Effect of oxygen sublattice order on conductivity in highly defective fluorite oxides

The study of ionic transport in cubic bismuth oxides is important technically because these materials exhibit the highest oxygen–ion conductivity of any material known to date. From a scientific point of view, the study of ionic transport in cubic bismuth oxides also provides an understanding of how anion transport in oxides with the fluorite structure is influenced by high vacancy concentration and how this is influenced by local structure. Bismuth oxide doped with isovalent rare earth cations retains the high temperature defective fluorite structure upon cooling down to room temperature. However, these doped materials undergo an order–disorder transition of the oxygen sublattice at about 600 °C. When annealed at temperatures less than the transition temperature the oxygen sublattice continues to order, and consequently oxygen ion conductivity undergoes a decay. However, the conductivity activation energies of the ordered structures after extended aging at 500 °C were observed to be lower than those of the structures prior to aging. Modeling of ordered structures based on TEM diffraction patterns indicates a<111> vacancy ordering in the anion sublattice (occupancy ordering). Neutron diffraction studies show additional structural changes in the oxygen sublattice due to positional ordering. These studies indicate that the ionic conductivity is dependent on the distribution of oxygen ions between the regular 8c sites and the interstitial 32f sites in the fluorite structure. Based on the TEM and neutron diffraction studies and conductivity of ordered and disordered structures the influence of local structure on conductivity is described. These results indicate that ordering of anion vacancies in <111> is common to fluorite oxides at high vacancy concentrations. Further, that the tendency to order depends on the dopant radii and polarizability.     

Ordering of Octahedral Vacancies in Transition Aluminas

The microstructure of transition aluminas obtained via the dehydration of boehmite has been characterized by using transmission electron microscopy (TEM). The presence of γ-, δ-, and θ-aluminas was identified by using selected-area electron diffraction. Modifications that resulted from the reordering of aluminum vacancies on octahedral sites in a cubic close-packed oxygen network have been detected and analyzed by using high-resolution transmission electron microscopy (HRTEM) combined with image simulations. A good correspondence of the observed and calculated images confirmed the ordering of vacant octahedral sites located on {011} and {011} planes that formed a zigzag configuration along the <010> direction. Two more arrangements of empty octahedral sites, but now concentrated on {001} planes, have been determined in the sintered powder-gel agglomerates. Structure analysis suggested that the modifications are all associated with the rearrangement of vacant sites during the phase transformation from gamma-alumina to δ-alumina, and further to theta-alumina, and may be driven by configuration entropy minimization.     

Microstructure of Lanthanum Magnesium Niobate at Elevated Temperature

Microstructural studies of the complex perovskite compound La(Mg_2/3Nb_1/3)O₃ (LMN) were conducted using transmission electron microscopy (TEM) and X-ray diffractometry (XRD) at elevated temperatures. 1:1 chemical ordering of B-site cations and tilting of oxygen octahedra were observed in LMN. Three types of superlattice reflections, [1—2]{111}, [1—2]{110}, and [1—2]{100} were observed at room temperature and at 800°C in electron diffraction patterns. In the XRD experiments, the [1—2]{210} and [1—2]{300} extra peaks disappeared at temperatures >1200°C. However, the intensity of the superlattice [1—2]{111} peak did not change with increased temperature up to 1400°C. These results strongly indicated that the origin of superlattice reflection [1—2]{111} was different from that of the other superlattice reflections. It was mainly caused by the 1:1 chemical ordering of magnesium and niobium atoms. The TEM image observed at 800°C showed the ordered domain structures separated by the antiphase boundaries.     

