LaFe3/4Ni1/4O3 was subjected to oxygen near edge x-ray absorption fine structure (NEXAFS) spectroscopy for 300 K < T < 773 K. The spectra show in the pre-edge a small hole doped peak originating from Ni substitution. The relative spectral weight of t
his transition to the weight of the hybridized O(2p) - Fe(3d) transitions scales with T and has a maximum at around 600 K. 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.
Iron oxide nano particles with nominal Fe2O3 stoichiometry were synthesized by a wet, soft chemical method with the heat treatment temperatures from 250{deg}C to 900{deg}C in air. The variation in the structural properties of the nano particles with
the heat treatment temperature was studied by X-ray diffraction and Fe K shell X-ray absorption study. X-ray diffractograms show that at lower annealing temperatures nano particle comprises both maghemite and hematite phases. With increasing temperature, the remainder of the maghemite phase transformed completely to hematite. Profile analysis of the leading Bragg reflections reveals that the average crystallite size increases from 50 nm to 150 nm with increasing temperature. The mean primary particle size decreased from 105 nm to 90 nm with increasing heat treatment temperature. The X-ray diffraction results are paralleled by systematic changes in the pre-edge structure of the Fe K-edge X-ray absorption spectra, in particular by a gradual decrease of the t2g/eg peak height ratio of the two leading pre-edge resonances, confirming oxidation of the Fe from Fe2+ towards Fe3+. Transmission electron microscopy (TEM) on the samples treated at temperatures as high as 900{deg}C showed particles with prismatic morphology along with the formation of stacking fault like defects. High resolution TEM with selected area electron diffraction (SAED) of samples heat treated above 350{deg}C showed that the nano particles have well developed lattice fringes corresponding to that of (110) plane of hematite.
Electrochemical oxidation of hematite ({alpha}-Fe2O3) nano-particulate films at 600 mV vs. Ag+/AgCl reference in KOH electrolyte forms a species at the hematite surface which causes a new transition in the upper Hubbard band between the Fe(3d)-O(2p)
state region and the Fe(4sp)-O(2p) region, as evidenced by oxygen near edge x-ray absorption fine structure (NEXAFS) spectra. The electrochemical origin of this transition suggests that it is related with a surface state. This transition, not known for pristine {alpha}-Fe2O3 is at about the same x-ray energy, where pristine 1% Si doped Si:Fe2O3 has such transition. Occurrence of this state coincides with the onset of an oxidative dark current wave at around 535 mV - a potential range, where the tunneling exchange current has been previously reported to increase by three orders of magnitude with the valence band and the transfer coefficient by a factor of 10. Oxidation to only 200 mV does not form such extra NEXAFS feature, supporting that a critical electrochemical potential between 200 and 600 mV is necessary to change the electronic structure of the iron oxide at the surface. Decrease of the surface roughness, as suggested by visual inspection, profilometry and x-ray reflectivity, points to faceting as potential structural origin of the surface state.
Iron resonant valance band photoemission spectra of Sr substituted LaFe0.75Ni0.25 O3-{delta} have been recorded across the Fe 2p - 3d absorption threshold to obtain Fe specific spectral information on the 3d projected partial density of states. Compa
rison with La1-xSrxFeO3 resonant VB PES literature data suggests that substitution of Fe by Ni forms electron holes which are mainly O 2p character. Substitution of La by Sr increases the hole concentration to an extent that the eg structure vanishes. The variation of the eg and t2g structures is paralleled by the changes in the electrical conductivity.
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 el
ectron 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.
