Selma Erat

Pre-edges in oxygen (1s) x-ray absorption spectra: A spectral indicator for electron hole depletion and transport blocking in iron perovskites

Partial substitution of Fe by Ta in the hole-doped La0.5Sr0.5FeO3 decreases the electric conductivity by up to three orders of magnitude. This decrease is in immediate correlation with a decrease of the electron hole concentration and a shift of the spectral weight within the O(1s)-Fe(3d) mixed states from eg bands near the Fermi energy to t2g bands. Corresponding differences in the Fe(2p) and O(1s) X-ray absorption spectra reveals formation of states with increased covalency in the initial state and states which contain O(2p) character, and that hole states are responsible for transport changes in the material. The intensity ratio of eg and t2g bands appears to be a spectral indicator for hole formation.The eg↑/(t2g↓+eg↓) band ratio in cation-substituted La–Fe oxides is identified in Ou2009(1s) x-ray absorption spectra as a linear spectral indicator for conducting electron holes. The t2g↓ and eg↓ bands act as a conductivity inhibitor by ferromagnetic double exchange coupling on the eg↑ electron. Disorder induced by substitution appears to modulate the hole conduction such that an exponential relation is found between the conductivity and the eg↑/(t2g↓+eg↓) ratio and hole concentration. The quantitative correlation of conductivity and x-ray absorption spectra from heterovalent-substituted LaFeO3 lets substitution-driven metal insulator transitions appear in a new light.

Yttrium and hydrogen superstructure and correlation of lattice expansion and proton conductivity in the BaZr0.9Y0.1O2.95 proton conductor

Bragg reflections in Y-resonant x-ray diffractograms of BaZr0.9Y0.1O2.95 (BZY10) reveal that Y is organized in a superstructure. Comparison with neutron diffraction superstructure reflections in protonated/deuterated BZY10 suggests that both superstructures are linked, and that protons move in the landscape imposed by the Y. The thermal lattice expansion decreases abruptly for protonated BZY10 at T≥648±20u2002K, coinciding with the onset of lateral proton diffusion and suggesting a correlation of structural changes and proton conductivity. The chemical shift in the Y L1-shell x-ray absorption spectra reveals a reduction from Y3+ toward Y2+ upon protonation.

Correlation of O (1s) and Fe (2p) near edge x-ray absorption fine structure spectra and electrical conductivity of La1−xSrxFe0.75Ni0.25O3−δ

A-site substitution of La3+ by Sr2+ in polaron conducting ABO3-type perovskite La1−xSrxFe0.75Ni0.25O3−δ causes oxidation of Fe3+ toward Fe4+ and formation of conducting electron holes, as evidenced by Fe (2p) and O (1s) near edge x-ray absorption fine structure spectra. Hole doping is reflected by linear variation of the prepeak ratio eg(↑)/[t2g(↓)+eg(↓)] of oxygen spectra, along with increased conductivity. The significant increase in conductivity due to NiO doping in La1−xSrxFeO3−δ is caused by increased overlap between Fe (3d) and O (2p) and charge transfer from the O (2p) to the Ni (3d) states, as concluded from near edge x-ray absorption fine structure spectra and ligand field multiplet calculations.The conductivity of the electron hole and polaron conductor La1-xSrxFe0.75Ni0.25O3-{delta}, a potential cathode material for intermediate temperature solid oxide fuel cells, was studied for 0<x<1 and for temperatures 300 K<T<1250 K. In LaSrFe-oxide, an ABO3 type perovskite, A-site substitu-tion of the trivalent La3+ by the divalent Sr2+ causes oxidation of Fe3+ towards Fe4+, which forms conducting electron holes. Here we have in addition a B-site substitution by Ni. The compound for x = 0.5 is identified as the one with the highest conductivity ({sigma} ~ 678 S/cm) and lowest activation energy for polaron conductivity (Ep = 39 meV). The evolution of the electronic structure was monitored by soft x-ray Fe and oxygen K-edge spectroscopy. Homogeneous trend for the oxida-tion state of the Fe was observed. The variation of the ambient temperature conductivity and activation energy with relative Sr content (x) shows a correlation with the ratio of (eg/eg+t2g) in Fe L3 edge up to x=0.5. The hole doping process is reflected by an almost linear trend by the variation of the pre-peaks of the oxygen K-edge soft x-ray absorption spectra.

