SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Transition metal interstitial compounds thin film preparation and superconducting properties, emphasizing high critical temperature, rock salt structure and NbN base
Small transition metal clusters were studied at a high level of approximation, including all the valence electrons in the calculation and extensive electron correlation, in order to understand the electronic structure of these small metal clusters. By comparison of dimers, trimers, and possibly higher clusters, the information obtained was used to provide insights into the electronic structure of bulk transition metals. Small metal clusters are currently of considerable experimental interest and some information is becomming available both from matrix electron spin resonance studies and from gas phase spectroscopy. Collaboration between theorists and experimentalists is thus expected to be especially profitable at this time since there is some experimental information which can serve to guide the theoretical work.
Small transition metal clusters at a high level of approximation i.e. including all the valence electrons in the calculation and also including extensive electron correlation were studied. Perhaps the most useful end result of these studies is the qualitative information about the electronic structure of these small metal clusters, including the nature of the bonding. The electronic structure studies of the small clusters are directly applicable to problems in catalysis. From comparison of dimers, trimers and possibly higher clusters, it is possible to extrapolate the information obtained to provide insights into the electronic structure of bulk transition metals and their interaction with other atoms and molecules at both surface and interior locations.
It is pointed out that transition metals and transition metal (TM) compounds are currently of considerable interest because of their relevance to catalysis and to materials science problems such as hydrogen embrittlement and crack propagation in metals. The present paper is concerned with complete active space Self-Consistent Field (SCF) externally contracted configuration interaction (CASSCF/CCI) calculations for the low-lying states of Sc3 and Sc3(+). A comparison is conducted regarding the bonding in the Ca3, Sc3, and Cu3 molecules. This comparison makes it possible to predict general trends for the TM trimers. Attention is given to the qualitative features of the bonding in the TM trimers, the basis sets and other technical details of the calculations, the calculated results for Sc3 and Sc3(+), and conclusions from this work.
Transition metal complexes of 8-aminoquinoline
The transition-metal oxides are quite ionic; Mulliken population analyses for several oxides give a negative charge of about 0.7 electrons for oxygen. When the transition-metal d shell is only partially filled, the orbitals are involved in covalent bonds with O; both two-electron bonding (2)-antibonding (0) and one-electron bonding (2)-antibonding (1) bonds are formed. These covalent bonds occur in addition to the ionic bonding. There is d-sigma-O2 p sigma repulsion, and this repulsion is reduced when the d-sigma electron is promoted into an orbital which has dominantly 4sp-sigma character and is polarized away from O.
Transition-metal additives in cathodes of Na/NiCI(2) high-temperature, rechargeable electrochemical cells found to slow premature fading of charge/discharge capacity. Decline in capacity of cell attributed to agglomeration of Ni particles at cathode: this agglomeration reduces electrochemical area of cathode. Depending on choice of transition-metal additive for particular cell, additive might even participate in desired electrochemical reactions in cell, contributing to specific energy of cell.
The vibrational Raman spectrum of solid hydrogen has been measured from 77 to 295 K in the vicinity of the recently observed insulator-metal transition and low-temperature phase transition at 150 gigapascals. The measurements provide evidence for a critical point in the pressure-temperature phase boundary of the low-temperature transition. The result suggests that below the critical temperature the insulator-metal transition changes from continuous to discontinuous, consistent with the general criteria originally proposed by Mott (1949) for metallization by band-gap closure. The effect of temperature on hydrogen metallization closely resembles that of the lower-pressure insulator-metal transitions in doped V2O3 alloys.
The determination of accurate spectroscopic parameters for molecular systems containing transition metal atoms is shown to require extensive data sets and a high level correlation treatment, and techniques and their limitations are considered. Extensive results reported on the transition metal atoms, hydrides, oxides, and dimers makes possible the design of a calculation to correctly describe the mixing of different atomic asymptotes, and to give a correct balance between molecular bonding and exchange interactions. Examples considered include the dipole moment of the 2Delta state of NiH, which can help determine the mixture of 3d(8)4s(2) and 3d(9)4s(1) in the NiH wavefunction, and the bonding in CrO, where an equivalent description of the relative energies associated with the Cr 3d-3d atomic exchange and the Cr-O bond is important.
