Phase-transition studies of Ba(0.90)Ca(0.10)(Ti(1-x)Zr(x))O(3) ferroelectric ceramic compounds

Perovskite-structured Ba(0.90)Ca(0.10)(Ti(1-x)Zr(x))O(3) ceramics were prepared in this work and subsequently studied in terms of composition-dependent dielectric and high-resolution long-range order structural properties from 30 to 450 K. The dielectric response of these materials was measured at several frequencies in the range from 1 kHz to 1 MHz. Combining both techniques, including Rietveld refinement of the X-ray diffraction data, allowed observing that, when increasing Zr(4+) content, the materials change from conventional to diffuse and relaxor ferroelectric compounds, the transition occurring spontaneously at the x = 0.18 composition. Interestingly, this spontaneous transition turned out to be prevented for a further increase of Zr(4+). On the basis of all the dielectric and structural results processed, a phase diagram of this system is presented. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Major- and trace-element composition of olivine, perovskite, clinopyroxene, Cr-Fe-Ti oxides, phlogopite and host kamafugites and kimberlites, Alto Paranaiba, Brazil

An integrated whole-rock petrographic and geochemical study has been carried out on kamafugites and kimberlites of the Late Cretaceous Alto Paranaiba igneous province, in Brazil, and their main minerals, olivine, clinopyroxene, perovskite, phlogopite, spinels and ilmenite. Perovskite is by far the dominant repository for light lanthanides, Nb, Ta, Th and U, and occasionally other elements, reaching concentrations up to 3.4 x 10(4) chondrite values for light lanthanides and 105 chondrite for Th. A very strong fractionation between light and heavy lanthanides (chondrite-normalized La/Yb from similar to 175 to similar to 2000) is also observed. This is likely the first comprehensive dataset on natural perovskite. Clinopyroxene has variable trace-element contents. likely due to the different position of this phase in the crystallization sequence; Sc reaches values as high as 200 ppm whereas the lanthanides show very variable enrichment in light over heavy REE, and commonly show a negative Eu anomaly. The olivine, phlogopite (and tetra-ferriphlogopite), Cr-Ti oxide and ilmenite are substantially barren minerals for lanthanides and most other trace elements, with the exception of Ba, Cs and Rb in mica, and V, Nb and Ta in ilmenite. Estimated mineral/whole-rock partition coefficients for lanthanides in perovskite are similar to previous determinations, though much higher than those calculated in experiments with synthetic compositions, testifying once more to the complex behavior of these elements in a natural environment. The enormous potential for exploitation of lanthanides, Th, U and high-field-strength elements in the Brazilian kamafugites, kimberlites and related rocks is clearly shown.

Study of Water-Gas-Shift Reaction over La((1-y))Sr(y)Ni(x)Co((1-x))O(3) Perovskite as Precursors

The performance of La((1-y))Sr(y)Ni(x)Co((1-x))O(3) perovskites for the water gas shift reaction (WGSR) was investigated. The samples were prepared by the co- precipitation method and were performed by the BET method, XRD, TPR, and XPS. The catalytic tests were performed at 300 and 400 A degrees C and H(2)O(v)/CO = 2.3/1 (molar ratio). The sample with the highest surface area is La(0.70)Sr(0.30)NiO(3). The XRD results showed the formation of perovskite structure for all samples, and the La(0.70)Sr(0.30)NiO(3) sample also presented peaks corresponding to La(2)NiO(4) and NiO, indicating that the solubility limit of Sr in the perovskite lattice was surpassed. The replacement of Co by Ni favored the reduction of the species at lower temperatures, and the sample containing Sr presented the highest amount of reducible species, as identified by TPR results. All samples were active, the Sr containing perovskite appearing the most active due to the highest surface area, presence of the La(2)NiO(4) phase, and higher content of Cu in the surface, as detected by XPS. Among the samples containing Co, the most active one was that with x = 0.70 (60% of CO conversion).

Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation

Hollow fibre membranes of mixed conducting perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) were prepared via the combined phase inversion and sintering technique. The fibres were tested for air separation with a home-made reactor under the oxygen partial pressure gradient generated by the air/He streams. Some fibres were in situ activated by introducing methane in the He sweeping gas at high temperatures. The activated membranes with new morphology were created by transforming the inner densified surface layer to a porous structure. Compared to the original membranes, the activated gave appreciable higher oxygen fluxes. At 800 °C, the oxygen fluxes were increased by a factor of 10 after activation was carried out at 1000 °C for 1 h.

Fabrication and visible-light photocatalytic behavior of perovskite praseodymium ferrite porous nanotubes

Perovskite praseodymium ferrite (PrFeO3) porous nanotubes are prepared by electrospinning of the precursor solution into nanofibers, subsequently by annealing the precursor fibers at a low temperature (e.g. 40 °C) and finally by calcination at a high temperature. The low temperature annealing treatment is found to play a key role in the formation of porous nanotube. The porous tubes show a perovskite-type PrFeO3 crystal characteristic with high optical absorption in the UV-visible region and an energy band gap of 1.97 eV. When compared with PrFeO3 porous nanofibers and PrFeO3 particles, the PrFeO3 porous nanotubes show better visible-light photo-catalytic ability to degrade Rhodamine B in aqueous phase because of the increased surface area and more active catalytic sites on the inner walls and outer surfaces.

