Biblioteca Digital

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...

Neste trabalho foram avaliadas as atividades de catalisadores do tipo perovskita LaNi1-xCoxO3 frente à reação de reforma a vapor de etanol. Devido à baixa área superficial, característica de óxidos do tipo perovskita, esses foram suportados em SiO2, Al2O3 e ZrO2, a fim de verificar o efeito do suporte na atividade catalítica.
Os catalisadores foram preparados pelo método da co-precipitação e caracterizados por espectrometria de emissão atômica por plasma induzido, difração de raios X pelo método do pó, adsorção de nitrogênio pelo método B.E.T, redução a temperatura programada e espectroscopia de absorção de raios X.
Para estudar o processo de redução e a possibilidade de oxidação durante a reação catalítica, foram realizados estudos in situ da reação de reforma a vapor, através da espectroscopia de absorção de raios X. Estes dados foram comparados com os resultados de aplicação das técnicas de RTP e DRX às amostras parcial e totalmente reduzidas e foi proposto um mecanismo de redução do óxido do tipo perovskita durante o processo de ativação do catalisador.
Todos os catalisadores mostraram-se ativos nas reações de reforma a vapor de etanol e a seletividade dos produtos foi dependente do tipo do catalisador avaliado. De acordo com os resultados obtidos...

The nonohmic electrical features of (Ca-1/4,Cu-3/4)TiO3 perovskite ceramics, which have very strong gigantic dielectric is believed originate from potential barriers at the grain boundaries. In the present study, we used the admittance and impedance spectroscopy technique to investigate (Ca-1/4,Cu-3/4)TiO3 perovskite ceramics with low nonohmic electrical properties. The study was conducted under two different conditions: on as-sintered ceramics and on ceramics thermally treated in an oxygen-rich atmosphere. The results confirm that thermal treatment in oxygen-rich atmospheres influence the nonohmic properties. Annealing at oxygen-rich atmospheres improve the nonohmic behavior and annealing at oxygen-poor atmospheres decrease the nonohmic properties, a behavior already reported for common metal oxide nonohmic devices and here firstly evidenced for the (Ca-1/4,Cu-3/4)TiO3 perovskite related materials. The results show that oxygen also influences the capacitance values at low frequencies, a behavior that is indicative of the Schottky-type nature of the potential barrier. (c) 2006 Elsevier Ltd. All rights reserved.

The mechanical activation is one of the most effective method for obtaining highly disperse system due to mechanical action stress fields form in solids during milling procedure. This effect results in changes of free energy, leading to release of heat, formation of a new surface, formation of different crystal lattice defects and initiation of solid-state chemical reaction. The accumulated deformation energy determines irreversible changes of crystal structure and consequently microstructure resulting in the change of their properties. Mechanochemical processing route has been developed recently for the production of intermetallic and alloy compounds. The intrinsic advantage of this process is that the solid-state reaction is activated due to mechanical energy instead of the temperature. It was shown that the chemical reactivity of starting materials could be improved significantly after mechanochemical activation and, subsequently, the calcination temperature was reduced. Besides, it was apparent that the mechanochemical treatment could enhance the reactivity of constituent oxides; however, the sintering process could not be avoided to develop the desired ceramics. A novel mechanochemical technique for synthesis of fine-grained perovskite structured powders has shown that it is possible to form perovskite at room temperature. The effect of milling on the formation of perovskite structure of barium titanate (BT)...

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Strontium stannate (SrSnO(3) a perovskite material, was synthesized by the polymeric precursor method, with different routes to eliminate organic matter. The utilization of pure oxygen for the elimination of organic matter changed the Raman spectra, especially in the low frequency region. Some peaks, which were previously assigned to the perovskite phase, were not noticed when the carbonate amount was lower. on the other hand, the profile of the IR spectra and XRD patterns did not change. These results suggest that carbonate may be present inside the perovskite lattice. (C) 2008 Elsevier B.V. All rights reserved.

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); In a previous article, we used Hartree-Fock (HF) theory to study the piezoelectricity in BaTiO(3). In this paper, we applied the Douglas-Kroll-Hess second order scalar relativistic method to investigate the possible piezoelectric properties in the perovskite YFeO(3) structure, which has not yet been studied experimentally. The 30s20p13d and 31s21p17d Gaussian basis sets for the Fe ((5)D) and Y ((2)D) atoms, respectively, were built with the Generator Coordinate HF method. After contraction to [13s7p5d] and [13s8p7d], in combination with the 20s14p/6s4p basis set for the O ((3)P) atom from literature, they had their quality evaluated using calculations of the total and the orbital energies for the (2)FeO(+1) and (1)YO(+1) fragments. The dipole moment, the total energy, and the total atomic charges in YFeO(3) in C(s) space group were calculated. The results and the analysis lead us to believe that the perovskite YFeO(3) does not present piezoelectric properties.

