Metal D-states Research Articles

Periodic trends of oxygen vacancy formation and C–H bond activation over transition metal-doped CeO2 (1 1 1) surfaces

Substitutional transition metal dopants in the cerium oxide surface can alter the surface reducibility and catalytic activity for hydrocarbon conversion. Density functional theory (DFT+U) methods are used to examine the electronic and structural effects of transition metal dopants (groups IV–XII) in the CeO2 (111) surface. Surface reducibility (oxygen vacancy formation) and dissociative adsorption of methane (forming H* and CH3*) are considered. Both the methane dissociative adsorption energy and activation barriers correlate linearly with the surface oxygen vacancy formation energy. Charge analysis is used to determine the role of dopant metal in serving as a reduction center or altering the Ce reducibility. Dopants in groups IV and V alter the reducibility of the surface and dopants in groups X–XII become the reduction center. The dopant plays the same role in both oxygen vacancy formation and methane adsorption. A Brønsted–Evans–Polanyi relationship is established between the methane activation barrier, through H-abstraction, and the dissociative adsorption energy. The sensitivity of quantitative and qualitative trends to the inclusion of U terms for the dopant transition metal d-states is considered. The optimal M/CeO2 dopant for methane conversion to CO or CO2 follows a volcano relationship with oxygen vacancy formation: Highly reducible surfaces will be limited by re-oxidation, whereas surfaces difficult to reduce will show high barriers for C–H bond activation. Transition metal dopants near the peak region of the volcano are Pd, Co, Ni, and Mn.

Full-Potential Study of Half-Metallic Ferromagnetism in CdTMO<SUB>2</SUB> (TM=Fe, Co and Ni)

In this work, we present an ab initio calculation for the structural, electronic and magnetic properties of CdTMO2 (TM=Fe, Co and Ni) compound. The calculations are performed by a developed full-potential augmented plane wave plus local orbitals (FP-L/APW+lo) method within the spin density functional theory. As exchange-correlation potential we used the generalized gradient approximation GGA form and GGA+U (where U is the Hubbard correlation terms). We have performed total energy calculations of these compounds in the chalcopyrite structure. We present a comparative study between the electronic structures, total and partial densities of states and local moments calculated within GGA and GGA+U. Furthermore, we predict the values of spin-exchange splitting energies Δx(d) and Δx(p-d) and exchange constants N0α and N0β produced by the transition metal 3d states. Our calculations show the half-metallic behavior of these ferromagnetic compounds.

Communications: Exceptions to the d-band model of chemisorption on metal surfaces: The dominant role of repulsion between adsorbate states and metal d-states

We show that there is a family of adsorbate-substrate systems that do not follow the trends in adsorption energies predicted by the d-band model. A physically transparent model is used to analyze this phenomenon. We found that these adsorbate-substrate pairs are characterized by the repulsive interaction of the substrate d-band with the renormalized adsorbate states. The exceptions to the d-band model are mainly associated with the adsorbates having almost completely filled valence shell, and the substrates with nearly fully occupied d-band, e.g., OH, F, or Cl adsorption on metals and alloys characterized by d(9) or d(10) substrate surface atoms.

Electronic Structure of Few-Layer Epitaxial Graphene on Ru(0001)

The electronic structure of epitaxial monolayer, bilayer, and trilayer graphene on Ru(0001) was determined by selected-area angle-resolved photoelectron spectroscopy (micro-ARPES). Micro-ARPES band maps provide evidence for a strong electronic coupling between monolayer graphene and the adjacent metal, which causes the complete disruption of the graphene pi-bands near the Fermi energy. However, the perturbation by the metal decreases rapidly with the addition of further graphene sheets, and already an epitaxial graphene bilayer on Ru recovers the characteristic Dirac cones of isolated monolayer graphene. A graphene trilayer on Ru behaves like free-standing bilayer graphene. Density-functional theory based calculations show that this decoupling is due to the efficient passivation of metal d-states by the interfacial graphene layer.

Electronic structural changes of the electrochemically delithiated LiFe 0.5Co 0.5PO 4 cathode material studied by X-ray absorption spectroscopy

In order to study the electronic structure changes of the electrochemically delithiated Li 1− x Fe 0.5Co 0.5PO 4 system, in situ Fe and Co K-edge XAS and ex situ P K-edge XAS have been carried out during the first charging process. The Fe and Co K-edge XAS results showed that the major charge compensation at the metal sites during charge is achieved by the oxidation of Fe 2+ ions at lower potential plateau (∼3.6 V) and the oxidation of Co 2+ ions at higher potential plateau (∼5.0 V). The gradual shift of main edge features in P K-edge XANES spectra showed that P O bonds become less covalent during delithiation, due to the increased covalency of Fe 3+ O bonds via the inductive effect. From the observation of pre-edge peaks, it is concluded that the electrochemical delithiation of Li 1− x FePO 4 result in the hybridization of P 3p states with the metal 3d states.

