Interrelationship between Interphase Boundaries and Phase Contents near the Critical Compositions of Lead-Free Ferroelectric (Na0.5Bi0.5)TiO3-BaTiO3

Heterophase structures in lead-free perovskite-type ferroelectric solid solutions of (1 - z)(Na0.5Bi0.5)TiO3 - zBaTiO(3) are analysed for a few critical compositions near the morphotropic phase boundary (z = 0.05-0.07). Examples of the phase coexistence and elastic matching of the phases from different symmetry groups are considered to find optimum volume fractions of specific domain types and coexisting phases at the complete stress relief in two-phase samples. Some interrelations between these volume fractions are described using variants of the domain arrangement at changes in the composition and unit-cell parameters. The evaluated room-temperature volume fractions of the ferroelectric monoclinic (Cm symmetry) and tetragonal (P4mm symmetry) phases near the morphotropic phase boundary are in agreement with experimental data.

Order-disorder phase transition and multiferroic behaviour in a metal organic framework compound (CH3)(2)NH2Co(HCOO)(3)

We have investigated the multiferroic and glassy behaviour of metal-organic framework (MOF) material (CH3)(2)NH2Co(CHOO)(3). The compound has perovskite-like architecture in which the metal-formate forms a framework. The organic cation (CH3)(2)NH2+ occupies the cavities in the formate framework in the framework via N-H center dot center dot center dot O hydrogen bonds. At room temperature, the organic cation is disordered and occupies three crystallographically equivalent positions. Upon cooling, the organic cation is ordered which leads to a structural phase transition at 155 K. The structural phase transition is associated with a para-ferroelectric phase transition and is revealed by dielectric and pyroelectric measurements. Further, a PE hysteresis loop below 155 K confirms the ferroelectric behaviour of the material. Analysis of dielectric data reveal large frequency dispersion in the values of dielectric constant and tan delta which signifies the presence of glassy dielectric behaviour. The material displays a antiferromagnetic ordering below 15 K which is attributed to the super-exchange interaction between Co2+ ions mediated via formate linkers. Interestingly, another magnetic transition is also found around 11 K. The peak of the transition shifts to lower temperature with increasing frequency, suggesting glassy magnetism in the sample. (C) 2016 AIP Publishing LLC.

Fracture of materials undergoing solid-solid phase transformation

A large number of technologically important materials undergo solid-solid phase transformations. Examples range from ferroelectrics (transducers and memory devices), zirconia (Thermal Barrier Coatings) to nickel superalloys and (lithium) iron phosphate (Li-ion batteries). These transformations involve a change in the crystal structure either through diffusion of species or local rearrangement of atoms. This change of crystal structure leads to a macroscopic change of shape or volume or both and results in internal stresses during the transformation. In certain situations this stress field gives rise to cracks (tin, iron phosphate etc.) which continue to propagate as the transformation front traverses the material. In other materials the transformation modifies the stress field around cracks and effects crack growth behavior (zirconia, ferroelectrics). These observations serve as our motivation to study cracks in solids undergoing phase transformations. Understanding these effects will help in improving the mechanical reliability of the devices employing these materials.

In this thesis we present work on two problems concerning the interplay between cracks and phase transformations. First, we consider the directional growth of a set of parallel edge cracks due to a solid-solid transformation. We conclude from our analysis that phase transformations can lead to formation of parallel edge cracks when the transformation strain satisfies certain conditions and the resulting cracks grow all the way till their tips cross over the phase boundary. Moreover the cracks continue to grow as the phase boundary traverses into the interior of the body at a uniform spacing without any instabilities. There exists an optimal value for the spacing between the cracks. We ascertain these conclusion by performing numerical simulations using finite elements.