The phase evolution of lead titanate processed by the polymeric precursor method was investigated by thermal analysis, X-ray diffraction, and high-resolution transmission electron microscopy. The results showed that the cubic perovskite PbTiO3 (PT) phase is formed from an inorganic amorphous precursor at a temperature of 444 °C. A gradual transition from cubic to tetragonal perovskite PT was observed with the increase of calcination time at this temperature. HRTEM results showed that the cubic PT particles have a size of around 5 nm. The identification of cubic PT as an intermediate phase supports the hypothesis that the chemical homogeneity was kept at the molecular level during the synthesis process, with no cation segregation.

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.

Um grande número de óxidos de metais de transição se forma em estrutura perovskita simples ou em uma de suas variantes, e muitos deles apresentam propriedades físicas interessantes, como supercondutividade, magneto-resistência colossal e ferroeletricidade. Uma das variantes da estrutura perovskita simples, a perovskita dupla ordenada (PDO), pode se cristalizar em simetria cúbica ou em variantes distorcidas, e possui fórmula geral A2B?B" O6, em que o íon A ocupa os vértices do cubo enquanto os cátions B? e B?? se alternam nos centros dos octaedros de oxigênio. Dentre os compostos com estrutura perovskita dupla já reportados na literatura, Sr2FeRe O6e Sr2FeMoO6são particularmente interessantes devido à suas interessantes propriedades físicas e à sua potencialidade como dispositivos magneto-eletrônicos. As propriedades estruturais, de transporte e magnéticas desses materiais estão altamente conectadas, e acredita-se que essas propriedades das PDO sejam fortemente dependentes do grau de hibridização dos orbitais d dos cátions B" (ex.: Re e Mo). Portanto, para se comprovar a potencialidade dos compostos PDO como dispositivos magnetoeletrônicos, além entender os mecanismos microscópicos responsáveis por suas propriedades magnéticas e eletrônicas...

Escócia; The perovskite-type oxide with general formula ABO3 is a promising material as catalyst for the ethanol conversion reactions to hydrogen. These mixed oxides are able to produce highly dispersed metallic particles upon reduction, which inhibits the formation of carbon [1]. Recently, we have studied the performance of LaNiO3 perovskite-type oxide precursor as a catalyst for both steam reforming (SR) and oxidative steam reforming (OSR) of ethanol [2]. Catalyst deactivation took place depending on the reaction conditions and it was attributed to the deposition of carbon on the surface of the catalyst. The aim of this work was to study the effect of the preparation method of LaNiO3 perovskite-type oxide precursor on catalyst stability during SR and OSR of ethanol.

In this study, we investigated iron-magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO3 and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in

The Fe3+/∑Fe ratios in both (Al, Fe)-bearing MgSiO3 post-perovskite phase and Ca-ferrite-type Al-phase, synthesized in a natural mid-oceanic ridge basalt (MORB) composition at 113 GPa and 2240 K, were determined by electron energy-loss near-edge structu

This is the final version of the article. It was first available from ACS via http://dx.doi.org/10.1021/acsami.5b08038; Organometal halide (hybrid) perovskite solar cells have been fabricated following four different deposition procedures and investigated in order to find correlations between the solar cell characteristics/performance and their structure and composition as determined by combining depth-resolved imaging with time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-Ray photoelectron spectroscopy (XPS) and analytical scanning transmission electron microscopy (STEM). The interface quality is found to be strongly affected by the perovskite conversion methodology and in particular from the environment where the conversion is performed (air, nitrogen or vacuum). The conversion rate of the precursors into the hybrid perovskite is compared between the different solar cells by looking at the ToF-SIMS intensities of the characteristic molecular fragments from the perovskite and the precursor materials. Energy dispersive X-ray spectroscopy in the STEM confirms the macroscopic ToF-SIMS findings and allows elemental mapping with nanometer resolution. A clear evidence for iodine diffusion has been observed and related to the fabrication procedure.; We acknowledge Lucio Cin?...

This is the final version of the article. It first appeared from ACS via http://dx.doi.org/10.1021/acs.nanolett.5b00235; Electroluminescence in light-emitting devices relies on the encounter and radiative recombination of electrons and holes in the emissive layer. In organometal halide perovskite light-emitting diodes, poor film formation creates electrical shunting paths, where injected charge carriers bypass the perovskite emitter, leading to a loss in electroluminescence yield. Here, we report a solution-processing method to block electrical shunts and thereby enhance electroluminescence quantum efficiency in perovskite devices. In this method, a blend of perovskite and a polyimide precursor dielectric (PIP) is solution-deposited to form perovskite nanocrystals in a thin-film matrix of PIP. The PIP forms a pinhole-free charge-blocking layer, while still allowing the embedded perovskite crystals to form electrical contact with the electron- and hole-injection layers. This modified structure reduces nonradiative current losses and improves quantum efficiency by 2 orders of magnitude, giving an external quantum efficiency of 1.2%. This simple technique provides an alternative route to circumvent film formation problems in perovskite optoelectronics and offers the possibility of flexible and high-performance light-emitting displays.; The authors acknowledge funding from the Gates Cambridge Trust...

