3d Transition Metal Compounds Research Articles

Differences between L3 and L2 x-ray absorption spectra of transition metal compounds

The differences between L3 and L2 edges of 3d and 4d transition metal complexes and compounds in octahedral symmetry are discussed. The main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions. The 3d and 4d spin–orbit coupling is a second origin of difference. For 3d systems the multiplet effects dominate all other interactions and the L3 and L2 edge are completely mixed and reordered. For 4d systems the core hole spin–orbit coupling is large and the L3 and L2 are separated by about 100 eV with a ratio close to 2:1. The differences between the L3 and L2 edge originate from the weight transfer between the t2g and eg peaks due to the multiplet effect. This weight transfer is about 25% for the L3 edge and about 5% for the L2 edge, which implies that for a comparison to single-particle calculations the L2 edge is preferable to use. Partly filled 4d systems are low-spin and the occupation of the t2g states implies a decrease of the first peak. This decrease is stronger for the L2 edge, implying an increase in the L3:L2 ratio. For 4d5 systems transitions to the t2g hole are only possible at the L3 edge due to the combined effects of 4d spin–orbit coupling and the dd multiplet effects.

Electronic structure of 3d transition metal compounds: systematic chemical trends and multiplet effects

The local electronic structure of 3d transition metal compounds is characterized by a few parameters, namely the ligand p to cation d charge transfer energy Δ, the dd Coulomb repulsion energy U, and the pd transfer integrals T. Values for these parameters deduced from the cluster model analysis of cation core level photoemission spectra are shown to exhibit systematic chemical trends as functions of cation atomic number, ligand, and cation valence. Physical properties of these compounds such as the magnitudes of the band gaps, pd covalency and the character of doped carriers, however, are not necessarily smooth functions of those variables but depend also on the nominal d electron number n due to the multiplet effects leading to the stabilization of the Hund's rule ground state. As an illustrative example, the electronic structure of valence-control Mn and Fe oxides is discussed.

Systematic Aspects of the Electronic Structure of 3d Transition-Metal Compounds

Systematic changes in the electronic structure of 3d transition-metal compounds with varying chemical compositions are studied by photoemission spectroscopy and configuration-interaction cluster-model analysis. The changes consist of (i) the smooth variation of model parameters such as the charge-transfer energy Δ and the d-d Coulomb repulsion energy U with atomic number and valence and (ii) the apparently irregular multiplet corrections to Δ and U, which are functions of the nominal electron number. These systematics are reflected upon the band gaps, covalency and character of doped carriers in transition-metal oxides and chalcogenides, and upon the optical absorption spectra and the ionization energies of substitutional transition-metal impurities in semiconductors.

Predicative Approach to the Entropy of Manganese Nitrides and Calculation of the Mn-N Phase Diagram

The thermodynamic and phase equilibrium information on the Mn-N system is insufficient for a reliable assessment of the properties of the system. The present work demonstrates how such an assessment can be supported by estimating the entropy of various phases. The estimate is made with a recent method based upon a consideration of the systematics of bonding properties and vibrational entropy of 3d-transition metal compounds. A thermodynamic description of the system is obtained, which is used to generate the Mn-N phase diagram in reasonable agreement with the experimental information

Systematics in the electronic structure of 3d transition-metal compounds

We have studied a wide range of 3d transition-metal compounds by a cluster configuration-interaction analysis of the metal 2p core-level photoemission spectra. Deduced values for the charge-transfer energy Δ and the d-d Coulomb repulsion U, defined with respect to the multiplet-averaged energies, show a smooth variation as functions of cation atomic number, ligand and cation valence. Many physical properties, however, show apparently irregular variation, which we attribute to d-d exchange or multiplet effects, which also reflect upon Δ eff and U eff defined with respect to the lowest multiplet energies.

The 2p absorption spectra of 3d transition metal compounds in tetrahedral and octahedral symmetry

The authors present the 2p (L2,3) absorption spectra of first-row transition metal ions in tetrahedral and octahedral crystal field symmetry. These have been calculated using a localized description for the 3dn to 2p53dn+1 excitation including electrostatic and spin-orbit interactions. The spectra are significantly different from those already presented where the 3d spin-orbit interaction was neglected. The spectral shape provides a valence- and symmetry-selective probe. Whereas it changes gradually with the crystal field, abrupt changes in the spectra are indicative of high-spin to low-spin transitions. These spin transitions are accompanied by a strong decrease in the 2p branching ratio. The calculated spectra provide a basis for the use of L2,3 absorption spectroscopy in materials science and biological science. The limitations of these calculations and the use of configuration interaction are discussed.

