E. Z. Kurmaev

Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO•7Al 2O 3

Natural 12CaO·7Al2O3 (C12A7) is a wide bandgap insulator, but conductivity can be realized by introducing oxygen deficiency. Currently, there are two competing models explaining conductivity in oxygen-deficient C12A7, one involving the electron transfer via a “cage conduction band” inside the nominal band gap, the other involving electron hopping along framework lattice sites. To help resolve this debate, we probe insulating and conducting C12A7 with X-ray emission, Xray absorption, and X-ray photoemission spectroscopy, which provide a full picture of both the valence and conduction band edges in these materials. These measurements suggest the existence of a narrow conduction band between the main conduction and valence bands common in both conducting and insulating C12A7 and support the theory that free electrons in oxygen-deficient C12A7 occupy the low-energy states of this narrow band. Our measurements are corroborated with density functional theory calculations.

Tuning the electronic structure of graphene through nitrogen doping: experiment and theory

Tuning the electronic properties of graphene by doping atoms into its lattice makes it more applicable for electronic devices. We present a study of nitrogen doped graphene samples grown using chemical vapor deposition with a variety of synthesis conditions. Soft X-ray absorption and emission spectroscopy, which are techniques sensitive to the unoccupied and occupied partial electronic density of states, respectively, were used to study the electronic structure of N-doped graphene. Complementary full-potential, all-electron density functional theory calculations of the measured spectra reveal the existence of graphitic, pyridinic and nitrilic-like sites in the samples studied. Although our study shows that the defect type is sensitively related to the synthesis conditions, the graphitic configuration is found to be the most dominant one in each system studied. The dependence of the defect type on the sample growth conditions and the corresponding shifts in the Fermi energy level from the Dirac point, result in n- or p-type material being obtained.

Effect of additives on titanium-hydrogen interaction under ball milling of Ti powder probed by hard x-ray emission spectroscopy

Titanium is well known as a light-weight hydrogen storage material that is applied as a component of hydrogen storage composites together with Mg and other metals. Amorphous boron, boron nitride, and graphite were used as additives to improve Ti-H2 reactivity during ball-milling due to its anti-sticking and matrix-forming properties. The chemical state and local electronic structure of Ti atoms were studied by hard x-ray emission spectroscopy (XES). We have measured fluorescent Ti Kβ5 (4p→1s transition) x-ray emission spectra, which are very sensitive to the local surroundings of exciting atoms, and found additional features coinciding in energy with spectra of reference samples TiB2, TiN, and TiC. Based on these measurements, it is concluded that atoms of additives form chemical bonding with Ti due to the occupation of interstitials in the host Ti-lattice.

Electronic structure ofBiMO3multiferroics and related oxides

We have performed a systematic study of the electronic structures of BiMeO3 (Me = Sc, Cr, Mn, Fe, Co, Ni) series by soft X-ray emission (XES) and absorption (XAS) spectroscopy. The band gap values were estimated for all compounds in the series. For BiFeO3 a band gap of ~0.9 eV was obtained from the alignment of the O Ka XES and O 1s XAS. The O 1s XAS spectrum of BiNiO3 indicates that the formation of holes is due to a Ni2+ valency rather than a Ni3+ valency. We have found that the O Ka XES and O 1s XAS of BiMeO3 probing partially occupied and vacant O 2p states, respectively, are in agreement with the O 2p densities of states obtained from spin-polarized band structure calculations. The O Ka XES spectra show the same degree of Bi 6s--O 2p hybridization for all compounds in the series. We argue herein that the stereochemical activity of Bi 6s lone pairs must be supplemented with inversion symmetry breaking to allow electric polarization. For BiMnO3 and BiFeO3, two cases of multiferroic materials in this series, the former breaks the inversion symmetry due to the antiferromagnetic order induced by particular orbital ordering in the highly distorted perovskite structure and the latter has rhombohedral crystal structure without inversion symmetry.

LOCAL ELECTRONIC STRUCTURE OF DOPING ATOMS IN MA2Can-1CunO2n+3 HIGH-Tc SUPERCONDUCTORS WITH [M-12(n-1)n] TYPE STRUCTURES

Results of X-ray fluorescence measurements of MA2Can-1CunO2n + 3 compounds with [M-12(n-1)n] type structures, (C0.8N0.2Sr2CuO5.3, [Cu0.25(C,N)0.75]Sr2CaCu2Oy, Cu0.5V0.5Sr2CaCu2O7.5, Cu0.5V0.5Sr2Ca2Cu3O9.4, Cu0.5V0.5Sr2(Ca3.6Sr0.4)Cu5O13.5 and Cu0.5V0.5Sr2(Ca4.7Sr0.3)Cu6O15.3, are presented. The experiments show that C-atoms form a distorted CO3 oxyacid group with a partial substitution of copper and nitrogen atoms in the M-sites. We conclude that the CO32- group in (C,N) Sr2Can-1CunOy and (Cu,C,N) Sr2Can-1CunOy is substituted by the NO2- which induces the creation of the hole carriers in these compounds. We have found that V-atoms in the given compounds form (VO4)3- tetrahedrons and have a pentavalent state.

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.

Electronic structure of Co_xTiSe_2 and Cr_xTiSe_2

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.