This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/aelm.201500008; Organometallic lead perovskite-based solar cells can be converted to light-emitting diodes by engineering the current density. Diodes are fabricated with adjacent perovskite and conjugated polymer layers using orthogonal solvents. Under forward bias, these devices show simultaneous emission from both the luminescent conjugated polymer and the perovskite, providing direct information on electron and hole recombination as a function of device architecture and bias voltage.; We gratefully acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC). A.K. acknowledges NRF-Singapore for a scholarship.

We have demonstrated that SrCoO3-δ can be stabilised into phase pure perovskite forms by the introduction of small amounts ∼5% of certain rare earth ions (Sm3+-Yb3+). At the same doping levels, La3+ and Pr3+ crystallise with the same isostructural trig

The elastic moduli of ScAlO3 perovskite, a very close structural analogue for MgSiO3 perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al2O3 measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO3 perovskite, the value of (partial differentialK(S)/partial differentialT)(P) is - 0.033 GPa K-1 and (partial differentialG/partial differentialT)(P) is -0.015 GPa K-1. The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895-1912.] that ζ(partial differentialK(S)/partial differentialT)(P)ζ is typically about twice ζ(partial differentialG/partial differentialT)(P)ζ. The value of (partial differentialK(S)/partial differentialT)(P) for ScAlO3 is intermediate between those inferred less directly from V(P,T) studies of Fe-free and Fe- and Al-bearing MgSiO3 perovskites [Wang...

The low-frequency mechanical properties of single crystal LaAlO3 have been investigated as a function of temperature, frequency and applied force using the technique of dynamical mechanical analysis (DMA) in three-point bend geometry. LaAlO3 undergoes a cubic to rhombohedral phase transition below 550°C. The mechanical response in the low-temperature rhombohedral phase is shown to be dominated by the viscous motion of transformation twin domain walls, resulting in a factor of 10 decrease in the storage modulus relative to the high-temperature cubic phase (super-elastic softening) and a significant increase in attenuation. Super-elastic softening is observed down to 200 °C, below which the mobility of the domain walls decreases markedly, causing a rapid increase in storage modulus and a pronounced peak in attenuation (domain wall freezing). The frequency dependence of the storage modulus close to the freezing temperature is accurately described by a modified Burgers model with a Gaussian distribution of activation energies with mean value 84.1(1) kJ/mol and S.D. 10.3(1)kJ/mol. This activation energy suggests that domain walls are pinned predominantly by oxygen vacancies. Detailed analysis of the dynamic force-deflection curves reveals three distinct regimes of mechanical response. In the elastic regime...

We report structural analysis, magnetic ordering, ferroelectric behavior, and the electronic structure of SR2TIMNO6 complex perovskite. Samples were produced by the solid state reaction recipe. Crystallographic analysis was performed by Rietveld refinement of experimental X-ray diffraction patterns. Results show that this material crystallizes in a perovskite tetragonal structure which corresponds to the space group I4/mmm. We obtain a matching of 99% between structural properties predicted by the density functional theory and structural ordering, found from the Rietveld refinement method. From field cooling measurements of susceptibility as a function of temperature, we determine the occurrence of a magnetic ordering for a critical temperature of 44.8 K. From a fitting with the molecular field theory of ferrimagnetism, we establish that the ordering corresponds to a paramagnetic-ferrimagnetic transition, which is corroborated by the behavior of the inverse of susceptibility as a function of temperature. The Curie constant allowed for us to obtain an effective magnetic moment of 3.5 mB. The result of magnetization as a function of the applied field, measured at T = 40 K, shows a ferromagnetic-hysteretic behavior. The curve of polarization as a function of applied voltage evidences a ferroelectric character. It is shown experimentally that below 44.8 K this perovskite material evidences a multiferroic feature. Ab initio calculations of density of states and band structure for this manganite-like material were carried out by means of the density functional theory (DFT) and the linearized augmented plane waves method for both spin orientations. For calculations...

LaNiO3 perovskite was prepared by the self-combustion method and tested as catalyst for CH4 activation using the oxygen lattice at 700°C and 800°C. Based on the non-stoichiometry experiments, the perovskite formula is written as La3+Ni3+0.37Ni2+0.63O2.68. When unreduced LaNiO3-d perovskite is used, only b-type oxygen species were responsible for the partial CH4 oxidation. Over non reduced perovskite high CH4 conversions to H2 and CO were obtained. CH4 conversion and H2, CO and CO2, selectivities under quasi-stationary conditions were 100%, 80%, 98% and 2 % respectively at 800°C. At the beginning of the reaction, a complete oxidation of methane to CO2 and H2O took place, whereas a partial oxidation to CO and H2 was observed after that period. In such conditions the H2/CO molar ratio obtained was 1.7, indicating a contribution of the parallel reverse water gas shift reaction