Development of electron holes across the temperature-induced semiconductor–metal transition inBa1−xSrxCo1−yFeyO3−δ(x, y = 0.2–0.8): a soft x-ray absorption spectroscopy study

Abstract

The x-ray absorption spectra ofBa1−xSrxCo1−yFeyO3−δ (BSCF) powders quenched in air from 623 and 1173 K were measured at the oxygenK and transitionmetal LII,III edges. All the samples show a predominantly Fe high spin ground state of3d5L character,while the 3d6L Co ions are intermediate spin at 623 K and high spin at 1173 K. Further changes in the metalLII,III peaks caused by higher temperature quenching are attributed to changesin symmetry around the cations associated with oxygen loss. The oxygenK spectra show the development of unoccupied states just abovethe Fermi level for samples quenched from 1173 K. At 1173 K,Ba1−xSrxCo1−yFeyO3−δ shows metallic conductivity, while at 623 K it is a semiconductor; the states developed at hightemperature with strong oxygen character are pathways for hole conductivity. Splitting ofthe transition metal 3d energy levels by the ligand field was observed in the oxygenK spectra, andthe range for 10Dq is 1.6–1.8 eV, while the 3d bandwidth is 1.1–1.4 eV in samples quenched from623 K. On the basis of the soft x-ray absorption results, the classification ofBa1−xSrxCo1−yFeyO3−δ as a material with a negative charge-transfer energy is proposed.