Theoretical Study on the Structure, Stability, and Electronic Properties of the Guanine−Zn−Cytosine Base Pair in M-DNA

M-DNA is a type of metalated DNA that forms at high pH and in the presence of Zn, Ni, and Co, with the metals placed in between each base pair, as in G-Zn-C. Experiments have found that M-DNA could be a promising candidate for a variety of nanotechnological applications, as it is speculated that the metal d-states enhance the conductivity, but controversy still clouds these findings. In this paper, we carry out a comprehensive ab initio study of eight G-Zn-C models in the gas phase to help discern the structure and electronic properties of Zn-DNA. Specifically, we study whether a model prefers to be planar and has electronic properties that correlate with Zn-DNA having a metallic-like conductivity. Out of all the studied models, there is only one which preserves its planarity upon full geometry optimization. Nevertheless, starting from this model, one can deduce a parallel Zn-DNA architecture only. This duplex would contain the imino proton, in contrast to what has been proposed experimentally. Among the nonplanar models, there is one that requires less than 8 kcal/mol to flatten (both in gas and solvent conditions), and we propose that it is a plausible model for building an antiparallel duplex. In this duplex, the imino proton would be replaced by Zn, in accordance with experimental models. Neither planar nor nonplanar models have electronic properties that correlate with Zn-DNA having a metallic-like conductivity due to Zn d-states. To understand whether density functional theory (DFT) can describe appropriately the electronic properties of M-DNAs, we have investigated the electronic properties of G-Co-C base pairs. We have found that when self-interaction corrections (SIC) are not included the HOMO state contains Co d-levels, whereas these levels are moved below the HOMO state when SIC are considered. This result indicates that caution should be exercised when studying the electronic properties of M-DNAs with functionals that do not account for strong electronic correlations.

Ultraviolet photoemission of Ni 2MnZ (Z = Ga, In and Sn)

Ultraviolet photoelectron spectroscopy (UPS) was performed on Ni Mn based Heusler-type intermetallic compounds, including the mother material for the ferromagnetic shape memory. Although UPS is relatively surface-sensitive, it is useful to explore the compositional dependence of the electronic states. Significant changes in the band structure are found between Ni 2MnGa and Ni 2MnIn, that are interpreted in terms of the increase of the hybridization between the neighboring transition metal 3d states. An increase of the band filling is found in going from Ni 2MnIn to Ni 2MnSn, which is interpreted by the increase of the number of 5p electrons in changing from In to Sn.

Theoretical photoelectronic spectra of molybdenum, tungsten, and rhenium disilicides

Photoelectronic spectra of volume samples of molybdenum, tungsten, and rhenium disilicides are theoretically investigated as functions of the excitation energy (hv = 12–1486 eV) by the method of linearized attached plane waves. The main contribution to the spectrum at low (hv = 12–52 eV) and very high (hv = 1486 eV) excitation energies is given by the metal d-states, while the contribution of the Si p-states is dominant at intermediate excitation energies. An analysis of the calculated results demonstrates that the main peak of the photoelectronic spectra is displaced by 0.2 eV toward the bottom of the valence band when going from MoSi2 to WSi2; it is displaced by 0.3 eV when going from WSi2 to ReSi1.75.

A theoretical investigation on Pt 3Sn(1 0 2) surface alloy and CO–Pt 3Sn(1 0 2) system

