Electronic structure of the 4d transition metal carbides: Dispersed fluorescence spectroscopy of MoC, RuC, and PdC (original) (raw)
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First spectroscopic investigation of the 4d transition metal monocarbide MoC
Journal of Chemical …, 1998
The first optical spectroscopic investigation of MoC has revealed a complicated vibronic spectrum consisting of about 35 bands between 17 700 and 24 000 cm Ϫ1. Analysis has shown the ground state to be the ⍀ϭ0 ϩ spin-orbit component of a 3 ⌺ Ϫ state that derives from a 10 2 11 2 5 4 2␦ 2 configuration. The X 3 ⌺ 0 ϩ Ϫ rotational constant for 98 Mo 12 C was determined to be B 0 ϭ0.553 640 Ϯ0.000 055 cm Ϫ1 , giving r 0 ϭ1.687 719Ϯ0.000 084 Å. Consideration of spin-uncoupling effects in the X 3 ⌺ Ϫ state requires that this value be revised to r 0 ϭ1.6760 Å, which represents our best estimate of the true Mo-C bond length. Spectroscopic constants were also extracted for six other major isotopic modifications of MoC in this mass resolved experiment. All rotationally resolved transitions were found to originate from the ground state and terminate in electronic states with ⍀ϭ1. An attempt is made to classify the observed transitions into band systems, to rationalize the complexity of the spectrum, and to understand the bonding from a molecular orbital point of view.
A comparative study of electronic structure and bonding in transition metal monocarbides
Journal of Physics and Chemistry of Solids, 2012
The structural, electronic, elastic and bonding properties of four transition metal carbides, ScC, YC (group III), VC and NbC (group V), have been investigated systematically using the first principles density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange correlation has been used for the calculation of the total energy. The ground state properties, such as equilibrium lattice constant, bulk modulus, are computed and compared with theoretical and experimental data. The electronic and bonding patterns of the two groups of compounds have been analyzed quantitatively and compared with the available data. It is clear from band structures that all the four transition metal monocarbides are metallic in nature. Analysis of elastic constants reveals that the carbides of group III are ductile in nature while those of group V are brittle.
Phase stabilities and homogeneity ranges in 4d-transition-metal carbides: A theoretical study
Physical Review B, 2001
First-principles full-potential linear muffin-tin orbital calculations have been used to study the 4d-transition-metal carbides ZrC, NbC, and MoC. The experimental phase diagrams at Tϭ0 of the refractory compounds ZrC, NbC, and MoC have been reproduced with great accuracy from first principles theory. The energy of formation for these compounds has been calculated for several phases and stoichiometries in order to understand the differences in phase stabilities and the changes in homogeneity ranges found between these systems is explained. The results can be regarded as theoretical zero-temperature phase stability diagrams for the three compounds containing not only the experimentally verified but also hypothetical phases and many of the experimental properties and trends are reproduced and explained. A study of the changes and differences in electronic structure and bonding of the studied compounds, phases and stoichiometries is also presented. As a part of this study the hexagonal Me 2 C ͑Me being Zr, Nb, or Mo͒ phases were studied and the theoretical structures, with relaxed interlayer distances and lattice parameters, were obtained. The phase stabilities and electronic structure of the experimentally reported orthorhombic Nb 2 C and Mo 2 C phases were also studied.
Trends in bulk electron-structural features of early transition-metal carbides
2010
A detailed and systematic density-functional theory (DFT) study of a series of early transition-metal carbides (TMC's) in the NaCl structure is presented. The focus is on the trends in the electronic structure and nature of bonding, which are essential for the understanding of the reactivity of TMC's. The employed approach is based on a thorough complementary analysis of the electron density differences, the density of states (DOS), the band structure, and the real-space wave functions to gain insight into the bonding of this class of materials and get a more detailed picture of it than previously achieved, as the trend study allows for a systematic identification of the bond character along the different bands. Our approach confirms the presence of both the well-known TM--C and TM--TM bonds and, more importantly, it shows the existence and significance of direct C--C bonds in all investigated TMC's, which are frequently neglected but have been recently identified in some cases [Solid State Commun. 121, 411 (2002); Phys. Rev. B 75, 235438 (2007)]. New information on the spatial extent of the bonds, their \textit{k}-space location within the band structure, and their importance for the bulk cohesion is provided. Trends in covalency and ionicity are presented. The resulting electron-structural trends are analyzed and discussed within a two-level model.
Optical spectroscopy of tungsten carbide (WC)
The Journal of Chemical Physics, 2002
Resonant two-photon ionization spectroscopy has been used to study the diatomic transition-metal carbide, WC. A low-resolution scan revealed a five-member vibrational progression beginning with the 0-0 band at 17 585 cm Ϫ1. Analysis of this progression yielded a vibrational frequency of e Ј(184 W 12 C͒ϭ752.6͑4.9͒ cm Ϫ1 and a bond length of r e Ј(184 W 12 C͒ϭ1.747͑4͒ Å. Several unassigned bands were also rotationally resolved and analyzed. All of the observed bands are ⍀Јϭ2←⍀Љϭ1 transitions, confirming the predicted ground state of 3 ⌬ 1 arising from a 14 2 8 4 15 2 4␦ 1 16 1 configuration. The measured line positions in these bands were simultaneously fitted to provide B 0 Љϭ0.509 66(10) cm Ϫ1 for 184 W 12 C, corresponding to r 0 Љ(184 W 12 C͒ϭ1.713 5͑2͒ Å. These values are corrected for spin-uncoupling effects in the ground state and represent our best estimate of the true bond length of WC. Dispersed fluorescence studies provide the ground-state vibrational constants of e ϭ983(4) cm Ϫ1 and e x e ϭ11(1) cm Ϫ1 , and have also permitted the low-lying ͓1.2͔ 3 ⌬ 2 and ͓4.75͔ states to be located and characterized. These results on WC are discussed in relation to the isovalent molecule MoC and other transition-metal carbides.