Thermodynamic properties of scandium, lanthanum, neodymium and gadolinium silicides and germanides

Abstract The Gibbs free energy, enthalpy and entropy of formation of the compounds ScSi, Sc5Si3, ScGe2, ScGe, Sc11Ge10, Sc5Ge4, Sc5Ge3, LaSi2-x, LaGe2-x, LaGe, La5Ge4, NdSi2-x, NdGe2-x, GdSi2-x, GdSi1.5, GdSi, Gd5Si4, Gd5Si3, GdGe2-x, GdGe1.5, GdGe, Gd5Ge4, Gd5Ge3 were determined by measuring the e.m.f. of high-temperature galvanic cells. All the compounds investigated are thermodynamically stable. Their formation is followed by significant exothermic effects and a decrease in entropy. Absolute values of ΔGfo and ΔHfo increase in the sequence of silicides and germanides with increasing content of the rare earth element and reach a maximum for the compound R5X3. The thermodynamic stability of rare earth silicides and germanides is much greater compared with the appropriate compounds of 3d transition metals. Rare earth metal germanides are somewhat more stable than the corresponding silicides, whereas for 3d metals the inverse relationship is observed, silicides being more stable than germanides.

The undulator beamline at the SRS Daresbury

The design and performance of the undulator beamline 5U.1 at the SRS used for soft x-ray spectroscopy is reported and compared with the theoretical expectation of the intensity, resolution, and polarization. The beamline consists of a ten-period permanent magnet variable gap undulator followed by an entrance slitless plane grating monochromator. The energy region covered is from 0.1 to 1 keV. The design is similar to the SX700 but uses a spherical instead of an ellipsoidal focusing mirror. To eliminate the large horizontal beam width at the exit slit the beamline has been modified to include a post-focusing mirror. The spherical focusing mirror has been changed to one with a larger radius resulting in a decreased demagnification, thereby essentially eliminating primary coma. The monochromator contains an ion-etched and lamellar profile 1200-lines/mm plane grating of Pt-coated CVD SiC on a graphite substrate. The high flux of the undulator makes the beam line suitable for SEXAFS studies and for work involving soft x-ray fluorescence detection. The combination of high flux and high resolution is especially advantageous for atomic and molecular spectroscopy and to resolve the quasiatomic structures which are observed in localized 3d transition metal compounds.

Characterization of the resolving power of a double multilayer monochromator in the energy range of 600–900 eV

Two different pairs of x-ray reflecting multilayer coatings have been tested in a double crystal monochromator configuration. The two pairs consisted of Ni (0.4 nm)–C (1.85 nm) and NiSi (0.4 nm)–C (1.85 nm), both with 225 periods. In order to determine the resolution in the energy range of 600–900 eV, the photoelectron yield of several 3d transition metal compounds was recorded as well as the throughput around the Ni L lines. We found for both pairs a resolving power of 220. Simulations taking into account the angular divergence of the x-ray beam yield a resolution of 250. The difference with the observed value can be due to the nonhomogeneity of the multilayer coating.

Thermodynamic properties of ni nitrides and phase stability in the Ni-N system

The thermodynamics of the Ni-N system is poorly known from experiments, and there is a need of information on the stability of the various nitride phases and the Ni-N phase diagram. This kind of information has been obtained by us, by combining the few measurements available with predictions, based on recently reported regularities in bonding properties and vibrational entropy of 3d transition metal compounds. A calculated Ni-N phase diagram is presented. A certain range of homogeneity for the hexagonal nitride phase is obtained, which is comparable to that of other 3d transition metal-nitrogen systems. The question of the possible existence of a stable “Ni4N” phase is examined. According to our results, Ni4N is metastable in the Ni-N system.

Calculation of magnetic circular dichroism in core level photoemission spectroscopy

Magnetic X-ray dichroism in core level photoemission is calculated using a localized model. It is shown that the angular dependent part of the electrostatic interaction between the core hole and the 3d electrons determines the magnetic dichroism in the 2p photoemission of 3d transition metal compounds. The circular dichroism is expected to be very strong in compounds with localized 3d electrons.

2p x-ray absorption of 3d transition-metal compounds: An atomic multiplet description including the crystal field.

