Origin of the pre-peak features in the oxygen K-edge x-ray absorption spectra ofLaFeO3 andLaMnO3 studied by Ga substitution of the transition metal ion

Abstract

We report on experimental oxygen K-edge x-ray absorption near edge structure (XANES) spectra ofthe LaFe1 − xGaxO3 and LaMn1 − xGaxO3 series. Transition metal substitution by the 3d full shell Ga atom is mainly reflected in asystematic decrease of the pre-edge structures in the XANES spectra of the two series.This result shows that the associated states originate from the hybridization ofoxygen 2p and unoccupied Fe (or Mn) 3d states. In order to gain insight intothe states associated with the pre-edge spectral features, we have performed abinitio theoretical calculations based on multiple scattering theory. Simulationswith variable cluster size and composition around the absorber oxygen in theLaFeO3 andLaMnO3 crystal structures were carried out. We find that the low-energy pre-peak is reproducedonce the absorbing oxygen and the two nearest neighbour Fe (or Mn) ions are considered inthe cluster. Conversely, higher energy pre-peaks only arise when the full oxygencoordination geometry around the two metal sites is taken into account, implyingthat their energy distance is a reflection of the strength of the oxygen ligandfield. Substitutions of the two nearest neighbours by Ga atoms in the cluster ofcalculation lead to changes in the theoretical spectra that reasonably agree with theevolution of the pre-peaks in the experimental XANES spectra of both the series.