Late 3d Transition Metals Research Articles

Oxide formation at the surface of late 4d transition metals: insights from first-principles atomistic thermodynamics

Using density-functional theory we assess the stability of bulk and surface oxides of the late 4d transition metals in a ``constrained equilibrium'' with a gas phase formed of O2 and CO. While the stability range of the most stable bulk oxide extends for ruthenium well into gas phase conditions representative of technological CO oxidation catalysis, this is progressively less so for the 4d metals to its right in the periodic system. Surface oxides could nevertheless still be stable under such conditions. These thermodynamic considerations are discussed in the light of recent experiments, emphasizing the role of (surface) oxides as the active phase of model catalysts formed from these metals.

Generation of [Mm-phenyl]– (M = Mn–Cu) complexes in the gas phase: Metal cluster anions inducement of a selective benzene C–H cleavage

This article reports on the generation of [Mm-phenyl]− (M = Mn–Cu) complexes, which are very rare cases in chemistry. Experimental results showed that the typical cation products for the reactions of all 3d transition metal clusters (Sc–Cu) with benzene were different [Mm(C6H6)n]+ (M = Sc–Cu) species, while the typical anion products for the late 3d transition metals were [Mm–C6H5]− (M = Mn–Cu) complexes. Their formation mechanisms, which involve anion metal clusters inducing a selective benzene C–H cleavage in the gas phase, were proposed.

Morphology of nanoscaled LaMO 3-particles (M=Mn, Fe, Co, Ni) derived by citrate precursors in aqueous and alcoholic solvents

LaMO 3-type perovskites are promising ceramic materials used as electrode materials in fuel cells or as the catalytic active component in motorcar exhaust catalysts. In this work we present a systematic study on the morphology of LaMO 3-perovskites containing late 3d-transition metals prepared by thermal decomposition of the corresponding citrate precursors at the lowest possible temperature. The calcination temperature was determined by thermogravimetric analysis and XRD. The specific surface areas as well as the particle morphologies of the perovskite powders are investigated by SEM and compared with Brunauer-Emett-Teller (BET) surface area measurements. The effect of the solvent, which was used for the precursor synthesis as well as the citrate stochiometry on the nanoparticle morphology will be taken into evaluation.

Role of subsurface oxygen in oxide formation at transition metal surfaces.

We present a density-functional theory trend study addressing the incorporation of oxygen into the basal plane of the late 4d transition metals (TMs) from Ru to Ag. Occupation of subsurface sites is always connected with a significant distortion of the host lattice, rendering it initially less favorable than on-surface chemisorption. Penetration starts only after a critical coverage theta(c), which is lower for the softer metals towards the right of the TM series. The computed theta(c) are found to be very similar to those above which the bulk oxide phase becomes thermodynamically more stable, thus suggesting that the initial incorporation of O actuates the formation of a surface oxide on TM surfaces.

Ultra-fast diffusion mechanism of the late 3d transition metal impurities in silicon

Based upon ab initio electronic structure calculations with super-cells and an FLAPW method, we discuss the mechanism of the ultra-fast diffusion and the reduction of the migration barriers of the late 3d transition atom impurities of Co, Ni and Cu in silicon. The reduction mechanism of the migration barriers of the late 3d transition metal impurities is due to the pseudo-Jahn-Teller interaction, in which the six-fold-coordinated bonds are formed at the interstitial D 3d symmetry site and the energy gain by these bonds overcomes that of the four-fold-coordinated bonds at the tetrahedral T d interstitial site. For Sc, Ti, V, Cr, Mn and Fe impurities in silicon, the T d interstitial site is more stable than the D 3d site, however, for Co, Ni and Cu impurities, the D 3d interstitial site becomes more stable than the T d interstitial site. Calculated migration barriers are in reasonably good agreement with the experimental data. We discuss the systematic variation of the chemical trend of the migration barrier of 3d transition metal impurities in silicon based upon the calculation.

Evidence for correlation shifts in the photoemission spectrum of late 4d transition metal compounds

Photoemission spectra of the Rh 4d contribution to the valence band in diluted Rh-rare earth compounds (with Ce and Lu) deviate substantially from standard local density approximation (LDA) predictions, at variance with the results on Rh-rich phases. We attribute these deviations to incomplete screening of the final 4d hole due to the narrowing of the 4d band, related to the change in structure. We propose a scheme that improves considerably the coincidence between LDA and experiment by taking into account final state relaxation effects.