The metal 2p x-ray-absorption spectra (or ${\mathit{L}}_{2,3}$ edges) of 3d transition-metal compounds are calculated, using atomic multiplet theory with inclusion of the cubic crystal field. A general overview of the effect of the cubic crystal field on the shape of the 3${\mathit{d}}^{\mathit{N}}$ to 2${\mathit{p}}^{5}$3${\mathit{d}}^{\mathit{N}+1}$ excitation spectrum is given for 14 common valencies of 3d transition-metal ions. Comparison to some high-resolution 2p x-ray-absorption spectra shows excellent agreement, which confirms the validity of the approach. Possible refinements of the theory, including lower-symmetry calculations and the inclusion of configuration interaction, are discussed.

Optical evidence of the bipolaronic ground state in the low temperature phase of Ti 4O 7 and Fe 3O 4

We discuss our thorough optical investigations on the 3d transition metal compounds Fe 3O 4 and Ti 4O 7 within the bipolaron model of Chakraverty. Of particular interest are the sharp lines overlapping the phonon modes. These are ascribed to the fingerprints of a charge ordering phase. Futhermore, we present the phonon spectrum of rutile, in order to review the vibrational states of magnetite and above all of Ti 4O 7.

Systematics in band gaps and optical spectra of 3D transition metal compounds

In this paper we discuss the systematics in the transition metal d- d Coulomb interactions and the anion to cation charge transfer energies, and relate these to systematics in observed band gaps. In addition, we discuss the nature of the optical thresholds and their dependence on the cation and anion electronegativity.

Polaronic satellites in x-ray-absorption spectra.

Satellite structure in the 2p absorption spectra of early 3d transition-metal compounds is reported. The spectral shape depends strongly on the local symmetry of the excited atom and gives evidence that in Ti oxides the excitation on the metal is screened by an exciton transition in the oxygen band and not by a charge-transfer or Mott-Hubbard transition as in the case of narrow-band metal compounds. The relative satellite intensities can be used to assess the anisotropy of the local electronic structure and the influence of the excited electron on the screening of the core-hole potential.

Photoemission and the Electronic Structure of Transition Metal Compounds

In this paper we study the valence electronic structure of 3d transition metal compounds in an Anderson impurity approximation. Using simplified models we arrive at a classification scheme (in terms of several parameters) for the nature of the band gaps in these materials. Mott-Hubbard insulators, in the simple sense, are special cases in this scheme. Another important class of insulators is characterized by light holes of ligand p character and heavy d-like electrons (charge-transfer semiconductors). Within the same theoretical basis we develop the theory describing the photoemission and inverse photoemission spectra from which one can obtain values for the parameters in the Anderson impurity Hamiltonian. Using an exact two particle Green's function formalism the photoemission of CuCl2 is studied and it is shown that this compound is a charge-transfer semi-conductor. For the more general case an approximate, easy to use method is developed and is applied to the (inverse) photoemission of NiO. It is shown that NiO can be characterized as being intermediate between the Mott-Hubbard and charge-transfer regimes.

Photoconductivity of MnO

The photoconductivity spectra and the carrier drift mobilities of MnO single crystals are measured at various temperatures from 83 to 299 K in the spectral region from 300 to 650 nm by the synchronous detection and transit time methods. The photoconductivity spectra show the photoconductivities associated with the intrinsic inter-band excitations and with the crystal-field adsorptions of Mn2+ ions. The photocarriers are both electrons and holes with high drift mobilities. The results are discussed in terms of recent theories describing the band gaps and the electronic structures of the 3d transition-metal compounds.

Magnetic susceptibilities of actinide 3d-metal intermetallic compounds

We have numerically calculated the magnetic susceptibilities which appear in the Hartree–Fock instability criterion for actinide 3d transition-metal intermetallic compounds. This calculation is based on a previous tight-binding description of these actinide-based compounds [A. Troper and A. A. Gomes, Phys. Rev. B 34, 6487 (1986)]. The parameters of the calculation, which starts from simple tight-binding d and f bands are (i) occupation numbers, (ii) ratio of d-f hybridization to d bandwidth, and (iii) electron-electron Coulomb-type interactions.

Electronic band structures of intercalation compounds of 3d transition metals with TiS2

APW band calculations are made for the non-magnetic states of intercalation compounds MxTiS2 (M = Fe, Co; x = 1, ⅓). The band structures of the ferromagnetic state of FeTiS2 and Fe⅓TiS2 are also calculated. The results are discussed in connection with chemical bonding and magnetic ordering.

Itinerant magnetic character in 3d transition-metal intercalation compounds MxTiS2

The magnetization and specific heat data, together with the transport and photoemission data, of 3d metal intercalation compounds MxTiS2 with the 1T-CdI2 type layered structure, show that an itinerant electron or band picture is valid in this material system, in agreement with band calculations using a self-consistent APW method.