Electron-phonon interaction superconductivity in transition metals
Theoretical dissociation energies for the first- and second-row transition metal hydride positive ions are critically compared against recent experimental values obtained from ion beam reactive scattering methods. Theoretical spectroscopic parameters and dipole moments are presented for the ground and several low-lying excited states. The calculations employ large Gaussian basis sets and account for electron correlation using the single-reference single- and double-excitation configuration interaction and coupled-pair-functional methods. The Darwin and mass-velocity contributions to the relativistic energy are included in the all-electron calculations on the first-row systems using first-order perturbation theory, and in the second-row systems using the Hay and Wadt relativistic effective core potentials. The theoretical D(0) values for the second-row transition metal hydride positive ions should provide a critical measure of the experimental values, which are not as refined as many of those in the first transition row.
Sputtering technique forms transition-metal interstitial compounds into superconducting thin films having transition temperatures similar to those of the bulk materials. Since the magnetic-field and current-carrying properties of the films exceed those of the bulk materials, they may have applications other than in tunneling devices.
Metal-nonmetal transition in n- and p-type Si from high magnetic field galvanometric measurements, noting abnormally large magnetoresistance accompanied by Hall coefficient increase
In approaching the extremely involved and complex problem of the origin of life, consideration of the coordination chemistry appeared not only as a possibility but as a necessity. The first model experiments appear to be promising because of prebiotic-type synthesis by means of transition-metal complexes. It is especially significant that in some instances various types of vitally important substances (nucleic bases, amino acids) are formed simultaneously. There is ground to hope that systematic studies in this field will clarify the role of transition-metal complexes in the organizatorial phase of chemical evolution. It is obvious that researchers working in the fields of the chemistry of cyano and carbonyl complexes, and of the catalytic effect of transition-metal complexes are best suited to study these aspects of the attractive and interesting problem of the origin of life.
The electronic structure of the transition metal (TM) trimers was studied by comparing the bonding in the Ca3, Sc3, and Cu3 molecules. The complete active space SCF/externally contracted configuration interaction (CI) ratio for the low-lying states of Sc3 and Sc3(+) and the SCF/CI ratio for Ca3 and Cu3 trimers, all for near equilateral triangle geometries, were calculated. In addition, vertical excitation energies for Cu3 were computed, leading to a new assignment of the upper state in the resonant two-photon ionization spectrum. Based on these studies, bonding in other TM trimers was discussed.
Spectroscopic parameters (D sub e, r sub e, mu) are determined for the first-row transition metal hydrides using better than DZP basis sets at the modified coupled pair functional (MCPF) level. Extensive comparisons between MCPF and complete-active space self-consistent field (CASSCF)/MRCI calculations with natural orbital iterations, and studies with more extensive basis sets, show this level of treatment to supply an accurate and cost-effective treatment of these systems. For the transition metal hydrides, the bonding can arise from either the 3d(n)4s(2) or 3d(n + 1)4s(1) atomic asymptotes, or a mixture of both. Since the dipole moment arising from these two bonding mechanisms is very different, the dipole moment is found to be directly related to the 3d population. Thus, the magnitude of the dipole moments provides a sensitive test of the wave function, and gives insight into the nature of the bonding.
Crystal field theory applied transition metal impurities in semiconductors
Ab initio calculations have been carried out on the first- and second-row transition-metal mono- and dicarbonyl positive ions. The bonding in these systems is discussed in detail. Trends in the series of mono- and dicarbonyl ions and between the first- and second-row transition metals are explained in terms of a dominantly electrostatic bonding interaction and differences in metal ion state separations, ionization potentials, and s and d orbital sizes. Dissociation energies are presented and a detailed comparison is made with experimental data. Where reliable experimental data exists, agreement with the theoretical results is generally good.