The microstructure and ferroelectric property of Nd-doped multiferroic ceramics Bi0.85Nd0.15FeO3

The microstructures of Bi_0.85Nd_0.15FeO₃ ceramics were investigated by using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), convergent-beam electron diffraction (CBED) and X-ray energy dispersive spectrometry (EDS). The superstructure phase related to 1/4(hh0)_p and 1/4(00 l)_p diffraction spots were observed in the samples. It is found that the superstructure phase can co-exist with R3c phase in a single grain. The bright and dark field images accompanied with SAED patterns provided evidences of the superstructure phase dispersed into the matrix with R3c symmetry and evolution of the domain. The ferroelectric domain wall was observed according to the displacement of Fe³⁺ ions respect to Bi³⁺ sub-lattice in the ferroelectric R3c phase based on HRTEM observations. The space group Pnam of the superstructure was identified by combining SAED and CBED techniques. EDS measurement revealed that the concentration of Nd in the Pnam phase is higher than that in R3c phase. This might mean that the transition from R3c to Pnam structure is due to the inhomogeneous distribution of the Nd concentration and the weakened stereochemical activity of Bi³⁺ lone electron pair, arisen from the increase in Nd content.     

Microstructural Investigations of Barium Titanate-Based Material for Base Metal Electrode Ceramic Multilayer Capacitor

The microstructures of BME-X7R ceramics have been investigated by imaging and analytical TEM. By varying the sintering conditions (pressing, sintering temperature and atmosphere), the microstructure changes drastically, and dislocation loops can be observed. These defects most probably are caused by an ordering of oxygen vacancies in the lattice, and the density appears to be correlated to the oxygen partial pressure applied during the reoxidation step. For pressed and unpressed ceramic foils, a model for different sintering behaviors according to liquid-phase assistance is given that is based on a kinetically determined microstructure rather than thermodynamic equilibrium.     

Crystal Structure and Defects of Ordered (Pb1-xCax)TiO3

Crystal structure and defects of ordered ((Pb_{1- x} Ca _x )TiO₃ ceramics have been investigated by transmission electron microscopy. The structure is determined to be tetragonal, belonging to point group 4 mm . Pb and Ca are in an ordered fcc arrangement on the A sites. In addition to the chemical ordering, there is ordering due to atomic shuffling or what has been referred to as electrical ordering. Two distinct sets of planar defects are observed in the same region. One set of planar defects are identified as antiphase boundaries that are the results of chemical ordering and the other set are displacement boundaries that are the results of electrical ordering.     

Atomistic process of electron-stimulated structural ordering in tetrahedral amorphous carbon

To have insight to the atomistic process of graphitic ordering in tetrahedral amorphous carbon films which is induced by irradiation of high-energy electrons, the role of temperature in the graphitization was experimentally studied. The change of electron diffraction patterns before and after irradiation at room temperature indicated graphitic ordering, but irradiation at − 170 °C resulted in disordering. Systematic measurements of the temporal evolution of electron energy loss spectra with irradiation at various elevated temperatures and data analysis based on the Johnson–Mehl–Avrami model yielded the activation energy for electron-stimulated ordering of as small as ∼ 0.09 ± 0.01 eV. The most plausible model to account for all the experimental facts is the dissociative diffusion that is triggered by the electron-stimulated displacement of carbon atoms at the initial sites.     

TEM study of a fluorite-type (1 − x)Bi2O3·xFe2O3 superstructure in BiFeO3 ceramics synthesized by the rapid liquid-phase sintering method

A commensurate modulated structure of (1 ? x)Bi₂O₃·xFe₂O₃ is found in the BiFeO₃ ceramics synthesized by the rapid liquid-phase sintering method. Transmission electron microscopy studies show that the superstructure is based on fluorite-type δ-Bi₂O₃ and modulated probably by oxygen vacancy ordering. The existence of such a superstructure may increase the leakage current of BiFeO₃ ceramics.     

TEM and SEM studies on the formation of superconducting phases in the Bi-based system

We present the formation mechanism and structure of the superconducting phases in the Bi–Sr–Ca–Cu–O system by TEM and SEM observations. Oxalate coprecipitation was used as preparation method. The average particle size, distribution and the presence of different phases and shapes of the particles were evaluated. All samples were well crystallized, showing grains in submicronic range. After thermal treatments, the complex samples showed mica-like crystals uniform dispersed and also an orientation tendency. The superlattice was composed of building blocks and successive planes. The TEM observations were in agreement with the diffraction patterns that revealed a mixture of 2212 and 2223 superconducting phases. With the building blocks in the system, it is possible to form many incommensurate phases that destroy the symmetry. Careful control of the preparation conditions is essential to produce a well-ordered commensurate phase, suitable for the precise determination of the complete superstructure by X-ray diffraction.     

