Hole-state density of La1-xSrxCoO3-δ(0≤x≤0.5) across the insulator/metal phase boundary

Oxygen K near-edge x-ray-absorption fine-structure (NEXAFS) of polycrystalline and film La1-xSrxCoO3-delta at room temperature was studied using fluorescence yield techniques. The bulk-sensitive nature of fluorescence yield (relative to electron yield), combined with the 0.2 eV incident photon resolution, allow us to accurately determine the evolution with x and delta of the oxygen K NEXAFS. For LaCoO3, a complex prepeak is centered near an energy E = 530.5 eV. This peak, which has a full width at half maximum of about 3.0 eV, has been identified with oxygen bonding to Co 3d electrons. With Sr substitution we observe a distinct oxygen hole peak with an intensity proportional to x, centered near E = 528.8 eV, with a full width at half maximum of 1.2 eV. In oxygen-reduced samples the hole peak tends to decrease in intensity as oxygen is removed. Comparisons are made with theoretical work and with earlier oxygen K near-edge studies on similar materials, most of which utilized electron yield. The behavior of the prepeak features with Sr (hole) doping parallels that observed in the oxide superconductors. Oxygen K NEXAFS of La0.5Sr0.5CoO3-delta thin films is compared with bulk results to estimate relative oxygen contents of the films.