XANES study of the role of Sr doping in LaCoO3

With the comparisons of experimental XAFS data of Sr doped LaCoO3 samples taken at room temperature and LDA+U (with U = 0,4eV and 8eV, respectively) model calculations of LaCoO3, it is found that the experimental XAFS data of Sr doped samples can be well reconstructed if the samples are phase separated. When U takes 4eV or 8eV, the ground state of LaCoO3 is Mott insulator and the curve shape of calculated XAFS data is even the same as that of the experimental data; when U=0 the HOMO-LUMO band gap vanishes and the calculated XAFS data gets an extra peak. The curve can be well fitted with experimental data of Sr doped LaCoO3 sample. It shows that the doped LaCoO3 sample is not traditional Mott insulator but with some properties of charge transfer type, that is, when Sr ions are doped in LaCoO3, the trivalent Co ions become tetravalent and leave an extra hole in each Co eg orbital. The charge carriers can hop back and forth between trivalent Co ions and tetravalent ones in which way close the band gap.