Sub-solidus phase equilibria in the YO1.5-TaO2.5 system

The phase equilibria in the YO_1.5-TaO_2.5 binary system were studied in air from 1250?°C to 1700?°C across the entire composition range, clarifying discrepancies in previous investigations. Materials were synthesized by precursor routes and phases identified by X-ray diffraction and Raman spectroscopy. Solubility limits were determined using TEM/EDS and EPMA. The fluorite phase field was shown to extend down to at least 1250?°C. Ordering phenomena in two closely related phases near Y₃TaO₇ were investigated, suggesting a second-order ordering relationship between them. The order-disorder transformation to fluorite was observed to occur through formation of a superstructure. The homogeneity range for YTa₇O₁₉ was expanded based on chemical analysis, while YTa₃O₉ was shown to be a line compound. The emerging understanding provides valuable insight into related ternary systems as well as inputs for thermodynamic assessments.     

Hydrothermal synthesis of uniform Fe-doped Gd(OH)3 nanorods and their magnetic properties: Phase conversion from paramagnetism to ferromagnetism

High quality pure and Fe-doped Gd(OH)₃ nanorods were fabricated through a template-free hydrothermal method for the first time. Analysis of XRD indicates that Fe³⁺ was incorporating in the interstitial sites rather than occupying the substitutional sites, forming pure hexagonal structure of Gd(OH)₃ without any other impurity phase. TEM characterizations show that all the samples perform uniform rod-like morphologies with similar diameter and length, which suggests that the Fe doping has little influence on the morphologies of samples. ICP and XPS spectra suggest that the dopant Fe³⁺ is incorporated into the inner body sites, not on the surface of nanorods. Magnetic studies show that the magnetic phase can be converted from paramagnetism to room-temperature ferromagnetism by doping Fe³⁺ ions into the Gd(OH)₃ nanorods. The saturation magnetization (Ms) is sensitive to the amount of Fe dopants, and the Ms for Fe_0.03Gd_0.97(OH)₃ nanorods reaches the maximum value of 0.184 emu/g. It is considered that the ferromagnetic ordering is possibly originated from the exchange interaction of Fe³⁺ through the oxygen vacancies, leading to the formation of point defect-mediated bound magnetic polarons (BMPs). Ruling out the affect of morphologies and secondary magnetic phase on the magnetic properties, the ferromagnetic ordering in uniform Fe-doped Gd(OH)₃ nanorods, in which the dopant Fe³⁺ is incorporated into the inner body sites of nanorods, are of great importance to deeply understand the rare earth-based DMS/DMD systems and have potential applications in spintronic devices.     

Solid Solution Range and Microstructures of Melt-Grown Mullite

The solid solution range of melt-grown mullite was examined by crystal-chemical methods. The maximum Al₂O₃ content as determined by EDX was ∼83.6 wt%, 75 mol%, or the nominal composition 3Al₂O₃.SiO₂. For samples of overall composition 81 to 83 wt% Al₂O₃, extra lines indicating crystallographic superstructure appeared in Guinier X-ray patterns. The corresponding TEM microstructure consisted of a mullite matrix finely twinned on (001), the twins being 0.02 to 0.10 μm wide, with oriented exsolution of α-A1₂O₃, often twinned, also being present. The analogy between mullite superstructure and that of plagioclase feldspars, as well as the relevance of these findings to the SiO₂-Al₂O₃ metastable phase equilibria are discussed.     