Second, we model the effect of the semiconducting nature and dopants on cracks in ferroelectric perovskite materials, particularly barium titanate. Traditional approaches to model fracture in these materials have treated them as insulators. In reality, they are wide bandgap semiconductors with oxygen vacancies and trace impurities acting as dopants. We incorporate the space charge arising due the semiconducting effect and dopant ionization in a phase field model for the ferroelectric. We derive the governing equations by invoking the dissipation inequality over a ferroelectric domain containing a crack. This approach also yields the driving force acting on the crack. Our phase field simulations of polarization domain evolution around a crack show the accumulation of electronic charge on the crack surface making it more permeable than was previously believed so, as seen in recent experiments. We also discuss the effect the space charge has on domain formation and the crack driving force.

Single sublattice endotaxial phase separation driven by charge frustration in a complex oxide.

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO3 perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO3 network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO3 network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation.

Structure and oxygen permeability of a dual-phase membrane

The dual-phase membrane of La0.15Sr0.85Ga0.3Fe0.7O3-delta-Ba0.5Sr0.5Fe0.2Co0.8O3-delta (LSGF-BSCF) was prepared successfully. This membrane was characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro-analyzer (EPMA). This membrane has a dense dual-phase structure: LSGF being the dense body of this membrane and BSCF as another phase running along the LSGF body. This structure is favorable for the oxygen permeation through the membrane. The oxygen permeation test shows that the oxygen permeation flux of LSGF-BSCF membrane (Jo(2) = 0.45 ml/min cm(2), at 915 degreesC) is much higher than that of LSGF membrane (Jo(2) = 0.05 ml/min cm(2)). Thickness dependence of oxygen permeation indicates that the oxygen permeation is controlled by the bulk diffusion. Compared to pure BSCF the dual-phase membrane of LSGF-BSCF is stable in reducing atmosphere. (C) 2003 Elsevier B.V. All rights reserved.

Phase behavior of rare earth manganites

Among complex oxides containing rare earth and manganese BaLn(2)Mn(2)O(7)( Ln = rare earth) with the layered perovskite type and Ln(2)(Mn, M)O-7 with pyrochlore-related structure were studied since these compounds show many kinds of phases and unique phase transitions. In BaLn(2)Mn(2)O(7) there appear many phases, depending on the synthetic conditions for each rare earth. The tetragonal phase of so-called Ruddlesden-Popper type is the fundamental structure and many kinds of deformed modification of this structure are obtained. For BaEu2Mn2O7 at least five phases have been identified from the results of X-ray diffraction analysis with the space group P4(2)/mnm, Fmmm, Immm and A2/m in addition to the fundamental tetragonal I4/mmm phase. In the pyrochlore-related type compounds, Ln(2)Mn(2-x)M(x)O(7)(M = Ta, Nb, W etc), there also appear several phases With different crystal structures. With regard to every rare earth, Ln(2)MnTaO(7) phase is stable only for excess Ta and can be obtained under high oxygen partial pressure process. This group has trigonal structure with zirkelite type ( P3(1)21 space group).

Y-Ba-Cu-O mixed oxides for production of diphenols in liquid-solid phase

Superconductor mixed oxides were often used as catalysts at higher temperature in gas phase oxidations, and considered not suitable for lower temperature reactions in the liquid-solid phase; here the catalysis of YBa2Cu3O7+/-x and Y2BaCuO5+/-x in the phenol hydroxylation at lower temperature with H2O2 as oxygen donor was studied, and found that the superconductor YBa2Cu3O7+/-x, has no catalytic activity for phenol hydroxylation, but Y2BaCuO5+/-x does, even has better catalytic activity and stability than most previously reported ones. With the studies of catalysis of other simple metal oxides and perovskite-like mixed oxides, a radical substitution mechanism is proposed and the experimental facts are explained clearly, and draw a conclusion that the perovskite-like mixed oxides with (AO)(ABO(3)) and (AO)2(ABO(3)) structure have better catalytic activity than the simple perovskite oxides with (ABO(3))(3) structure alone, and (AO) structure unit is the key for the mixed oxides to have the phenol hydroxylation activity. No pollution of this process is very important for its further industrial application.

