Reaction temperature variations on the crystallographic state of spinel cobalt aluminate

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 (Co2+) and aluminum (Al3+) 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 degrees C). Crystallographic and thermal analyses suggest that the low-temperature-sintered products contain Co3+ ions stabilized by chemisorbed water and/or hydroxide groups, which was not observed for products sintered at temperatures higher than 1000 degrees C. The color of the products turned from clear blue (Thenard's blue) to dark green when sintering temperatures were below 1000 degrees 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. Co2+ and Co3+, and unique cation distributions at the different crystallographic sites in the spinel structure, especially in the products sintered at lower temperatures.