Estudo DFT para a interação das moléculas de CO e NO com perovskitas de LaFePdO3 dopadas com Mn, Co e Ni

Recentemente, vem sendo desenvolvido o uso de catalisadores de metais preciosos suportados por óxidos do tipo perovskita em automóveis. Tais sistemas catalíticos são conhecidos como catalisadores Inteligentes. A tecnologia dos catalisadores inteligentes aponta para um novo futuro na catálise automotiva e surge como um promissor substituinte para os catalisadores convencionais. O entendimento dos principais fatores que levam a auto regeneração destes catalisadores é um passo fundamental no processo de evolução desta tecnologia. O mecanismo de auto regeneração é responsável diretamente pelo aumento considerável do tempo de vida útil destes catalisadores perante aos convencionais. Consequentemente, o seu custo é bem mais baixo comparado ao convencional. Outro fator relevante é a durabilidade estrutural e o grande número de possibilidade de combinações possíveis das perovskita que fazem delas excelentes estruturas para estudo. O objetivo do trabalho é entender o processo auto regenerativo do catalisador automotivo a base de perovskita dopadas com um átomo de cobalto, manganês e níquel e quando expostas a um ambiente com uma molécula de NO e CO , através da análise da interação desses átomos dopantes em relação a estrutura da perovskita e como se comportará o átomo de paládio ao entrar em contato com a molécula de NO e CO

Magnetocaloric effect in nano- and polycrystalline manganite $La_{0.7}Ca_{0.3}MnO_3$

$La_{0.7}Ca_{0.3}MnO_3$ samples were prepared in nano- and polycrystalline forms by sol-gel and solid state reaction methods, respectively, and structurally characterized by synchrotron X-ray diffraction. The magnetic properties determined by ac susceptibility and dc magnetization measurements are discussed. The magnetocaloric effect in this nanocrystalline manganite is spread over a broader temperature interval than in the polycrystalline case. The relative cooling power of the poly- and nanocrystalline manganites is used to evaluate a possible application for magnetic cooling below room temperature.

Magnetocaloric effect in La0.75Sr0.25MnO3 manganite

The polycrystalline manganite La0.75Sr0.25MnO 3 prepared by an alternative carbonate precipitation route reveals the rhombohedral perovskite structure. Magnetization isotherms measured up to 2 T are used to determine Curie temperature of 332 K by means of Arrott plot. Maximum of magnetic entropy change is found at Curie temperature. The relative cooling power equal to 64 J/kg for 1.5 T magnetic field, is superior as compared to the manganite with the same chemical composition from the solgel method. © 2010 Elsevier B.V. All rights reserved.

Local structure of Mn in (La1-xHox) 2/3Ca1/3MnO3 studied by X-ray absorption fine structure

Results of X-ray absorption fine structure measurements in manganites (La1-xHox)2/3Ca1/3MnO3 with 0.15 < x < 0.50 are presented. When LaMnO3 is doped with a, divalent element such as Ca2+, substituting for La3+, holes are induced in the filled Mn d orbitais. This leads to a, strong ferromagnetic coupling between Mn sites. Ca ions in La1-xCa xMnO3 introduce a distortion of the crystal lattice and mixed valence Mn ions (Mn3+ and Mn4+). On the other hand, in manganites (La1-xHox)2/3Ca 1/3MnO3 the substitution of La for Ho causes a lattice distortion and induces a disorder, which reduces a magnetic interaction. The ferromagnetic transition temperature and conductivity decrease very quickly with increasing x. The magnetic and transport properties of compounds depend on the local atomic structure around Mn ions. The information on the bond lengths and Debye-Waller factor are obtained from the extended X-ray absorption fine structure (EXAFS) data analysis. The charge state of Mn is determined from the position of the absorption edge in X-ray absorption near edge structure (XANES) data. XAFS results are in good agreement with magnetic characteristics of the studied materials.

Magnetocaloric effect in nano- and polycrystalline manganite La 0.7Ca0.3MnO3

La0.7Ca0.3MnO3 samples were prepared in nano- and polycrystalline forms by the sol-gel and solid state reaction methods, respectively, and structurally characterized by synchrotron X-ray diffraction. The magnetic properties determined by ac susceptibility and dc magnetization measurements are discussed. The magnetocaloric effect in this nanocrystalline manganite is spread over a broader temperature interval than in the polycrystalline case. The relative cooling power of the poly- and nanocrystalline manganites is used to evaluate a possible application for magnetic cooling below room temperature. © 2007 Springer-Verlag.