Electron hole–phonon interaction, correlation of structure, and conductivity in single crystal La0.9Sr0.1FeO3−δ

Electric conductivity and structural details of the hole-doped polaron conductor La0.9Sr0.1FeO3 (LSF10) are reported. The conductivity of a single crystal shows an exponential increase with temperature with a maxi-mum of 100 S/cm at 700 K and with activation energy of about 375 meV, and a decrease for higher temperatures that follows a power law. The exponential increase of the electric conductivity for 300 K {leg} T {leg} 700 is accompanied by a shift of spectral weight in the photoemission valence band towards the Fermi level, indicative of a strong electron-phonon interaction. The subsequent decrease of the conductivity for T>700 K is accompanied by a reversible phase transformation from orthorhombic to rhombohedral symmetry. The decreasing conductivity for T>700 K is likely due to the reduction of the iron due to oxygen loss causing a decreasing hole concentration, as evidenced by a substantial chemical shift in the Fe K-shell x-ray absorption spectra. Two additional fine structures in the conductivity data at 357 K, this is, a small temperature reversible jump in the conductivity, and at 573 K, a slight reversible increase of the polaron activation energy, are correlated with an exceptionally strong decrease in spectral valence band intensity near the Fermi level, and with the onset of a corresponding structural transition.The conductivity and structure of the hole-doped polaron conductor La0.9Sr0.1FeO3−δ are reported for elevated temperatures. The conductivity increases exponentially with temperature to a maximum and decreases for T>700K following a power law, accompanied by a shift of spectral weight in the photoemission valence band. The conductivity decrease is accompanied by a phase transformation, due to the reduction of Fe, as evidenced by x-ray absorption spectra. Additional fine structures in the conductivity are correlated with a strong decrease in valence band intensity near EF and with the onset of a corresponding structural transition.

The effect of annealing temperature on the structural, optical, and electrical properties of CdS films

Cadmium sulfide (CdS) photocatalyst films were grown on glass by chemical bath deposition (pH 9.4, 70 °C) and then annealed in nitrogen from 423 K to 823 K in steps of 100 K. The XRD crystallite size increases in a sigmoidal manner from 60 nm to 100 nm while the optical band gap energy decreases from 2.42 eV to 2.28 eV. This trend is paralleled by the decreasing Urbach energy, but only up to 623 K, where it increases again. This is the temperature where the Cd effectively surpasses the phase transformation from cubic to hexagonal, and the activation energy for electronic transport drops by a factor of nearly two.

Entanglement of charge transfer, hole doping, exchange interaction, and octahedron tilting angle and their influence on the conductivity of La1−xSrxFe0.75Ni0.25O3−δ: A combination of x-ray spectroscopy and diffraction

Substitution of La by Sr in the 25% Ni doped charge transfer insulator LaFeO3 creates structural changes that inflect the electrical conductivity caused by small polaron hopping via exchange interactions and charge transfer. The substitution forms electron holes and a structural crossover from orthorhombic to rhombohedral symmetry, and then to cubic symmetry. The structural crossover is accompanied by a crossover from Fe3+-O2--Fe3+ superexchange interaction to Fe3+-O2--Fe4+ double exchange interaction, as evidenced by a considerable increase of conductivity. These interactions and charge transfer mechanism depend on superexchange angle, which approaches 180{deg} upon increasing Sr concentration, leading an increased overlap between the O (2p) and Fe/Ni (3d) orbitals.