Atomic Resolution Analysis of the Defect Chemistry and Microdomain Structure of Brownmillerite-Type Strontium Cobaltite

To investigate the fundamental aspects of vacancy ordering in oxygen-transporting ceramic membranes, we have performed atomic resolution analysis of individual domains in brownmillerite-type SrCoO_3−δ. Electron energy loss spectroscopy indicates that the Co valence state in adjacent planes can be 2+ and 4+. This charge localization is accompanied by oxygen deficiency and the formation of ordered octahedral and tetrahedral coordinated Co sites. At microdomain boundaries, Z -contrast images reveal a structural relaxation of the octahedral site with the reduction of the Co valence state from 4+ to 3+ and the incorporation of extra oxygen vacancies.     

Defect Chemistry of Relaxor Ferroelectrics and the Implications for Dielectric Degradation

Degradation of the electrical resistance of perovskite-related dielectrics under voltage–temperature stress is a function of the concentration of the most mobile ionic species, the oxygen vacancy. Sources of oxygen vacancies in simple perovskite oxides are reviewed, and the effects of the compositional complexity of the relaxor ferroelectrics are discussed. In particular, the possibility of compositional inhomogeneity that results from partial ordering of the cations on the octa hedral sites can greatly affect the properties of the continuous, disordered matrix that controls the degradation behavior. [Key words: ferroelectrics, electrical properties, defects, capacitors, perovskites.]     

In situ wet-cell TEM observation of gold nanoparticle motion in an aqueous solution

In situ wet-cell transmission electron microscopy (TEM) technology enables direct observation of nanomaterials in a fully hydrated environment with high spatial and temporal resolution, which can be used to address a wide range of scientific problems. In this paper, the motions of approximately 5-nm sized gold nanoparticles in an aqueous solution are studied using the wet-cell TEM technology. It is observed that gold nanoparticles can be either in a single particle or cluster forms, and dynamic displacement and rotation motions are observed for both forms in the solution. Under electron beam irradiation, nanoparticles in some clusters gradually fused together; sometimes they also showed dramatic growth behavior. Mechanisms for the motion and growth of the particles/clusters are discussed.     

Investigation of nanostructures and properties of sulfonated poly(arylenethioethersulfone) copolymer as proton conducting materials by small angle neutron scattering

Sulfonated poly(arylenethioethersulfone) copolymer (SPTES-50), a promising candidate material for proton exchange membrane fuel cell (PEMFC), exhibited excellent thermal stability, high proton conductivity (135 mS/cm at 85 °C, 85% relative humidity), and electrochemical property. Small angle neutron scattering (SANS) of fully hydrated SPTES-50 membranes revealed the presence of embedded spherical nanodomains containing ionic group and water within the polymer membranes. The polydispersity of the nanoscale structure limited scattering contrast between the polymer backbone and sulfonated groups, and precluded analysis of intermediate and large scattering vectors in terms of the polymer-water interface structure. Inter-cluster correlations associated with the large extent of water absorption in the fully hydrated SPTES-50 membranes were accounted by Percus-Yevick liquid-like ordering of polydispersed hard sphere model with Schulz polydispersity approximation. Approximation of their low q upturn with an exponential decay results in a decay of −3 at 25 °C accounted for inter-cluster correlations which changed to a decay of −1.1 at 55 °C and 77 °C. This indicated a change in morphology upon increase of temperature such as to fractal morphology or an interconnected cylindrical network. The scattering patterns don't exhibit any further changes within examined range of q when the temperature increased from 55 °C to 77 °C. The number density of ionic clusters remained approximately constant (∼1.1818 × 10¹⁷ cm³), which indicated that additional water adsorbed by the polymer at the elevated temperature did not result in substantial coalescence of the clusters. Transmission electron microscopy (TEM) observation of the silver exchanged SPTES-50 membranes exhibited aggregates of Ag⁺ embedded within the dry membranes which can be approximated by isolated spheres.     