STUDIES ON THE PEROVSKITE-TYPE MIXED CATALYSTS OF TITANIUM SYSTEM FOR OXIDATIVE COUPLING OF METHANE - THE INFLUENCE OF CALCINATION TEMPERATURE

The correlations of the calcination temperature, structure and catalytic activity for the oxidative coupling of methane on the LiLa0.5Ti0.5O2+lambda catalysts whose main phase and major active phase is Perovskite-type ternary complex oxide LaTi1-yLiyO3-lambda have been studied. The surface and bulk structures of the catalysts were characterized by means of XRD, XPS, IR, BET and so on, The results cleary indicated that the effect of calcination temperature on the activity for the oxidative coupling of methane is twofold. On one hand, it is favorable for Li+ substitution for Ti3+ to enter into the lattice of LaTiO3 and produce more oxygen vacancies in which active oxygens are formed; however, excessively high calcination temperature make the amount of Li+ substitution for Ti3+ lower, due to a little change of structure or phases for the catalyst. On the other hand, the conversion of CH4 drops because of the decrease of surface area, when the calcination temperature is raised.

RAMAN-SPECTROSCOPIC STUDY OF THE PEROVSKITE LAYER STRUCTURE COMPOUNDS - [N-CNH2N+1NH3]2ZNCL4

The C-H stretching, C-H bending, C-C stretching and the low-frequency vibrational regions have been investigated by Raman spectroscopy for [n-CnH2n+1NH3]2ZnCl4 with n = 7-12, 16. The frequencies and relative intensities are related to the length of the carbon chain in the molecules and present the odd-even effect of the carbon atom numbers in the chains. Some changes in the spectra are interpreted in terms of the different molecular packing.

Low temperature synthesis of perovskite oxide using the adsorption properties of cellulose

La-0.8Sr(0).2CoO(3) (LSCO) oxide powder was prepared using the adsorption properties of cellulose. The preparation process was studied by XRD, FTIR, TG-DTA and CO2-TPD techniques. The results of XRD, IR and TG-DTA testified that cellulose could successfully reserve the homogeneity of the solution system to the solid precursor. During the early stage of pyrolysis, cellulose was partially oxidized, and some COO- groups appeared in its texture, which were then complexed with the adsorbed metal ions, and effectively suppressed the aggregation of metal ions. Formation of a pure perovskite and the properties of the powder resulted were found to be significantly influenced by the cellulose to metal nitrate ratio. Also the properties of the resulting powder were greatly influenced by the calcination conditions. If the produced carbon dioxide could not be eluted in time, carbonate would be formed in the bulk. Hence, a high calcination temperature (> 800 degreesC) was needed to acquire a pure phase LSCO. At optimized conditions, nano-crystal LSCO could be obtained at as low as 500 degreesC.

In situ high temperature X-ray diffraction studies of mixed ionic and electronic conducting perovskite-type membranes

Phase structure and stability of three typical mixed ionic and electronic conducting perovskite-type membranes, SrCo0.8Fe0.2O3-delta (SCF), Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) and BaCo0.4Fe0.4Zr0.2O3-delta (BCFZ) were studied by in situ high temperature X-ray diffraction at temperatures from 303 to 1273 K and under different atmospheres (air, 2% O-2 in Ar and pure Ar) at 1173 K. By analyzing their lattice parameters the thermal expansion coefficients (TECs) of BSCF, SCF and BCZF are obtained to be 11.5 x 10(-6) K-1, 17.9 x 10(-6) K-1 and 10.3 x 10(-6) K-1, respectively. A relationship between phase stability and TEC was proposed: the higher is the TEC, the lower is the operation stability of the perovskite materials. (C) 2005 Elsevier B.V. All rights reserved.

