Formation and Color of the Solid Solution in CoO-MgO-ZnO System

Abstract

This study was concerned with the formation and color development of the solid solution in CoO-MgO-ZnO system.
Specimens were prepared by calcining the oxide mixture at 1200, 1300, 1400 and 1500°C for 1h respectively.
Formation of solid solution was examined by X-ray analysis, and the lattice parameter were calculated. Furthermore, the color was discussed by measuring the spectral reflectance.
The results obtained at 1400°C, were summarized as follows.
1) Having electronic configuration 3 d⁷, Co²⁺ revealed appreciable tetrahedral preference in addition to the octahedral preference. Therefore, CoOCo₂O₃ (spinel) was observed with (Co, Mg, Zn)O solid solution (rock salt type structure) in CoO rich region.
2) Solid solution limits of approximately 35mol% ZnO into MgO, and approximately 10mol% MgO into ZnO were obtained.
3) ZnO was incorporated into CoO approximately 30mol%, and CoO was incorporated into ZnO approximately 20mol%.
4) The formation range of (Co, Mg, Zn)O solid solution with rock salt type structure was fairly wide, whereas in the case of wurtzite type structure was narrow.
5) Obedience to Vegard's law was observed in mutual replacement of Co²⁺ and Zn²⁺ in the solid solution with rock salt type structure, whereas in CoO-MgO system, the linearity of the lattice parameter was not revealed, owing to the change of bond character.
6) Color obtained in (Co, Mg, Zn)O solid solution with rock salt type structure was pink, and the absorption of octahedral Co²⁺ was observed. Brilliant hue, so-called cobalt green, resulted from the absorption of tetrahedral Co²⁺, developed in (Co, Zn)O and (Co, Mg, