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1.
Recent computational studies of models for manganese oxides have revealed a rich phase diagram, which was not anticipated in early calculations in this context performed in the 1950s and 1960s. In particular, the transition between the antiferromagnetic insulator state of the hole-undoped limit and the ferromagnetic metal at finite hole density was found to occur through a mixed-phase process. When extended Coulomb interactions are included, a microscopically charged inhomogeneous state should be stabilized. These phase separation tendencies, also present at low electronic densities, influence the properties of the ferromagnetic region by increasing charge fluctuations. Experimental data reviewed here by applying several techniques for manganites and other materials are consistent with this scenario. Similarities with results previously discussed in the context of cuprates are clear from this analysis, although the phase segregation tendencies in manganites appear stronger. 相似文献
2.
Wahlström E Vestergaard EK Schaub R Rønnau A Vestergaard M Laegsgaard E Stensgaard I Besenbacher F 《Science (New York, N.Y.)》2004,303(5657):511-513
Diffusion of oxygen molecules on transition metal oxide surfaces plays a vital role for the understanding of catalysis and photocatalysis on these materials. By means of time-resolved scanning tunneling microscopy, we provide evidence for a charge transfer-induced diffusion mechanism for O2 molecules adsorbed on a rutile TiO2(110) surface. The O2 hopping rate depended on the number of surface donors (oxygen vacancies), which determines the density of conduction band electrons. These results may have implications for the understanding of oxidation processes on metal oxides in general. 相似文献
3.
Much theoretical work has been devoted to understanding the role of strong electron correlations in high-temperature superconductivity mainly through magnetic interactions, but the possible role of electron correlation in ferroelectricity of metal oxides has not received attention. Diagonalization of a simple many-body, tight-binding Hamiltonian shows that the electron-lattice interaction is dramatically enhanced in some cases by strong electron correlation because of deformation-induced charge transfer. This effect may be closely related to ferroelectricity and superconductivity in transition metal oxides. 相似文献
4.
Jamieson JC 《Science (New York, N.Y.)》1963,140(3562):72-73
At high pressures, as determined by x-ray analysis, titanium and zirconium metal have a distorted, body-centered-cubic structure. This phase persists on pressure release. The normal hexagonal close-packed structures are recovered when the metals are heated. An electronic shift must occur in the transition. Hafnium metal showed no such transition. 相似文献
5.
A new class of electrode materials for high energy density, rechargeable batteries based on topochemical reactions of lithium and transition metal compounds is evolving. The physical and structural properties relevant to the ability of transition metal oxides with framework structures to topochemically incorporate lithium are discussed. Perovskite-related structures are particularly attractive hosts for lithium. 相似文献
6.
Nanobelts of semiconducting oxides 总被引：2，自引：0，他引：2
Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts. 相似文献
7.
Venkataramani S Jana U Dommaschk M Sönnichsen FD Tuczek F Herges R 《Science (New York, N.Y.)》2011,331(6016):445-448
Magnetic bistability, as manifested in the magnetization of ferromagnetic materials or spin crossover in transition metal complexes, has essentially been restricted to either bulk materials or to very low temperatures. We now present a molecular spin switch that is bistable at room temperature in homogeneous solution. Irradiation of a carefully designed nickel complex with blue-green light (500 nanometers) induces coordination of a tethered pyridine ligand and concomitant electronic rearrangement from a diamagnetic to a paramagnetic state in up to 75% of the ensemble. The process is fully reversible on irradiation with violet-blue light (435 nanometers). No fatigue or degradation is observed after several thousand cycles at room temperature under air. Preliminary data show promise for applications in magnetic resonance imaging. 相似文献
8.
应用S.Yu.Davydov提出的石墨烯态密度模型,求出吸附在石墨烯上的碱金属原子的有效电荷数,研究了吸附原子的电子能级、能级移动量、有效电荷数随金属原子元素的变化以及有效电荷数随电子能量的变化规律.结果表明:(1)被吸附的碱金属原子的电子能级和能级移动量随原子序数的变化为非线性,在Li,Na,K,Rb,Se,Fr这6种碱金属原子中,以Na原子的值为最小,其原因在于碱金属原子的电离能以及石墨烯与吸附原子的相互作用能均随原子序数的增大而减小;(2)石墨烯能带电子和吸附原子的局域态电子对有效电荷的贡献以及总有效电荷数,均随原子序数的增加而非线性地减小.其中,能带电子对有效电荷的贡献与电子能量无关,而吸附原子局域态电子的贡献与总有效电荷数和电子能量都有关,且随电子能量的变化有明显的局域特点,最可几电子能量随原子序数的增大而增大. 相似文献
9.