Atomically resolved Moiré-type superstructures in double-walled carbon nanotubes

Double-wall carbon nanotubes (DWCNTs) are investigated with high resolution by scanning tunneling electron microscopy at 78 and 4.5 K. Besides the atomic structure of the DWCNT surface, an additional honeycomb-type superstructure is revealed at these low temperatures. This periodic structure can be interpreted in terms of a Moiré pattern resulting from a rotation between the rolled up graphene sheets that constitute the DWCNT. Deviations of the observed Moiré patterns from the perfect hexagonal Moiré patterns that are commonly resolved for multi-graphene layer surfaces are found to be related to the intrinsic different curvature of the inner and outer tube of the DWCNT. Investigation of such Moiré patterns in the surface of DWCNTs allows one to gain more insight into the structure and chirality of the otherwise inaccessible inner shell of the DWCNTs.     

A simplified synthesis of N-doped zeolite-templated carbons, the control of the level of zeolite-like ordering and its effect on hydrogen storage properties

Nitrogen-doped, microporous carbon materials have been prepared using zeolite EMC-2 as a hard template and acetonitrile as the carbon source via chemical vapour deposition (CVD) in the temperature range 700–950 °C. The carbon products exhibited high surface areas (up to 3360 m²/g), high pore volumes (up to 1.71 cm³/g) and had zeolite-like structural ordering derived from the template. The carbons had XRD patterns that exhibited two well resolved peaks and TEM images that showed well ordered pore channels. A high proportion of porosity (up to 85% of surface area and 73% of pore volume) for the best ordered carbon arose from micropores that exhibited narrow size distribution in the range 5–15 Å. The carbons generally retained the morphology of the template with solid-core particles at CVD temperatures up to 900 °C and hollow shells at 950 °C. The carbons had total hydrogen storage capacities up to 6.0 wt.% at −196 °C and 20 bar. The hydrogen uptake was found to be dependent on the level of zeolite-like ordering and the resulting textural properties. Particularly, high levels of zeolite-like ordering favoured micropores of size <15 Å which are favourable for higher hydrogen uptake capacities.     

DC electric fields produce periodic bending of polyelectrolyte gels

The periodic bending of polyelectrolyte hydrogels made of polyacrylamide, polyacrylic and polymethacrylic acids with 10% ionization under the DC electric field was observed in 0.8 mM CaCl₂. When the field with intensity 13.5 V/cm was applied perpendicular to the longitudinal axis of the gel sample, the gel first bent to cathode, then more substantially to anode, then to cathode again and so on with damping amplitude. Within experimental error up to two periods of such oscillations were observed. The maximum amplitude of bending was approximately 30% of sample length. After 150-200 s, the gel sample finally reached a steady state slightly bent to anode. During these oscillations the volume of the sample gradually decreased by 10-30% depending on the gel. Kinetics of displacement of the free end of the gel sample could be fitted by a damped sine wave function, depending initial amplitude, final displacement, wave period, and decay time. All parameters except wave period depended on sample cross-section. The amplitude, wave period and decay time decrease with the increase of initial degree of gel swelling. The physical basis of the periodic bending is discussed based on the theoretical approach of Doi, Matsumoto and Hirose, although that theory can describe only initial cathodic and successive anodic displacement of gel.     

Rod-rod and rod-coil self-assembly and phase behavior of polypeptide diblock copolymers

Poly(γ-benzyl l-glutamate) (PBLG) forms a rigid helical rod in organic solvents. Cholesteric liquid crystalline ordering of these rods has been observed in PBLG solutions and cast films. In this research, peptidic block copolymers were created using PBLG in order to determine the effect of an added block on the classic cholesteric ordering. Peptide blocks with varied lengths and inherent secondary structures, random coil or rigid rod, were attached to PBLG molecules. The self assembly/liquid crystalline ordering of these molecules in films cast from various organic solvents was probed with transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). In pure PBLG and PBLG diblock copolymers with relatively small additional blocks, cholesteric liquid crystalline ordering was observed in bulk films. However, depending on the kinetics of film formation and the amount of non-PBLG block, significant changes in the nanostructure and microstructure were observed. These purely peptidic block molecules provide the opportunity to pattern materials with peptidic functionalities by taking advantage of block copolymer phase behavior and liquid crystal ordering.