title: Structural studies of a mixed-valence state in the incommensurate composite crystal Sr1.261CoO3.
abstract: The incommensurate modulated crystal structure of the hexagonal cobalt oxide Sr1.261CoO3has been studied using a four-dimensional (4D) superspace profile analysis of neutron powder diffraction data. Sr1.261CoO3is a composite crystal that consists of the [CoO3] and [2Sr] subsystems. The [CoO3] subsystem forms 1D chains that run parallel to thec-axis and consist of face-sharing CoO6polyhedra with octahedral (Oh) and trigonal prismatic (TP) coordinations. The structure analysis reveals that the [CoO3] chains contain 73.9%Ohand 26.1%TPsites, and that theTPsites have longer Co-O bonds than theOhsites:dav. =2.039(4) Å (TP) and 1.895(3) Å (Oh). The averaged Co bond valences are Co3.56(3)+in theOhsites and Co2.45(3)+in theTPsites, suggesting that a considerable amount of Co3+ions are mixed with Co4+ions in theOhsites and with Co2+ions in theTPsites. The observed magnetic susceptibility can be well explained assuming that the compound has the Co mixed-valence state with the spin configurations ofS=0 low-spin state for Co3+(dε6),S=1/2 low-spin state for Co4+(dε5) andS=3/2 high-spin state for Co2+(dε5dγ2). The Weiss temperature, approximately 0.8 K, implies that Sr1.261CoO3naturally assumes a Curie paramagnetic state, probably owing to the obstruction of the intrachain magnetic interaction by the nonmagnetic Co3+ions. These results suggest that the nonmagnetic Co3+ions play an essential role in the magnetism of Sr2γCoO3systems.
authors.affiliation: Superconducting Materials Center, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Isobe
authors.affiliation: Superconducting Materials Center, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Yoshida
authors.affiliation: Superconducting Materials Center, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Eiji Takayama
authors.initials: ET
authors.lastname: Muromachi
authors.affiliation: Institute for Materials Research (IMR), Tohokus University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Ohoyama
keywords: Rietveld analysis; cobalt oxide; hexagonal perovskite structure; modulated structure; neutron diffraction