The Anisotropy of the Crystal Growth of Fe3O4 in an Aqueous Solution

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Abstract
The anisotropy of the crystal growth of the spinel-type ferrites in an aqueous solution was studied using ferrite films piled up on the glass substrate by means of the thin liquid-film method (TLF method). From the difference between the values of the Oind (the parameter for the degree of the oxidation state), it was found that the Fe3O4 crystal grows in the (111) direction in the “oxidation state” (Oind=1.4×10−2 dm3 min−1), but in the (100) direction in the “Fe2+-rich state” (Oind=3.38×10−4 dm3 min−1). Although the reaction conditions for the formation of the Co(II)– and Zn(II)–ferrites were the same (both used in the “oxidation-state”; Oind=5.5–5.8×10−2 dm3 min−1), the anisotropy of the crystal growth was different between Co2+ [the (111) direction] and Zn2+ [the (100) direction]. These results were explained in terms of difference in the degree of the oxidation state (Oind). For the inverse spinel, the slower rate of the formation of the sublattice of the B site in the “oxidation-state” seems to cause the preferential formation of the A site, since the Fe3+ ions are preferentially incorporated into the A site. This results in the anisotropy in the (111) direction of Fe3O4 and Co(II)–ferrite (inverse spinel). On the contrary, in the “Fe2+-rich state”, the preferential formation of one of the two B sites (the Fe2+ occupies one of the two B sites in the inverse spinel) results in the anisotropy in the (100) direction of Fe3O4. For the normal spinel, the acceleration of the incorporation of the Fe3+ ions into the B site in the “oxidation-state” will result in the anisotropy in the (100) direction. This is the case for the anisotropy in the (100) direction of the Zn(II)–ferrite.

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