The adsorption properties of CO on experimentally verified stepped Pt 3Sn(1 0 2) surface were investigated using quantum mechanical calculations. The two possible terminations of Pt 3Sn(1 0 2) were generated and on these terminations all types of possible adsorption sites were determined. The adsorption energies and geometries of the CO molecule for all those sites were calculated. The most favorable sites for adsorption were determined as the short bridge site on the terrace of pure-Pt row of the mixed-atom-ending termination, atop site at the step-edge of the pure row of pure-Pt-ending termination and atop site at the step-edge of the pure-Pt row of the mixed-atom-ending termination. The results were compared with those for similar sites on the flat Pt 3Sn(1 1 0) surface considering the fact that Pt 3Sn(1 0 2) has terraces with (1 1 0) orientation. The LDOS analysis of bare sites clearly shows that there are significant differences between the electronic properties of Pt atoms at stepped Pt 3Sn(1 0 2) surface and the electronic properties of Pt atoms at flat (1 1 0) surface, which leads to changes in the CO bonding energies of these Pt atoms. Adsorption on Pt 3Sn(1 0 2) surface is in general stronger compared to that on Pt 3Sn(1 1 0) surface. The difference in adsorption strength of similar sites on these two surface terminations is a result of stepped structure of Pt 3Sn(1 0 2). The local density of states (LDOS) of the adsorbent Pt and C of adsorbed CO was utilized. The LDOS of the surface metal atoms with CO-adsorbed atop and of their bare state were compared to see the effect of CO chemisorption on the electron density distribution of the corresponding Pt atom. The downward shift in energy peak in the LDOS curves as well as changes in the electron densities of the corresponding energy levels indicate the orbital mixing between CO molecular orbitals and metal d-states. The present study showed that the adsorption strength of the sites has a direct relation with their LDOS profiles.

Theoretical characterization of charge transport in chromia (α-Cr2O3)

Transport of conduction electrons and holes through the lattice of alpha-Cr2O3 (chromia) is modeled as a valence alternation of chromium cations using ab initio electronic structure calculations and electron-transfer theory. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e., the reorganization energy and the electronic coupling matrix element that enter Marcus' theory. The calculation of the electronic coupling followed the generalized Mulliken-Hush approach using the complete active space self-consistent-field (CASSCF) method and the quasidiabatic method. Our findings indicate that hole mobility is more than three orders of magnitude larger than electron mobility in both (001) and [001] lattice directions. The difference arises mainly from the larger internal reorganization energy calculated for electron-transport relative to hole-transport processes while electronic couplings have similar magnitudes. The much larger hole mobility versus electron mobility in alpha-Cr2O3 is in contrast to similar hole and electron mobilities in hematite alpha-Fe2O3 previously calculated. Our calculations also indicate that the electronic coupling for all charge-transfer processes of interest is smaller than for the corresponding processes in hematite. This variation is attributed to the weaker interaction between the metal 3d states and the O(2p) states in chromia than in hematite, leading to a smaller overlap between the charge-transfer donor and acceptor wave functions and smaller superexchange coupling in chromia. Nevertheless, the weaker coupling in chromia is still sufficiently large to suggest that charge-transport processes in chromia are adiabatic in nature. The electronic coupling is found to depend on both the superexchange interaction through the bridging oxygen atoms and the d-shell electron-spin coupling within the Cr-Cr donor-acceptor pair, while the reorganization energy is essentially independent of the electron-spin coupling.

Monolayer of h-BN chemisorbed on Cu(1 1 1) and Ni(1 1 1): The role of the transition metal 3d states

The role of the transition metal 3d states in the adsorption of h-BN on Cu(1 1 1) and Ni(1 1 1) has been studied by NEXAFS and PE spectroscopies and by work function measurements. Based on the analysis of drastic differences in the spectra from h-BN/Cu(1 1 1) and h-BN/Ni(1 1 1), it has been concluded that the strength of chemical bonding at the interfaces under study is determined mainly by the strength of the TM 3d–h-BN π orbital hybridization. On Ni(1 1 1) h-BN is found to be strongly chemisorbed, while on Cu(1 1 1) only weak chemisorption is observed. On Ni(1 1 1) the h-BN monolayer is probably metallic. On Cu(1 1 1) the monolayer should possibly also be considered as metallic, however its metallic properties are much less pronounced, and the situation is somewhat uncertain.

Evolution of local electronic structure in alabandite and niningerite solid solutions [(Mn,Fe)S, (Mg,Mn)S, (Mg,Fe)S] using sulfurK- andL-edge XANES spectroscopy