Many commercially important catalysts consist of small metal particles dispersed on inorganic oxide surfaces. Although in most cases there is no significant interaction between the metal and the support, strong bonding can be demonstrated in a recently discovered class of supported-metal catalysts. These cases typically involve group VIII metals dispersed on transition metal oxides whose surfaces can be reduced to form cations with lower valences. Spectroscopic measurements indicate that an electron is transferred from the cation (such as Ti(3+) or Nb(4+)) to the metal particle. This, in turn, leads to profound changes in the catalytic and chemisorption properties and the morphology of the metal particles. 相似文献
10.
Younkin TR Connor EF Henderson JI Friedrich SK Grubbs RH Bansleben DA 《Science (New York, N.Y.)》2000,287(5452):460-462
More than half of the 170 million metric tons of polymers produced each year are polyolefins. Current technology uses highly active cationic catalysts, which suffer from an inability to tolerate heteroatoms such as oxygen, nitrogen, and sulfur. These systems require scrupulously clean starting materials and activating cocatalysts. A family of catalysts has been developed whose members are tolerant of both heteroatoms and less pure starting materials. These heteroatom-tolerant neutral late transition metal complexes are in fact highly active systems that produce high-molecular-weight polyethylene, polymerize functionalized olefins, and require no cocatalyst. 相似文献
11.
In a time of growing need for catalysts, perovskites have been rediscovered as a family of catalysts of such great diversity that a broad spectrum of scientific disciplines have been brought to bear in their study and application. Because of the wide range of ions and valences which this simple structure can accommodate, the perovskites lend themselves to chemical tailoring. It is relatively simple to synthesize perovskites because of the flexibility of the structure to diverse chemistry. Many of the techniques of ceramic powder preparation are applicable to perovskite catalysts. In their own right, they are therefore of interest as a model system for the correlation of solid-state parameters and catalytic mechanisms. Such correlations [See figure in the PDF file] have recently been found between the rate and selectivity of oxidation-reduction reactions and the thermodynamic and electronic parameters of the solid. For commercial processes such as those mentioned in the introduction, perovskite catalysts have not yet proven to be practical. Much of the initial interest in these catalysts related to their use in automobile exhaust control. Current interest in this field centers on noble metalsubstituted perovskites resistant to S poisoning for single-bed, dual-bed, and three-way catalyst configurations. The formulations commercially tested to date have shown considerable promise, but long-term stability has not yet been achieved. A very large fraction of the elements that make up presently used commercial catalysts can be incorporated in the structure of perovskite oxides. Conversely, it is anticipated that perovskite oxides, appropriately formulated, will show catalytic activity for a large variety of chemical conversions. Even though this expectation is by no means a prediction of commercial success in the face of competition by existing catalyst systems, it makes these oxides attractive models in the study of catalytic chemical conversion. By appropriate formulation many desirable properties can be tailored, including the valence state of transition metal ions, the binding energy and diffusion of O in the lattice, the distance between active sites, and the magnetic and conductive properties of the solid. Only a very small fraction of possible perovskite formulations have been explored as catalysts. It is expected that further investigation will greatly expand the scope of perovskite catalysis, extend the understanding of solid-state parameters in catalysis, and contribute to the development of practical catalytic processes. 相似文献
12.
The structural and electronic effects of lead substitution in the high-temperature superconducting materials Pb(x)Bi(2-x)Sr(2)CaCu(2)O(8) have been characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Large-area STM images of the Bi(Pb)-O layers show that lead substitution distorts and disorders the one-dimensional superlattice found in these materials. Atomic-resolution images indicate that extra oxygen atoms are present in the Bi(Pb)-O layers. STS data show that the electronic structure of the Bi(Pb)-O layers is insensitive to lead substitution within +/-0.5 electron volt of the Fermi level; however, a systematic decrease in the density of states is observed at approximately 1 electron volt above the Fermi level. Because the superconducting transition temperatures are independent of x(Pb) (x 相似文献
13.
Hagstrum HD 《Science (New York, N.Y.)》1972,178(4058):275-282
At this point it would be presumptuous to suggest either that we understand very much about the electronic structure of solid surfaces or that we can specify in detail exactly what each of our tools for studying such structure is telling us. I think it is fair to say, however, that FES, UPS, and INS do make it possible for us to determine energy level spectra which can with some confidence be ascribed to the resonances of electrons in surface orbitals. It is true that INS is the more surface-selective of the two electron spectroscopies capable of producing data over at least a 10-ev energy range. We have seen intriguing differences between INS and UPS which, when we come to understand them, will most certainly reveal important characteristics of surface electronic structure and greatly expand our ability to distinguish electronic states in the surface from those in the selvedge. Possibly it is not too much to hope that the combined use of INS and UPS with incidence angle as an independent variable will give us information on the geometrical extent of surface orbitals, as well as the net electrical charge and the electric potential gradient in the region of the surface in which the orbitals lie. 相似文献
14.