Microwave-hydrothermal synthesis of perovskite bismuth ferrite nanoparticles

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Effect of annealing atmosphere on phase formation and electrical characteristics of bismuth ferrite thin films

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Phase formation and characterization of BaBi2Ta2O9 obtained by mixed oxide procedure

Ferroelectric layefed-perovskite BaBi2Ta2O9 (BBT) has been prepared successfully by solid-state reaction. The influence of pressure and temperature/time annealing regime on the BBT phase formation was analyzed. The powders were characterized by thermal analysis and Xray diffraction and the sintered pellets by scanning electron microscopy. The crystalline BBT phase, free of secondary phases was obtained at 950 degreesC for 2 h. For an applied field strength of 380 kV/cm, a remnant polarization of 7.6 muC/cm(2) and an electric coercive field of 45.7 kV/cm were obtained. (C) 2004 Elsevier B.V. All rights reserved.

Ferroelectric phase transition in Pb0.60Sr0.40TiO3 thin films

Polycrystalline Pb-0.Sr-60(0).40TiO3 thin films with the tetragonal perovskite structure were grown on platinum-coated silicon substrates by a chemical method. Raman results reveal that A1 (1 TO) symmetry modes, also known as soft modes, persist above the phase transition 14 temperature. This is due to the high structural distortion caused by the substitution effect of Sr2+ for Pb2+ ions. In contrast, the E(1TO) symmetry mode vanishes at 498 K, characterizing the ferroelectric-paraelectric transition phase. However, the Raman spectra, as a function of temperature, reveal that the ferroelectric-paraelectric phase transition may be correlated with a diffuse phase transition. The experimental data obtained from measurements of the dielectric constant as a function of temperature and frequencies showed a classical behavior of ferroelectric phase transition in Pb-0.Sr-60(0).40TiO3 thin films, rather than a relaxor ferroelectric phase transition. (C) 2004 Elsevier B.V. All rights reserved.

Topotatic-like phase transformation of amorphous lead titanate to cubic lead titanate

The crystallization process of lead titanate (PT) prepared using the polymeric precursor method was investigated using X-ray diffractometry, Raman spectroscopy, electron microscopy, and X-ray absorption spectroscopy techniques. The results showed that amorphous PT was formed by an O-Ti-O structure composed of fivefold and sixfold oxygen-coordinated titanium. The local structure of the amorphous PT phase was similar to that of the cubic PT phase, i.e., similar coordination number and similar bond lengths, leading to a topotactic-like transformation during the phase transformation from amorphous to cubic perovskite PT. Because of the low crystallization temperature, every transformation observed during the crystallization process was associated with a short-range rearrangement process.

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298 - 673 K temperature range in polycrystalline Pb0.50Sr0.50TiO3 thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb0.50Sr0.50TiO3 thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO3 perovskite.

Study of potassium additive on the phase formation and ferroelectric properties of 0.9PMN-0.1PT ceramics

A recent and innovative method to include Ti into the columbite precursor has permitted to synthesize 0.9PMN-0.1PT powders with high homogeneity. The present work describes this methodology, named modified columbite method, showing that the reaction between MN(T)and PbO at 800 degrees C for 2 h results in perovskite single-phase. The crystal structure alterations in the columbite and perovskite phases obtained by this methodology and the effects of potassium doping were investigated by the Rietveld method. Changes in the powder morphology, density and weight loss during the sintering process were also studied. Conclusively, potassium does not affect significantly the perovskite amount, but reduces the particle and grain sizes. This dopant also changes the relaxor behavior of 0.9PMN-0.1 PT ceramic, reducing the dielectric loss and enhancing the diffuseness of the phase transition. (C) 2005 Published by Elsevier Ltd and Techna Gronp S.r.l.

Lanthanum cobaltite black pigments with perovskite structure

In this work La1-xCaxCoO3 (x = 0-0.4) pigments were synthesized by the polymeric precursor method with heat treatments at 700, 800 and 900 C for 4 h. The powders were characterized by colorimetry, UV-vis spectroscopy and powder X-ray diffraction (XRD). The X-ray diffraction patterns showed the presence of a single phase perovskite, changing its structure from rhombohedral to cubic, when calcium was added to the lattice. All of the pigments had a black colour with a strong absorption over the whole of the visible spectrum as a consequence of the different oxidation states of cobalt and the high short-range disorder. The substitution of Ca2+ for La3+ did not influence the pigment colour but decreased its final cost. © 2013 Elsevier Ltd. All rights reserved.

Structural refinement, optical and ferroelectric properties of microcrystalline Ba(Zr0.05Ti0.95)O-3 perovskite

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Modeling the crystallographic structure of Ho(Ni,Co,Mn)O3±δ perovskite-type manganite

Crystallographic and microstructural properties of Ho(Ni,Co,Mn)O3± perovskite-type multiferroic material are reported. Samples were synthesized with a modified polymeric precursor method. The synchrotron X-ray powder diffraction (SXRPD) technique associated to Rietveld refinement method was used to perform structural characterization. The crystallographic structures, as well as microstructural properties, were studied to determine unit cell parameters and volume, angles and atomic positions, crystallite size and strain. X-ray energies below the absorption edges of the transition metals helped to determine the mean preferred atomic occupancy for the substituent atoms. Furthermore, analyzing the degree of distortion of the polyhedra centered at the transitions metal atoms led to understanding the structural model of the synthesized phase. X-ray photoelectron spectroscopy (XPS) was performed to evaluate the valence states of the elements, and the tolerance factor and oxygen content. The obtained results indicated a small decrease distortion in structure, close to the HoMnO3 basis compound. In addition, the substituent atoms showed the same distribution and, on average, preferentially occupied the center of the unit cell.