title: Valence transitions in negative thermal expansion material SrCu₃Fe₄O₁₂.
abstract: The valence states of a negative thermal expansion material, SrCu3Fe4O12, are investigated by X-ray absorption and (57)Fe Mössbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr(2+)Cu(~2.4+)3Fe(~3.7+)4O12. The valence states continuously transform to Sr(2+)Cu(~2.8+)3Fe(~3.4+)4O12 upon cooling to ~200 K, followed by a charge disproportionation transition into the Sr(2+)Cu(~2.8+)3Fe(3+)(~3.2)Fe(5+)(~0.8)O12 valence state at ~4 K. These observations have established the charge-transfer mechanism in this compound, and the electronic phase transitions in SrCu3Fe4O12 can be distinguished from the first-order charge-transfer phase transitions (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in Ln(3+)Cu(2+)3Fe(3.75+)4O12 (Ln = trivalent lanthanide ions).
authors.affiliation: Nanoscience and Nanotechnology Research Center, Osaka Prefecture University , 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.
authors.forename: Ikuya
authors.initials: I
authors.lastname: Yamada
authors.forename: Kentaro
authors.initials: K
authors.lastname: Shiro
authors.forename: Hidenobu
authors.initials: H
authors.lastname: Etani
authors.forename: Shohei
authors.initials: S
authors.lastname: Marukawa
authors.forename: Naoaki
authors.initials: N
authors.lastname: Hayashi
authors.forename: Masaichiro
authors.initials: M
authors.lastname: Mizumaki
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Kusano
authors.forename: Shigenori
authors.initials: S
authors.lastname: Ueda
authors.forename: Hideki
authors.initials: H
authors.lastname: Abe
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Irifune