Vacancy effects on structural and electronic properties of 4d transition-metal carbides
Computational Materials Science, 2009
We present a study of the effect of the vacancies on the structural and electronic properties in substoichiometric NbC x and MoC x in the NaCl type structure using ab-initio full-potential linear augmented plane wave method (FP-LAPW). A model structure of 8 and 16 atom supercell with ordered vacancies within the carbon sublattices is used. We find that the lattice parameters of the studied stoichiometries in both MoC x and NbC x are smaller than that of ideal stoichiometric MoC and NbC. Our results are found to be in good agreement with experiment and other theoretical ab-initio calculations.
Electronic Structure of Cobalt Carbide, CoC
Journal of Physical Chemistry A, 2006
The ground and 18 low lying excited states of the diatomic molecule cobalt carbide, CoC, have been examined by multireference variational methods (MRCI) combined with quantitative basis sets. All calculated states are bound and correlate adiabatically to the ground-state atoms, Co(a 4 F) + C( 3 P). We report complete potential energy curves, equilibrium bond distances, dissociation energies (D e ), spectroscopic constants, electric dipole moments and spin-orbit splittings. The bonding character of certain states is also discussed with the help of Mulliken distributions and valence-bond-Lewis diagrams. We are practically certain that the ground state is of 2 Σ + symmetry with a state of 2 ∆ symmetry lying less than 3 kcal/mol higher, in agreement with the relevant experimental findings. Our best estimate of the X 2 Σ + dissociation energy is D e (D 0 ) ) 83(82) kcal/mol at r e ) 1.541 Å, 0.02 Å shorter than the experimental bond length.
Ab initio study of the electronic structure of manganese carbide
The Journal of Chemical Physics, 2006
We report electronic structure calculations on 13 states of the experimentally unknown manganese carbide ͑MnC͒ using standard multireference configuration interaction ͑MRCI͒ methods coupled with high quality basis sets. For all states considered we have constructed full potential energy curves and calculated zero point energies. The X state, correlating to ground state atoms, is of 4 ⌺ − symmetry featuring three bonds, with a recommended dissociation energy of D 0 = 70.0 kcal/ mol and r e = 1.640 Å. The first and second excited states, which also correlate to ground state atoms, are of 6 ⌺ − and 8 ⌺ − symmetry, respectively, and lie 17.7 and 28.2 kcal/ mol above the X state at the MRCI level of theory.
The computational description of the catalytic processes on the surface of transition metals (TMs) requires methods capable of accurate prediction of the bond forming and breaking between the atoms of metal and other elements. In our previous report [Goel and Masunov, J Chem Phys, 129, 214302, 2008], we studied TM hydrides and found that Boese-Martin functional for kinetics (BMK) combined with broken symmetry approach described dissociation process more accurately than multireference wavefunction theory (WFT) methods and some other functionals. Here, we investigate the binding energy, geometry, electronic structure, and potential energy curves for diatomic TM carbides using several exchange-correlation functionals. The functionals that include explicit dependence on the kinetic energy density (s-functionals) are considered, among others. We have found M05-2x performance to be the best, followed by BMK, when compared with experimental and high level WFT energetics. This agreement deteriorates quickly for other functionals when the fraction of the Hartree-Fock exchange is decreased. Scalar relativistic corrections yield mixed results for bond lengths and bond energies. The natural bond orbital analysis provides useful insight in description of stable spin state over others in these diatomics.
Trends in bulk electron-structural features of rocksalt early transition-metal carbides
Journal of Physics: Condensed Matter, 2010
A detailed and systematic density-functional theory (DFT) study of a series of early transitionmetal carbides (TMC's) in the NaCl structure is presented. The focus is on the trends in the electronic structure and nature of bonding, which are essential for the understanding of the reactivity of TMC's. The employed approach is based on a thorough complementary analysis of the electron density differences, the density of states (DOS), the band structure, and the real-space wave functions to gain insight into the bonding of this class of materials and get a more detailed picture of it than previously achieved, as the trend study allows for a systematic identification of the bond character along the different bands. Our approach confirms the presence of both the well-known TM-C and TM-TM bonds and, more importantly, it shows the existence and significance of direct C-C bonds in all investigated TMC's, which are frequently neglected but have been recently identified in some cases [Solid State Commun. 121, 411 (2002); Phys. Rev. B 75, 235438 (2007)]. New information on the spatial extent of the bonds, their k -space location within the band structure, and their importance for the bulk cohesion is provided. Trends in covalency and ionicity are presented. The resulting electron-structural trends are analyzed and discussed within a two-level model. PACS numbers: 71.,71.15.Mb,71.15.Nc