂B phase crystallized by heating at 700 °C, showing the largest size of crystallites of about 130 nm at 700 °C on the free (shiny) surface.     

Properties and intergranular phase analysis of a ZnO–CoO–Bi2O3 varistor

The electrical properties, microstructures and phase content of a ZnO–CoO–Bi₂O₃ varistor have been characterised. High-resolution TEM and selected area electron diffraction provided detailed information on the distribution and structure of the intergranular Bi-rich phase. Triple points between the ZnO matrix grains contained crystallites that could be indexed to tetragonal β-Bi₂O₃, cubic γ-Bi₂O₃ and δ-Bi₂O₃, or corresponding isostructural ZnO-Bi₂O₃ phases. Thin-film, 1–2 nm grain boundaries were found to be amorphous, but for the first time, crystalline precipitates (with the β-Bi₂O₃ type structure) were also detected in some thicker, 4–5 nm films.     

High-pressure synthesis of A15 Nb3Si phase from amorphous NbSi alloys

An amorphous phase has been produced in a binary Nb-19 at .% Si alloy by rapidly quenching from the molten state. Primary decomposition of the amorphous phase by annealing resulted in the production of a small quantity of the A15 cubic Nb₃Si phase. However, long annealing treatments led to the complete transformation to the equilibrium phases. Application of high pressure of 10 GPa during decomposition of the amorphous phase resulted firstly in the improved stability of the amorphous phase and secondly, in the production of a much larger quantity of the A15 cubic Nb₃Si phase. The lattice parameter of this phase was calculated to be 0.515 nm, in conformity with the previous investigations.