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Effect of annealing on cation distribution in cobalt aluminate

Cobalt aluminate, CoAl2O4, is an intense blue color pigment, known as Thenard blue, characterized by thermal and chemical stability, as well as stability to solar exposure and atmospheric agents. It possesses a normal spinel structure, the space group Fd-3m. The unit cell contains 32 oxygen atoms in cubic close packing, 8 tetrahedral cation sites occupied by divalent Zn2+ cations and 16 octahedral sites occupied by trivalent Al3+. However, spinels can also demonstrate some degree of inversion in the structure which is described by the inversion parameter, delta, defined as a fraction of trivalent metal cations on tetrahedral spinel sites. Elucidating a cation distribution between two cation sites is important, since it is a determining factor for controlling and forecasting the material properties. The aim of this investigation is to study the effect of annealing temperature on cation distribution in cobalt aluminte structure. Cobalt aluminate was prepared from aluminum-sec-butoxide and cobalt(II) nitrate hexahydrate by a sol-gel technique. The obtained material was grinded to fine powder and annealed in the furnace at 500, 600, 700, 800 and 900 °C for 2 h, then slowly cooled to RT. Heating to a chosen temperature was performed at the rate of 10 °C/min. The samples at T<700°C were green, while for samples annealed at higher temperatures the color changed to blue. Annealed samples were characterized by X-ray diffraction at RT using a Philips MPD 1880 counter diffractometer. Diffraction patterns indicated that all annealed samples had the spinel-type structure, no additional phases were detected. Unit-cell parameter a was determined using silicon powder as an internal standard, and refined by the whole-powder-pattern fitting method. It increased with the increase of annealing temperature. Rietveld refinement revealed that for the samples annealed below 700 °C the structure can be described with formula IV[Co2+] VI[Co3+Al3+] O4. At temperatures above 700°C cobalt is present only as Co2+.

Cobalt aluminate; Inverse spinel structure; X-ray diffraction; Rietveld method

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