Perovskite lead zirconium titanate nanorings: Towards nanoscale ferroelectric

Rings of perovskite lead zirconium titanate (PZT) with internal diameters down to similar to 5 nm and ring thicknesses of similar to 5-10 nm have been fabricated and structurally, crystallographically, and chemically characterized using an analytical transmission electron microscope. Ring fabrication involved conformal solution deposition of a thin layer of PZT on the inside of a thin film of anodized aluminum oxide nanopores, and subsequent sectioning of the coated pores perpendicular to their cylinder axes. Although the starting solution used for the solution deposition was made from morphotropic phase boundary PZT, the nanorings were found to be on the zirconium-rich side of the PZT phase diagram. Nevertheless, coatings were found to be of perovskite crystallography. The dimensions of these nanorings are such that they have the potential to demonstrate polarization vortices, as modeled by Naumov [Nature (London) 432, 737 (2004)], and moreover represent the perfect morphology to allow vortex alignment and the creation of the ferroelectric "solenoid" as modeled by Gorbatsevich and Kopaev [Ferroelectrics 161, 321 (1994)].

Room-temperature single phase multiferroic magnetoelectrics: Pb(Fe,M)x(Zr,Ti)(1−x)O3 [M=Ta, Nb]

We describe extensive studies on a family of perovskite oxides that are ferroelectric and ferromagnetic at ambient temperatures. The data include x-ray diffraction, Raman spectroscopy, measurements of ferroelectric and magnetic hysteresis, dielectric constants, Curie temperatures, electron microscopy
(both scanning electron microscope and transmission electron microscopy (TEM)) studies, and both longitudinal and transverse magnetoelectric constants a33 and a31. The study extends earlier work to lower Fe, Ta, and Nb concentrations at the B-site (from 15%–20% down to 5%). The magnetoelectric
constants increase supralinearly with Fe concentrations, supporting the earlier conclusions of a key role for Fe spin clustering. The room-temperature orthorhombic C2v point group symmetry inferred from earlier x-ray diffraction studies is confirmed via TEM, and the primitive unit cell size is found to be the basic perovskite Z¼1 structure of BaTiO3, also the sequence of phase transitions with increasing temperature from rhombohedral to orthorhombic to tetragonal to cubic mimics barium titanate.

Materiais à base de óxidos com estrutura do tipo perovskite e compósitos como ânodos de PCES

This work was focused on the analysis of transport, thermomechanical and electrochemical properties of a series of perovskite-like oxide materials and composites for potential applications as anodes of intermediate-temperature solid oxide fuel cells (SOFCs) with lanthanum gallate and silicate solid electrolytes. The primary attention was centered on A(Mn,Nb)O3-δ (A = Sr, Ca) and (La,Sr)(Mn,Ti)O3-based systems, lanthanum chromite substituted with acceptor-type and variable-valence cations, and various Ni-containing cermets. Emphasis was given to phase stability of the materials, their crystal structure, microstructure of porous electrode layers and dense ceramics, electronic conductivity, Seebeck coefficient, oxygen permeability, thermal and chemical induced expansion, and anodic overpotentials of the electrodes deposited onto (La,Sr)(Ga,Mg)O3- and La10(Si,Al)6O27- based electrolyte membranes. In selected cases, roles of oxygen diffusivity, states of the transition metal cations relevant for the electronic transport, catalytically active additives and doped ceria protective interlayers introduced in the model electrochemical cells were assessed. The correlations between transport properties of the electrode materials and electrochemical behavior of porous electrodes showed that the principal factors governing anode performance include, in particular, electronic conduction of the anode compositions and cation interdiffusion between the electrodes and solid electrolytes. The latter is critically important for the silicatebased electrolyte membranes, leading to substantially worse anode properties compared to the electrochemical cells with lanthanum gallate solid electrolyte. The results made it possible to select several anode compositions exhibiting lower area-specific electrode resistivity compared to known analogues, such as (La,Sr)(Cr,Mn)O3-δ.

Catalytic activity of some of the perovskite-type mixed oxides (ABO3) consisting of rare earth and 3d transition metals

The catalytic activity of some of the ABO3 (A = La, Pr and Sm, B= Cr, Mn, Fe, Co and Ni) perovskite-type oxides for the liquid phase reduction of ketone and oxidation of alcohol in 2-propanol medium has been studied. The data have been correlated with the surface electron donor properties of these oxides. The surface electron donor properties have been determined from the adsorption of electron acceptors of varying electron affinities on the oxide surface.