Cr 3d Electrons Research Articles

Structural and Electronic Properties of Reduced Transition Metal Oxide Clusters, M3O8and M3O8−(M = Cr, W), from Photoelectron Spectroscopy and Quantum Chemical Calculations

We report a comparative study of reduced transition metal oxide clusters, M(3)O(8)(-) (M = Cr, W) anions and their neutrals, via anion photoelectron spectroscopy (PES) and density functional theory (DFT) and molecular orbital theory (CCSD(T)) calculations. Well-resolved PES spectra are obtained for M(3)O(8)(-) (M = Cr, W) at 193 and 157 nm photon energies. Different PES spectra are observed for M = Cr versus M = W. Extensive DFT and CCSD(T) calculations are performed to locate the ground and low-lying excited states for the neutrals and anions. The ground states of Cr(3)O(8) and Cr(3)O(8)(-) are predicted to be the (3)B(2) and (4)B(2) states of a C(2v) structure, respectively, revealing ferromagnetic spin coupling for Cr 3d electrons. In contrast, the ground states of W(3)O(8) and W(3)O(8)(-) are predicted to be the (1)A' state (C(s) symmetry) and the (2)A(1) state (C(2v) symmetry), respectively, showing metal-metal d-d bonding in the anion. The current cluster geometries are in qualitative agreement with prior DFT studies at the PBE level for M = Cr and the B3LYP level for M = W. The BP86 and PW91 functionals significantly outperform the B3LYP functional for the Cr species, in terms of relative energies, electron detachment energies, and electronic excitation energies, whereas the B3LYP functional is better for the W species. Accurate heats of formation for the ground states of M(3)O(8) are calculated from the clustering energies and the heats of formation of MO(2) and MO(3). The energetics have been used to predict redox reaction thermochemistry.

Structural and Magneto-transport Properties of Cr-doped In2O3 Diluted Magnetic Semiconductors

AbstractWe report the experimental study of the structural and magnetotransport properties of chromium-doped indium oxide (In2O3:Cr) thin films using x-ray diffractometer, and by measuring the resistivity and Hall effect as a function of temperature in various magnetic fields. The In2O3:Cr diluted magnetic semiconductor thin films were grown under different partial oxygen pressures (Po2) on sapphire substrates using pulsed laser deposition (PLD) technique. Observed expansions in lattice parameter and crystal size in these films with increase in oxygen growth pressure are traceable to the reduction in oxygen vacancies. A redshift of the absorption edges of the samples with increase in oxygen growth pressure is attributed to the significant improvement in crystallinity. The exchange interaction between the electron spins in the conduction band and the spins of the Cr 3d electrons was evident in the anomalous Hall effect (AHE), which persisted up to 300 K. An analysis of the dc electrical transport in the films was carried out using hopping conduction and ionized impurity scattering models.

Correlation effect and magnetic moments in Cr2Te3

The electronic and magnetic structures of Cr2Te3 have been studied theoretically using the linearized muffin-tin orbitals band method. Experimental photoemission spectra and magnetic moments can be described better when the on-site Coulomb correlation U of Cr 3d electrons is considered using the local spin-density approximation+U method. The proper size of U is found to be U∼1.7eV. The complex magnetic behaviors of Cr2Te3 come from the degeneracy of parallel and antiparallel alignments of CrI spin to CrII and CrIII spins.

Orbital moments of CrO 2 and Fe 3O 4 studied by MCD in soft X-ray absorption

We present studies of orbital magnetic moments of CrO 2 and Fe 3O 4 by measuring magnetic circular dichroism (MCD) in soft X-ray absorption. The results show that Fe 3O 4 exhibits large orbital moments, as a consequence of the strong Coulomb interactions of Fe 3d electrons. In contrast, orbital moments of Cr in CrO 2 are nearly quenched, revealing that Cr 3d electrons are strongly hybridized with O 2p electrons and more delocalized as compared with those of Fe in Fe 3O 4. Comparing the MCD data with the band structure calculations based on local spin density approximation with on-site Coulomb energy U taken into account, we conclude that to include the on-site Coulomb interactions of 3d electrons is essential for adequately describing the electronic structure of CrO 2 and Fe 3O 4.

Electronic structures of layered compound LaCrSb3

We have investigated the electronic structure of LaCrSb 3 to explore the origin of the coexistence of the ferromagnetic (FM) and antiferromagnetic (AFM) spin sublattices. There exist both localized and itinerant Cr 3d electrons, suggesting the double exchange mechanism for the FM ordering. In the FM ground state, the 2D Fermi surfaces show the prominent nesting features, which is possibly related to the additional AFM ordering on top of the FM ordering. We also discuss the absence of Cr magnetic moments in RCrSb 3 with R=Gd, Tb, and Dy.

Surface magnetism during the early stages of oxygen-assisted growth of Cr on Fe(0 0 1): A SPMDS study

The surface magnetic properties of ultrathin Cr films grown on O(1×1)-Fe(0 0 1) have been studied by means of spin-polarised metastable de-excitation spectroscopy (SPMDS). The spin-selected surface density of states extracted from the experimental SPMDS data provides information on both composition and magnetic properties of the topmost layer. We find that oxygen floats on the surface of the growing Cr film and that Cr/Fe intermixing is restricted to the first Cr layer. Concerning the magnetic properties, a strong polarisation of Cr 3d electrons is found in agreement with theoretical calculations. Oxygen 2p states are polarised too and their polarisation is opposite to that of Cr 3d states. Surface magnetisation as a function of the Cr film thickness shows an oscillatory behaviour induced by the layered antiferromagnetic structure of Cr. At variance with the results of other experimental investigation in this case the layered antiferromagnetic structure of Cr is already present in the 3 ML thick film.

