Effect of charge transfer on EELS integrated cross sections in Mn and Ti oxides

The influence of the charge transfer between metal and oxygen atoms on the EELS integrated cross sections has been studied experimentally and theoretically in titanium and manganese oxides of different valence. It is demonstrated that the behavior of the metal L-2,L-3 integrated cross section can be connected with the formal valence of the compound. The charge depletion is most noticeable in the energy-loss region 10-20 eV above the threshold while, at greater energy windows, the increasing contribution of continuum states masks the effect of the charge transfer. This contribution of continuum states is independent of the local chemical environment and can be calculated in a free atom model. Complementary, the integrated cross section near the threshold can be reliably calculated by the LAPW method provided that the simulation of EELS accounts for the matrix elements and angular dependence of the scattering. The LAPW method is successful near the threshold, however, at higher energy-losses, the standard LAPW basis set is insufficient with respect to the continuum states. Extending the basis set with extra localized orbitals allows one to account partially for the contribution of the continuum states and therefore to enlarge the applicability range of the LAPW calculations. The created core hole might noticeably affect the intensity scale of integrated cross sections but this can be modeled by introducing a core-hole in calculations also. Finally it is demonstrated that the effect of charge transfer on EELS cross sections is adequately reproduced by the LAPW calculations.