Spin-cluster effect and lattice-deformation-induced Kondo effect, spin-glass freezing, and strong phonon scattering in La 0.7 Ca 0.3Mn 1-xCr xO 3

Besides the Kondo effect observed in dilute magnetic alloys, the Cr-doped perovskite manganate compounds La0.7 Ca0.3 Mn1-x Crx O3 also exhibit Kondo effect and spin-glass freezing in a certain composition range. An extensive investigation for the La0.7 Ca0.3 Mn1-x Crx O3 (x=0.01, 0.05, 0.10, 0.3, 0.6, and 1.0) system on the magnetization and ac susceptibility, the resistivity and magnetoresistance, as well as the thermal conductivity is done at low temperature. The spin-glass behavior has been confirmed for these compounds with x=0.05, 0.1, and 0.3. For temperatures above Tf (the spin-glass freezing temperature) a Curie-Weiss law is obeyed. The paramagnetic Curie temperature θ is dependent on Cr doping. Below Tf there exists a Kondo minimum in the resistivity. Colossal magnetoresistance has been observed in this system with Cr concentration up to x=0.6. We suppose that the substitution of Mn with Cr dilutes Mn ions and changes the long-range ferromagnetic order of La0.7 Ca0.3 MnO3. These behaviors demonstrate that short-range ferromagnetic correlation and fluctuation exist among Mn spins far above Tf. Furthermore, these interactions are a precursor of the cooperative freezing at Tf. The "double bumps" feature in the resistivity-temperature curve is observed in compounds with x=0.05 and 0.1. The phonon scattering is enhanced at low temperatures, where the second peak of double bumps comes out. The results indicate that the spin-cluster effect and lattice deformation induce Kondo effect, spin-glass freezing, and strong phonon scattering in mixed perovskite La0.7 Ca0.3 Mn1-x Crx O3. © 2005 American Institute of Physics.

Spin glass behaviour and spin-dependent scattering in la 0.7Ca 0.3Mn 0.9Cr 0.1O 3 perovskites

The magnetic, electrical and thermal transport properties of the perovskite La 0.7Ca 0.3Mn 0.9Cr 0.1O 3 have been investigated by measuring dc magnetization, ac susceptibility, the magnetoresistance and thermal conductivity in the temperature range of 5-300K. The spin glass behaviour with a spin freezing temperature of 70 K has been well confirmed for this compound, which demonstrates the coexistence and competition between ferromagnetic and antiferromagnetic clusters by the introduction of Cr. Colossal magnetoresistance has been observed over the temperature range investigated. The introduction of Cr causes the "double-bump" feature in electrical resistivity ρ(T). Anomalies on the susceptibility and the thermal conductivity associated with the double-bumps in ρ(T) are observed simultaneously. The imaginary part of ac susceptibility shows a sharp peak at the temperature of insulating-metallic transition where the first resistivity bump was observed, but it is a deep-set valley near the temperature where the second bump in ρ(T) emerges. The thermal conductivity shows an increase below the temperature of the insulating-metallic transition, but the phonon scattering is enhanced accompanying the appearance of the second peak of double-bumps in ρ(T). We relate those observed in magnetic and transport properties of La 0.7Ca 0.3Mn 0.9Cr 0.1O 3 to the spin-dependent scattering. The results reveal that the spin-phonon interaction may be of more significance than the electron (charge)-phonon interaction in the mixed perovskite system. © 2005 Chinese Physical Society and IOP Publishing Ltd.

Effects of silicon addition on the electrical and magnetic properties of copper-doped (La,Ca)MnO3 compounds

In this paper we report about the electrical properties of La 0.7Ca0.3MnO3 compounds substituted by copper on the manganese site and/or deliberately contaminated by SiO2 in the reactant mixture. Several phenomena have been observed and discussed. SiO2 addition leads to the formation of an apatite-like secondary phase that affects the electrical conduction through the percolation of the charge carriers. On the other hand, depending on the relative amounts of copper and silicon, the temperature dependence of the electrical resistivity can be noticeably modified: our results enable us to compare the effects of crystallographic vacancies on the A and B sites of the perovskite with the influence of the copper ions substituted on the manganese site. The most original result occurs for the compounds with a small ratio Si/Cu, which display double-peaked resistivity vs. temperature curves. © 2003 Elsevier B.V. All rights reserved.

Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C12H 25NH3)2PbI4(1-y)Br4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices. © 2013 AIP Publishing LLC.

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.

Studies of tetragonal PbTiO3 subjected to uniaxial stress along the c-axis

Tetragonal PbTiO3 under uniaxial stress along the c-axis is investigated from first-principles. The structural parameters, polarization, and squares of the lowest optical phonon frequencies for E(1TO) and A(1)(1TO) modes at Gamma show abrupt changes near a stress sigma(c) of 1.04 GPa, which is related to the dramatic change of elastic constant c(33) resulting from the uniaxial stress applied along the c-axis. We also find that the uniaxial compressive stress could enhance the piezoelectric stress coefficients, whereas the uniaxial tensile stress could enhance the piezoelectric strain coefficients. It is also found that when the magnitude of uniaxial compressive stress sigma(33) is greater than 12 GPa, PbTiO3 is transformed to the paraelectric tetragonal phase.

Third-order optical nonlinearities of lead-free (Na1-xKx)(0.5)Bi0.5TiO3 thin films

(Na1-xKx)(0.5)Bi0.5TiO3 (NKBT) (x = 0.1, 0.2, and 0.3) thin films with good surface morphology and rhombohedral perovskite structure were fabricated on quartz substrates by a sol-gel process. The fundamental optical constants (the band gaps, linear refractive indices and absorption coefficients) of the films were obtained through optical transmittance measurements. The nonlinear optical properties were investigated by Z-scan technique performed at 532 nm with a picosecond laser. A two-photon absorption effect closely related with potassium-doping content was found in thin films, and the nonlinear refractive index n(2) increases evidently with potassium-doping. The real part of the third-order nonlinear susceptibility chi((3)) is much larger than its imaginary part, indicating that the third-order optical nonlinear response of the NKBT films is dominated by the optical nonlinear refractive behavior. These results show that NKBT thin films have potential applications in nonlinear optics. (C) 2007 Elsevier B.V. All rights reserved.

钙钛矿过渡金属氧化物电、磁性质的理论研究

钙钛矿过渡金属氧化物已有大量实验和理论研究。本论文采用一般梯度近似（GGA）和GGA+U（U表示原位的库仑相互作用）下的第一性原理密度函数方法研究了双层，四层和含氧空位的钙钛矿过渡金属氧化物的晶体结构、电子结构以及电、磁性质。 从对双层钙钛矿Sr2FeMoO6和Sr2CoMoO6的研究，我们发现Sr2FeMoO6的四方相比立方相稳定，而且两种结构下它都显半金属特性；对于Sr2CoMoO6，原位的库仑相互作用决定了它的半导体性质。此外，我们还研究了实验上备受争议的Ba2YIrO6和Ba2LaIrO6在立方 Fm-3m, 菱形 R-3和单斜 P21/n三种结构下的相对稳定性。结果表明第一性原理与半经验的键价模型得到的结论相同，即Ba2YIrO6和Ba2LaIrO6的最稳定结构分别是单斜 P21/n和菱形R-3。 不同Mn-O-Mn角度下YBaMn2O5的电子结构和磁结构的计算结果表明，当Mn-O-Mn 角度处于实验所测的157.8o时，G-型反铁磁结构比A-型稳定，与实验结果相符。随着角度的增加，大约在170出现了磁结构转变。当角度大于170时，A型反铁磁结构比G型稳定，即YBaMn2O5从G型过渡到A型。此外，我们还研究了YBaMn2O5在不同磁结构以及不同角度下的导电性。 通过对四层钙钛矿化合物CaCu3M4O12 （M是3d过渡金属离子：Ti, V, Cr, Mn, Fe, Co）的能带结构计算研究了M离子的电子构型对其磁结构和导电性的影响。结果表明随着M电子数的增加，该系列化合物磁结构为：在CaCu3Ti4O12（Ti4+：d0）中Cu-Cu为反铁磁性耦合，即该物质为反铁磁体；在CaCu3M4O12 (M= V4+：d1, Cr4+：d2, Mn4+：d3, Fe4+：d4；dn，0