Resonant photoemission applied to transition-metals, rare-earths, and actinides

Abstract Recent experimental results concerning the application of resonant photoemission to late 3d-transition metals, rare-earth systems, and uranium metal are presented. Special attention is paid to angle-resolved valence-band studies, that allow a determination of spatial localization and angular momentum character of Bloch states across the Brillouin zone. For f-emissions, the influence of different initial and final state configurations on the resonant behavior is discussed. For 3d-transition metals, the question discussed is, whether the resonant enhancement of satellite structures should really be interpreted in terms of Fano-resonances, or the spectra should be described by an incoherent superposition of photoemission and Auger signals.

Adlayer core-level shifts of admetal monolayers on transition-metal substrates and their relation to the surface chemical reactivity.

Using density-functional theory, we study the electronic and structural properties of a monolayer of Cu on the fcc(100) and (111) surfaces of the late 4d transition metals, as well as a monolayer of Pd on Mo bcc(110). We calculate the ground states of these systems, as well as the difference of the ionization energies of an adlayer core electron and a core electron of the clean surface of the adlayer metal. The theoretical results are compared to available experimental data and discussed in a simple physical picture; it is shown why and how adlayer core-level binding energy shifts can be used to deduce information on the adlayer's chemical reactivity. \textcopyright{} 1996 The American Physical Society.

Some regularities in the thermochemistry of alloying of rare earths with late 3d-transition metals

The standard enthalpies of mixing measured directly by high temperature calorimetry are reported for binary alloys of Mn, Fe, Co, Ni and Cu with the whole range of trivalent rare earth (R) metals. Several regularities in the variations along R and transition metal (TM) rows were established. In particular, gradual growth of TM-R interactions (negative values of mixing enthalpies) along the R series was observed in all systems, with distinct anomalies found for Ce in alloys with Co and Ni. Mn → Fe → Co → Ni ← Cu trends in varying the interactions were established in alloys with different rare earths. The growth in interactions of dissimilar atoms along the R row was found to increase according to the scheme Cu → Mn ≈ Fe → Ni → Co.

Band theory and Mott insulators: Hubbard U instead of Stoner I.

We propose a form for the exchange-correlation potential in local-density band theory, appropriate for Mott insulators. The idea is to use the ``constrained-local-density-approximation'' Hubbard parameter U as the quantity relating the single-particle potentials to the magnetic- (and orbital-) order parameters. Our energy functional is that of the local-density approximation plus the mean-field approximation to the remaining part of the U term. We argue that such a method should make sense, if one accepts the Hubbard model and the success of constrained-local-density-approximation parameter calculations. Using this ab initio scheme, we find that all late-3d-transition-metal monoxides, as well as the parent compounds of the high-${\mathit{T}}_{\mathit{c}}$ compounds, are large-gap magnetic insulators of the charge-transfer type. Further, the method predicts that ${\mathrm{LiNiO}}_{2}$ is a low-spin ferromagnet and NiS a local-moment p-type metal. The present version of the scheme fails for the early-3d-transition-metal monoxides and for the late 3d transition metals.

Reentrant Kondo-mixed valent-Kondo regime behavior with T in the 3d CeT2Si2 series

In intermetallic compounds of Ce involving late 3d, 4d, and 5d row transition metals (T), the Ce valence state increases with decreasing T-d-band electron count. Neifeld, et al. [Phys. Rev. B 32, 6928 (1985)] have recently shown (using extensive Ce-L3-edge measurements) that this trend is reversed for the Ce-3d row compounds earlier than Co. This somewhat striking behavior is nowhere more apparent than in the ThCr2Si2 crystal structure series CeT2Si2 with T=Cu, Ni, Co, Fe, Mn, and (MnxCr1−x). In this paper, we will discuss the following: (1) the extension of this crystal structure to its stability limit in the T=(Mn,Cr) substituted system; (2), how this extension permits restoration of the Ce-valence state to the Kondo local-moment regime; (3) low-temperature electrical resistivity measurements which support first the Kondo to mixed valent followed by mixed valent to Kondo regime passage with decreasing 3d electron count in this series; and (4) finally the apparent compatibility of the Ce-Kondo effect with strong 3d antiferromagnetism for some of these materials.