Synchrotron radiation S K- and L-edge X-ray absorption near-edge structure (XANES) spectra are reported for the cubic, rocksalt (B1) structure sulfides niningerite (MgS), alabandite (MnS), and oldhamite (CaS), and for their solid solutions (Mn,Fe)S and (Mg,Mn)S, and S L-edge XANES spectra are reported also for (Mg,Fe)S solid solutions. Pre-edge features at the S K-edge are attributed to transition of S 1s electrons to the lowest available unoccupied S 3p σ* antibonding states hybridized with metal 3d(e g ) states, and at the S L-edge to transition of S 2p electrons to unoccupied S 3s σ*, 4s σ*, and 3d antibonding states hybridized with metal 3d(e g ) states, and to a lesser extent 3d(t 2g ) states. The S K-edge XANES spectra for the solid solutions show a progressive participation of 3d orbitals in metal-S bonding with increase in substitution by Fe in (Mn,Fe)S and (Mg,Fe)S and Mn in (Mg,Mn)S through progressive increase in the area of the pre-edge feature. However, the pre-peak area does not increase linearly in each solid solution series showing that a real change in bulk electronic properties has occurred. Increase in pre-peak area reflects an increase in overall attainability of metal 3d states for hybridization with S 3p σ* antibonding states as proportionally more metal 3d orbitals become available. The S L-edge XANES spectra show progressive evolution of pre-edge features at the L 3 - and L 2 -edges (a 1 and a 2 , respectively). Only a 2 is present in the S L-edge XANES spectrum of FeS (troilite), and with progressive decrease in Fe content in (Mn,Fe)S and (Mg,Fe)S solid solutions, a 1 first appears, then becomes dominant. Since a 1 is attributed to transition of S 2p 3/2 electrons to S 3s σ* states hybridized with metal 3d(e g ) and 3d(t 2g ) states, this appears to represent an increased contribution from metal-S π-bonding. The results show that the size and position of the pre-edge features to the S K- and L-edges are controlled more by the DOS of hybridized 3d(e g β ) and 3d(t 2g β ) states and nearest-neighbor coordination of the metal atoms than by the precise coordination of S and the extended structure of the sulfide. The full multiple scattering approach has been applied to the calculation of the S K-edge XANES spectra of MgS, MnS, and CaS. Results are consistent with experimental XANES spectra, especially for the pre-edge features, which are often neglected in such calculations.

Electronic band structure calculation and nuclear spin-lattice relaxation in chromium hydrides

The angular-momentum components of the density of states (DOS) were calculated by the FP-LAPW method for stoichiometric CrH having either hexagonal (anti-NiAs) or cubic (NaCl or ZnS) types of structure. The shape of the DOS of the metal 3d states is remarkably similar for the two models of cubic structure, and differs only slightly from that for the hexagonal structure. The DOSs have been used to calculate the 53Cr nuclear spin-lattice relaxation rates. It is found that that the s and p electron contributions to the relaxation rates are negligibly small due to their very low partial densities of states at the Fermi level. The d-orbital contributions are dominant and the core-polarisation and d-dipolar contributions play a minor role. The experimental relaxation rates measured in hexagonal CrH 0.97 and CrH 0.93 and cubic CrH 0.97 indicated the presence of magnetic impurities in the samples, the contribution of which was subtracted from measured relaxation rates. It was found that the intrinsic relaxation rates are reasonably well reproduced by theoretical calculations.

Transition metal 3d states in HgSe-based diluted magnetic semiconductors

The contributions of Mn3d, Fe3d and Co3d electrons to the valence band electronic structure of Hg 1− x TM x Se (TM=Mn, Fe, Co) crystals were investigated by means of resonant photoemission. The comparison of photoemission spectra with the results of theoretical calculations made by the CPA (coherent potential approximation) method shows that the valence band of Hg 1− x TM x Se diluted magnetic semiconductors is strongly influenced by TM3d electrons. We find out that for all TM incorporated in the HgSe matrix, 3d electrons with t 2g symmetry hybridize strongly with the valence band of the crystal, whereas 3d electrons with e 2g symmetry are not hybridized with the valence band. The contribution of 3d t2g electrons is spread over the valence band with the maximum at 3.4–3.8 eV below the Fermi level. The contributions of 3d e2g electrons were found in the middle part of the valence band and, in the Fe and Co case, also at the edge of the valence band.

Electron Density Calculations for Crystals with a NaCl Lattice

The ab initio local DFT method combined with the ab initio pseudopotential technique is used to calculate valence electron densities in series of crystal substances MA (M = Li, Na, K, Rb, Ag, Mg, Ca; A = F, Cl, Br, O, S) with a NaCl lattice. Systematic variations of valence electron density depending on the atomic number of anion and cation have been found. According to electron density distribution, the compounds are classified into three groups: a) oxides and fluorides; b) sulfides, chlorides, and bromides; c) noble metal halides. In oxides and fluorides, the maximum of valence electron density is in the middle of the M–M bond. In sulfides, chlorides, and bromides, the minimum density is in the middle of the M–M bond, with two symmetric maxima or two “shoulders” (depending on the atomic number of the cation) lying away from the center of the bond. In noble metal halides, the maximum of valence density is on the metal due to the presence of metal d-states, and the density map is rotated through 90° relative to the map of alkali and alkali earth metals, so that the Hal–Hal bond becomes an analog of the M–M bond.

