title: Reaction temperature variations on the crystallographic state of spinel cobalt aluminate.
abstract: In this study, we report a rapid and simple technique for obtaining cobalt aluminate having a spinel structure. The products were prepared from a hydroxide precursor synthesized by coprecipitation of cobalt (Co(2+)) and aluminum (Al(3+)) nitrates with an alkaline solution. The chosen precursor enabled low temperature fabrication of cobalt aluminate with a spinel structure by sintering it for 2 hours at low temperatures (>400 °C). Crystallographic and thermal analyses suggest that the low-temperature-sintered products contain Co(3+) ions stabilized by chemisorbed water and/or hydroxide groups, which was not observed for products sintered at temperatures higher than 1000 °C. The color of the products turned from clear blue (Thenard's blue) to dark green when sintering temperatures were below 1000 °C. Magnetic quantities, Curie constants, and Weiss temperatures show a strong dependence on the sintering temperature. These findings suggest that there are mixed valent states, i.e. Co(2+) and Co(3+), and unique cation distributions at the different crystallographic sites in the spinel structure, especially in the products sintered at lower temperatures.
authors.affiliation: Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan. mtaguchi@chuo-u.ac.jp
authors.forename: Minori
authors.initials: M
authors.lastname: Taguchi
authors.forename: Takayuki
authors.initials: T
authors.lastname: Nakane
authors.forename: Kenjiro
authors.initials: K
authors.lastname: Hashi
authors.forename: Shinobu
authors.initials: S
authors.lastname: Ohki
authors.forename: Tadashi
authors.initials: T
authors.lastname: Shimizu
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Sakka
authors.forename: Akiyuki
authors.initials: A
authors.lastname: Matsushita
authors.forename: Hiroya
authors.initials: H
authors.lastname: Abe
authors.forename: Toshitaka
authors.initials: T
authors.lastname: Funazukuri
authors.forename: Takashi
authors.initials: T
authors.lastname: Naka