A mechanism to enhance hardness in multilayer coatings is proposed. Using the technologically important hard transition metal carbides as prototypes, although the principle is transferable also to other systems, we demonstrate, from first-principles calculations, that by suitable alloying the energy difference between several competing structures in the transition metal carbides is small or tunable. This creates multiphase/polytypic compounds with a random or controllable layer stacking sequence, systems in which the propagation of dislocations can be strongly suppressed by a large number of interfaces between structures with different glide systems, accordingly allowing the possibility of a greatly enhanced hardness. With modern thin-film technologies, it should therefore be possible to deposit such materials that will express multilayer characteristics with only minor changes in the chemical constitution of the material, which is in contrast to conventional superlattices. 相似文献
15.
Leone SR 《Science (New York, N.Y.)》1985,227(4689):889-895
Lasers are used in increasingly sophisticated ways to carry out reactions between molecules in selected vibrational, rotational, and electronic states and to probe the product states of chemical reactions. Such investigations are providing unprecedented insights into chemical reaction dynamics, the study of the detailed motions that molecules undergo in simple chemical reactions. In many cases it is possible to describe the influence that specific types of molecular excitation have on reactive events. Experiments are also being carried out to leam about chemical reactivity as a function of the alignment of reagents. There is increasing excitement concerning the potential of laser methods to interrogate the transition states of molecular reactions. 相似文献
16.
铁（氢）氧化物和腐殖质是广泛分布在土壤中的重要天然活性物质,因其具有较大的比表面积并且铁（氢）氧化物表面的-OH 与腐殖质表面的-COOH、-OH 等活性官能团可通过静电作用、配体交换等多种机制对重金属离子产生较强吸附,从而影响重金属离子在环境中的迁移、转化和生物效应。深入了解重金属离子在铁（氢）氧化物-腐殖质复合体微界面相互作用的分子机理,对于阐明重金属离子在环境中的迁移、转化过程具有重要意义。本文综述了金属离子在铁（氢）氧化物与腐殖质上吸附机理和模型的研究进展,为重金属污染土壤的风险评估和控制提供理论依据。 相似文献
17.
The conventional theory of metals is in crisis. In the past 15 years, there has been an unexpected sprouting of metallic states in low-dimensional systems, directly contradicting conventional wisdom. For example, bosons are thought to exist in one of two ground states: condensed in a superconductor or localized in an insulator. However, several experiments on thin metal-alloy films have observed that a metallic phase disrupts the direct transition between the superconductor and the insulator. We analyze the experiments on the insulator-superconductor transition and argue that the intervening metallic phase is bosonic. All relevant theoretical proposals for the Bose metal are discussed, particularly the recent idea that the metallic phase is glassy. The implications for the putative vortex-glass state in the copper oxide superconductors are examined. 相似文献
18.
Anderson PW 《Science (New York, N.Y.)》2000,288(5465):480-482
The layer-structure cuprates with high superconducting transition temperatures T(c) exhibit a number of anomalous electronic properties in both superconducting and normal states. These anomalies are ascribed to the existence of independent spectra of excitations for charge and for spin, signaling a collective state, a "quantum protectorate." 相似文献
19.
目前，四环素类抗生素在绝大多数环境介质中能被检测到，该类污染物对人类健康和生态环境系统稳定具有潜在威胁，因此对环境中这类污染物的治理任重道远。研究者们发现锰氧化物普遍存在于土壤、沉积物及荒漠岩漆中，其具有电荷零点低、表面活性强、氧化还原电位高等优点，在治理四环素类污染物具有较大研究空间及应用前景。本文简述四环素的危害及其污染现状，着重综述不同类型锰氧化物对环境中四环素类污染物的去除效果、降解机制及其环境影响因素，并对今后锰氧化物去除该类污染物的研究进行了展望。 相似文献
20.
Weissmüller J Viswanath RN Kramer D Zimmer P Würschum R Gleiter H 《Science (New York, N.Y.)》2003,300(5617):312-315
Dimension changes on the order of 0.1% or above in response to an applied voltage have been reported for many types of materials, including ceramics, polymers, and carbon nanostructures, but not, so far, for metals. We show that reversible strain amplitudes comparable to those of commercial piezoceramics can be induced in metals by introducing a continuous network of nanometer-sized pores with a high surface area and by controlling the surface electronic charge density through an applied potential relative to an electrolyte impregnating the pores. 相似文献