层状钙钛矿结构化合物的合成、电性和磁性的研究

近年来，随着金属多层膜，磁隧道结和钙钦矿锰氧化物等材料中磁阻现象的发现，以研究磁阻效应的机理和应用为目的的磁电子学迅速发展。这其中钙钦矿结构的稀土锰氧化物以其超大的磁阻值和丰富的物理内涵而备受瞩目。尽管人们对此已做了大量的工作，但是对这类氧化物的深入认识直至超大磁电阻效应物理机制的合理解释仍需做艰苦细致的努力。本论文选择层状钙钦矿稀土锰氧化物作为研究对象，系统地研究了A位，B位的变化和B位原子之间的相互作用对氧化物的结构、磁性和磁阻性质的影响。希望得到钙钦矿中磁、电性质和磁阻与结构之间的关系，能对该系列化合物中电、磁性质的变化规律和相互作用机理作出合理的解释。基于LaSr2Mn2O7的结构特殊性，我们选择了它作为母体化合物。并通过过渡金属离子Cr，Ti，Ni，Fe对Mn离子取代来研究B位原子的变化对性质的影响。结果发现，Cr3＋因为与Mn4＋具有相同的电子形态而能够参与双交换作用，使掺杂Cr3＋的系列样品的磁化强度随cr含量的增加而增加。而掺杂讨＋，Ni2＋，Fe3＋离子的化合物虽然与Mn离子之间的相互作用各不相同，但引起的磁性变化却是相同的。这四种元素的掺杂都提高了体系的磁阻和电阻率。通过对这几种过渡金属取代的比较，发现在LaS2Mn2O7中对Mn离子进行取代的离子和Mn离子之间的交换作用对磁性质的影响并不起主要作用，掺杂引起的主要作用是致使Mn位的无序度增加和对双交换作用的稀释和阻碍。值得注意的是每个系列样品中都有一个样品的磁阻在高温时出现较大的正值，且随着温度的降低转变为负值。例如，在Fe掺杂的系列样品中，只有x＝0.2的样品表现出正磁阻，且MR在28OK时达到74％。这可能是因为掺杂导致的结构变化引起的。这种正磁阻对材料的应用意义重大。电荷有序对磁阻材料是一种很重要的状态，为了提高LasrZMn2O7的电荷有序温度，我们选择了具有孤对电子的Bi3+来取代Sr2+。结果发现，单相样品只能持续到x≤0.2。样品的电荷有序温度并没有象预想的那样有所提高。这是因为体系的二维结构抑止了Bi3＋离子的作用，同时由于体系中Bi3＋的含量较少没有达到提高电荷有序温度的程度。但Tco降低的程度相对于其它离子的取代效果（如Gd）要低。目前n=3的层状钙钦矿研究较少，但是由于该化合物具有结构可变性和理论上可以解释磁转换机理，我们对（La，ca）4kMn3O10进行了深入的研究。在La3-3xCa1＋3xMn3O10（0.5≤x≤1.0）中随La3＋含量的减少，该系列化合物经历了从铁磁性到顺磁性再到反铁磁性的转变，同时在磁阻上也经历了由负磁阻（x＝0.5～0.7）到正磁阻（x=0.8-1.0）的转变。根据磁性和电性的变化规律，我们认为这种正负磁阻的转变是由于体系中超交换和双交换作用的相互竞争引起的。La3＋含量多时，Mn3十离子含量较多，双交换作用占主导地位，产生负磁阻；随着Mn3+离子含量的减少，双交换作用逐渐减弱，Mn4+离子之间的反铁磁性超交换作用逐渐增强，产生了正磁阻。在低掺杂浓度时LaxCa4-xMn3O10（x＝0-0.9）经历了顺磁性到反铁磁性的转变，为了了解其磁性变化过程，我们进一步研究了富含Mn4＋的这一区间。发现磁化强度在x≤0.2的范围内随x增加而增强，在高于0.2的掺杂范围后随x的增加而逐渐降低。这是因为这一区间的磁结构由基态时的G型-AFM向x＝0.9时的C型-AFM的转变。而且这种转变与载流子浓度密切相关。

Electronic structure of microporous titanosilicate ETS-10

The electronic structure of a microporous titanosilicate framework, ETS-10 is calculated by means of a first-principles self-consistent method. It is shown that without the inclusion of the alkali atoms whose positions in the framework are unknown, ETS-10 is an electron deficient system with 32 electrons per unit cell missing at the top of an otherwise semiconductor-like band structure. The calculated density of slates are resolved into partial components. It is shown that the states of the missing electrons primarily originate from the Ti-O bond. The local density of states of the Ti-3d orbitals in the ETS-10 framework is quite different from the perovskite BaTiO3. The possibilities of ETS-10 crystal being ferroelectric or having other interesting properties are discussed.