Ultrafast Photophysics and Photochemistry of [Ni]-Bacteriochlorophyll a

The ultrafast photodynamics of bacteriochlorophyll with the central Mg2+-ion replaced by Ni2+ ([Ni]-BChl) in toluene and pyridine have been studied by femtosecond time-resolved fluorescence and absorption spectroscopy with a time resolution of 100 fs in the spectral range of 470−900 nm. Excitation of tetracoordinated [Ni]-BChl in toluene in its red-most absorption band leads to four transient states which decay single-exponentially with lifetimes of 100 fs, 450 fs, 4 ps, 25 ps. Except for the 4 ps component these kinetics arise from an internal conversion cascade conserving the overall multiplicity of the singlet configuration. The spectral characteristics of the intermediates indicate a deactivation process via several π- and metal-centered excited states. The 4 ps kinetic results from a relaxation within the metal d-states caused by a change of size of the central nickel ion upon electronic excitation. A different deactivation pattern is observed for [Ni]-BChl in pyridine, where the central metal is add...

Electronic structure systematics of 3d transition metal oxides

Abstract The electronic structure of a series of 3d transition metal oxides is investigated by core-level and valence band photoemission spectroscopy. The results are analyzed in terms of metal–ligand interactions and on-site correlation energies, using a simple cluster model approach. For some oxides we also present a comparison with band structure calculations. It is shown that the electronic structure of the early transition metal compounds is mainly determined by the strong hybridization between transition metal 3d states and oxygen 2p states, whereas correlation effects become more important for the late transition metals. Further evidence for a strong metal–ligand hybridization is supplied by LVV-Auger spectra of the nominal d0 compounds TiO2, V2O5 and PbCrO4, indicating a non-zero 3d occupation, in accordance with the cluster model results. We discuss briefly also the systematics in the exchange splitting of 3s spectra as function of the 3d occupation number.

Soft X-ray emission spectroscopy of early transition metal compounds

Abstract Soft X-ray emission studies for several early transition metal compounds, such as Sc, Ti and V compounds, have been studied. Raman scattering was found at the 2p→3d excitation in the transition metal compounds. The elementary excitation of the Raman spectrum is mainly elucidated by the d–d transition within the 3d bands, as well as the interband transition from the O 2p valence band to the conduction band, which consists mainly of transition metal 3d states. The charge transfer transition due to the localized electron picture is dominant in the soft X-ray spectra of Sc compounds. Raman scattering is a powerful tool in the study of the electronic structure of transition metal compounds.

Coherent quasiparticle evolution in charge transfer systems: A dynamical mean field theory

Using the two-band Hubbard Hamiltonian for transition metal-3d and oxygen-2p states with perovskite geometry, we investigate the evolution of coherent quasiparticles in charge transfer systems. It is shown that the recently observed change of the photoemission spectrum of Ca x Sr 1− x VO 3 upon Ca-doping can be understood as a consequence of the varying importance of charge transfer excitations compared to the excitations of the transition metal states. This demonstrates the dynamical character of the charge carriers near the metal-insulator instability and suggests that such systems are of charge transfer type rather than purely Mott-Hubbard. The calculations are based on a new approach of the dynamical mean field theory for transition metal compounds. The solution of the self-consistent dynamical mean field equations are performed using the non-crossing approximation.

Dynamical mean-field theory for perovskites.

Using the Hubbard Hamiltonian for transition metal-3d and oxygen-2p states with perovskite geometry, we present a dynamical mean field theory which becomes exactin the limit of large coordination numbers or equivalently large spatial dimensions $D$. The theory is based on a new description of these systems for large $D$ using a selective treatment of different hopping processes which can not be generated by a unique scaling of the hopping element. The model is solved using a perturbational approach and an extended non-crossing approximation. We discuss the breakdown of the perturbation theory near half filling, the origin of the various 3d and 2p bands, the doping dependence of its spectral weight, and the evolution of quasi particles at the Fermi-level upon doping, leading to interesting insight into the dynamical character of the charge carriers near the metal insulator instability of transition metal oxide systems, three dimensional perovskites and other strongly correlated transition metal oxides.