The effect of growing time and Mn concentration on the defect structure of ZnO nanocrystals: X-ray diffraction, infrared and EPR spectroscopy

ZnO nanopowder was synthesized via a hydrothermal route and characterized with several methods such as XRD, TG/DTA, FT-IR, FE-SEM, TEM and Electron Paramagnetic Resonance spectroscopy (EPR) in order to investigate the effect of growing time and Mn doping on the defects which occurred. The pure ZnO nanopowder was obtained in a hexagonal phase with (101) as the preferred orientation except for the one prepared for 36 h which is (002). The growing time does effect the orientation of the crystallite whereas the Mn-doping does not. The concentration of Mn2+ significantly increases the spin–spin interaction in the ZnOu2006:u2006Mn nanopowder. It was observed that there was a competition between intrinsic (Zn and O vacancies) and extrinsic (Mn2+ ion) structural defects but still the former defects are dominant in ZnOu2006:u2006Mn. The effect of growing time and concentration of Mn2+ on the activation energy of ZnO and the ZnOu2006:u2006Mn nanopowder are calculated by Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods.

Iron-resonant valence band photoemission and oxygen near edge x-ray absorption fine structure study on La1−xSrxFe0.75Ni0.25O3−δ

Iron resonant valence band photoemission spectra (VB PES) of Sr substituted LaFe0.75Ni0.25O3−δ have been recorded across the Feu20022p-3d absorption threshold to obtain Fe specific spectral information on the 3d projected partial density of states. Comparison with La1−xSrxFeO3 resonant VB PES literature data suggests that substitution of Fe by Ni forms electron holes which have mainly Ou20022p character. Substitution of La by Sr increases the hole concentration to an extent that the eg structure vanishes. The variation in the eg and t2g structures is paralleled by the changes in the electrical conductivity.

High temperature oxygen near edge x-ray absorption fine structure valence band spectra and conductivity of LaFe3/4Ni1/4O3 from 300 to 773 K

LaFe3/4Ni1/4O3 was subjected to oxygen near edge x-ray absorption fine structure spectroscopy for 300u2009Ku2009<u2009Tu2009<u2009773u2009K. The spectra show in the pre-edge a small hole doped peak originating from Ni substitution. The relative spectral weight of this transition to the weight of the hybridized O(2p)-Fe(3d) transitions scales with T and has a maximum at around 600u2009K. The characteristic energies of the thermal activated spectral intensity and conductivity suggest that the concentration of charge transferred electrons from O(2p) to Ni(3d) increases and that the pre-edges account in part for the polaron activated transport.

Correlation of high temperature x-ray photoemission valence band spectra and conductivity in strained LaSrFeNi oxide on SrTiO3(110)

Reversible and irreversible discontinuities at around 573 K and 823 K in the electric conductivity of a strained 175 nm thin film of (La0.8Sr0.2)0.95Ni0.2Fe0.8O3-{delta} grown by pulsed laser deposition on SrTiO3 (110) are reflected by valence band changes as monitored in photoemission and oxygen K-edge x-ray absorption spectra. The irreversible jump at 823 K is attributed to depletion of doped electron holes and reduction of Fe4+ to Fe3+, as evidenced by oxygen and iron core level soft x-ray spectroscopy, and possibly of a chemical origin, whereas the reversible jump at 573 K possibly originates from structural changes.Reversible and irreversible discontinuities at around 573 and 823 K in the electric conductivity of a strained 175 nm thin film of (La0.8Sr0.2)0.95Ni0.2Fe0.8O3−δ grown by pulsed laser deposition on SrTiO3 (110) are reflected by valence band changes as monitored in photoemission and oxygen K-edge x-ray absorption spectra (XAS). The irreversible jump at 823 K is attributed to depletion of doped electron holes concomitant with reduction of Fe3+ toward Fe2+, as evidenced by oxygen and iron core level soft XAS, and possibly of a chemical origin, whereas the reversible jump at 573 K possibly originates from structural changes.