title: Development of electron holes across the temperature-induced semiconductor-metal transition in Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ)(x, y = 0.2-0.8): a soft x-ray absorption spectroscopy study.
abstract: The x-ray absorption spectra of Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) (BSCF) powders quenched in air from 623 and 1173 K were measured at the oxygen K and transition metal L(II,III) edges. All the samples show a predominantly Fe high spin ground state of 3d(5)L character, while the 3d(6)L Co ions are intermediate spin at 623 K and high spin at 1173 K. Further changes in the metal L(II,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, Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) 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 Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) as a material with a negative charge-transfer energy is proposed.
authors.affiliation: Nonmetallic Inorganic Materials, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland.
authors.forename: A S
authors.initials: AS
authors.lastname: Harvey
authors.forename: Z
authors.initials: Z
authors.lastname: Yang
authors.forename: A
authors.initials: A
authors.lastname: Infortuna
authors.forename: D
authors.initials: D
authors.lastname: Beckel
authors.forename: J A
authors.initials: JA
authors.lastname: Purton
authors.forename: L J
authors.initials: LJ
authors.lastname: Gauckler