Electronic structure ofCoxTiSe2andCrxTiSe2

The results of investigations of intercalated compounds Cr_xTiSe_2 and Co_xTiSe_2 by X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) are presented. The data obtained are compared with theoretical results of spin-polarized band structure calculations. A good agreement between theoretical and experimental data for the electronic structure of the investigated materials has been observed. The interplay between the M3d--Ti3d hybridization (M=Cr, Co) and the magnetic moment at the M site is discussed. A 0.9 eV large splitting of the core Cr2p{3/2} level was observed, which reveals a strong exchange magnetic interaction of 3d-2p electrons of Cr. In the case of a strong localization of the Cr3d electrons (for x<0.25), the broadening of the CrL spectra into the region of the states above the nominal Fermi level was observed and attributed to X-ray re-emission. The measured kinetic properties are in good accordance with spectral investigations and band calculation results.

XPS study of electronic structure change of the Fe [sbnd]Cr system at temperatures below the brittleness threshold

A comparative X-ray photoelectron spectra (XPS) study of the electronic structure change and chemical bond change of the 3d-metal-based alloys at temperatures below the brittleness threshold has been carried out. It has been found that the redistribution of the interatomic bond takes place in the transition metal alloys. When decreasing the temperature below the brittleness range, the bond covalent component prevails, in which Fe and Cr 3d-electrons take part. It has been shown that the hybridization is affected by the Cr separation into layers. It is characteristic that the changes of the Fe Cr system chemical bond is reversible, i.e. on heating the system to room temperature, the hybridization decreases.

Electron spectroscopies and partial excitation spectra in Cr 2O 3

Polycrystalline Cr 2O 3 films grown on Cr metal have been measured by photoemission (PES), inverse photoemission (IPES, BIS) and electron-energy-loss spectroscopy (EELS). The high energy EELS data exhibit three structures in the range up to 15 eV loss energy which can be identified with transitions from occupied to unoccupied states in the combined XPS/BIS spectra. The valence band PES data measured at XPS, He-I and He-II photon energies are decomposed into the Cr-3d and O-2p contributions showing strong hybridizations over the whole valence band range and an occupation of about 3d 4.15 of the Cr-3d state. A valence band satellite observed at about 14 eV is similar to those in the Cr core level spectra and may therefore be identified as a many body satellite from photoemission of localized Cr-3d electrons.

Electron spectroscopies and partial excitation spectra in Cr2O3

Polycrystalline Cr 2 O 3 films grown on Cr metal have been measured by photoemission (PES), inverse photoemission (IPES, BIS) and electron-energy-loss spectroscopy (EELS). The high energy EELS data exhibit three structures in the range up to 15 eV loss energy which can be identified with transitions from occupied to unoccupied states in the combined XPS/BIS spectra. The valence band PES data measured at XPS, He-I and He-II photon energies are decomposed into the Cr-3d and O-2p contributions showing strong hybridizations over the whole valence band range and an occupation of about 3d 4.15 of the Cr-3d state. A valence band satellite observed at about 14 eV is similar to those in the Cr core level spectra and may therefore be identified as a many body satellite from photoemission of localized Cr-3d electrons.

Electronic and magnetic properties of the spinel semiconductor CdCr2Se4.

We present a first-principles study of the electronic and magnetic properties of the chromium spinel ${\mathrm{CdCr}}_{2}$${\mathrm{Se}}_{4}$, a ferromagnetic semiconductor, which, for its interesting magnetic and electric properties has been the object of many experimental studies. Using calculations based on the full-potential linearized augmented plane wave relativistic scheme we are able to reproduce the structural equilibrium properties and to explain many of the features measured by resonant photoemission spectroscopy. In particular, we analyze the contribution of the Cr 3d electrons in a density-of-states study including the two spin orientations and a comparison with the binary analog CdSe.

Valence band photoelectron spectra of CrBr3 insulators

The valence band structure of CrBr3 has been obtained by ultraviolet and X-ray photoelectron spectroscopy. The comparison of the X-ray photoemission valence band spectrum with published band calculations for the ground state within a one-electron formalism shows satisfactory agreement between observed and calculated structures. The contribution of the Cr 3d electrons relative to that of the Br 4p electrons in the photoemission energy distribution has been evaluated and compared with the ratio of photoionization cross sections for Kr 4p6 (calculated) and Cu 3d10 (experimental) at 40.8 eV. The degree of the p/d hybridization has then been estimated to be around 10-15%.

Photoemission and magnetic effects in NaCrS2

X-ray and angle-resolved ultraviolet photoemission data on the layered compound NaCrS2 are presented and related to the electronic structure. The data indicate that the Cr 3d electrons are essentially localised and give rise to a multiplet splitting of the Cr 3s emission line; the magnitude of the splitting is discussed in terms of the magnetic properties of the material and the net spin moment of the Cr ions.