Development of electron holes across the temperature-induced semiconductor-metal transition in Ba1-xSrxCo1-yFeyO3-δ (x, y=0.2-0.8): a soft x-ray absorption spectroscopy study

The x-ray absorption spectra of Ba1-xSrxCo1-yFeyO3-delta (BSCF) powders quenched in air from 623 and 1173 K were measured at the oxygen K and transition metal L-II,L-III edges. All the samples show a predominantly Fe high spin ground state of 3d(5)(L) under bar character, while the 3d(6)(L) under bar Co ions are intermediate spin at 623 K and high spin at 1173 K. Further changes in the metal LII, III peaks caused by higher temperature quenching are attributed to changes in symmetry around the cations associated with oxygen loss. The oxygen K spectra show the development of unoccupied states just above the Fermi level for samples quenched from 1173 K. At 1173 K, Ba1-xSrxCo1-yFeyO3-delta shows metallic conductivity, while at 623 K it is a semiconductor; the states developed at high temperature with strong oxygen character are pathways for hole conductivity. Splitting of the transition metal 3d energy levels by the ligand field was observed in the oxygen K spectra, and the range for 10Dq is 1.6-1.8 eV, while the 3d bandwidth is 1.1-1.4 eV in samples quenched from 623 K. On the basis of the soft x-ray absorption results, the classification of Ba1-xSrxCo1-yFeyO3-delta as a material with a negative charge-transfer energy is proposed.