Study on the Formation and the Color Development of Magnesium-Zinc Titanium Spinels containing Co²⁺ and Ni²⁺ ions

Abstract

Between the titanium spinels with the composition of xCoO⋅(2-x)MgO⋅TiO₂ and xCoO⋅(2-x)ZnO⋅TiO₂, remarkable difference of the color development is observed, especially in Co²⁺ ions poor region. The former is bluish green, and the latter brown. The same phenomenon is observed between the other spinels with the composition of xCoO⋅(1-x)NiO⋅MgO⋅TiO₂ and x CoO⋅(1-x)NiO⋅ZnO⋅TiO₂, namely, the former bluish green and the latter greyish or brownish green. This is due to the strong tetrahedral preference of Zn²⁺ ions, as the result of which Co²⁺ ions become absent from tetrahedral interstices in the spinels of x CoO⋅(2-x)ZnO⋅TiO₂ and x CoO⋅(1-x)NiO⋅ZnO⋅TiO₂.
To observe the influence of the preference of each cation such as Mg²⁺, Zn²⁺, Co²⁺ and Ni²⁺ upon the spinel formation and the color development, the gradual substitution of Zn²⁺ for Mg²⁺ in the spinels of CoO⋅MgO⋅TiO₂, NiO⋅MgO⋅TiO₂ and CoO⋅NiO⋅MgO⋅TiO₂ systems was carried out. The components were mixed by wet, calcined. at 1300°C for one hour. The reflectance between 400-760mμ was measured by a selfrecording photoelectric spectrometer to represent the result by C. I. E. color specification, and X-ray analysis was carried out to observe the spinel formation and to calculate the lattice constant. The results were summarized as follows.
1. CoO-MgO-ZnO-TiO₂ system.
The spinels with the composition of 0.2CoO⋅(1.8-x)MgO⋅xZnO⋅TiO₂, 0.5CoO⋅(1.5-x)MgO⋅xZnO⋅TiO₂ and CoO⋅(1-x)MgO⋅xZnO⋅TiO₂ were prepared. The absorption of tetrahedral Co²⁺ ion was observed, and a deep absorption ranging about 550-680mμ, characteristic of it, was also revealed when x=0. But with increasing the amount of x, this deep absorption diminished, and a new absorption about 530mμ due to octahedral Co²⁺ ion appeared. The color became bluish green to brown at the composition in which the tetrahedral Mg²⁺ ions were completely replaced by Zn²⁺ ions. For example, in 0.5CoO⋅(1.5-x)MgO⋅xZnO⋅TiO₂, the substitution proceeded as follows;
Co_0.5Mg_0.5[MgTi]O₄→Co_0.5Zn_0.5[MgTi]O₄→Zn[Co_0.5Zn_0.5Ti)O₄ (the bracket represents octahedral interstices). This substitution was reflected in the lattice constant too.
2. NiO-MgO-ZnO-TiO₂ system
The spinels with the composition of 0.2NiO⋅(1.8-x)MgO⋅xZnO⋅TiO₂ and NiO⋅(1-x)MgO⋅xZnO⋅TiO₂ were prepared. According to Romeijn, Dunitz et al., Ni²⁺ ion has the strong octahedral preference, and in most cases does not occupy tetrahedral interstices owing to its electron configuration d⁸. In this case Ni²⁺ ions occupy octahedral interstices continually. Therefore, there is no difference in color development between the spinels in NiO-MgO-TiO₂ and NiO-ZnO-TiO₂ systems. NiO⋅MgO⋅TiO₂ did not form the spinel structure owing to the difficulty of distribution of cations among tetrahedral and octahedral interstices in the spinel lattice. (Ni²⁺ and Ti⁴⁺ ions have octahedral preference, and Mg²⁺ ion is expected to be the same, because MgO is of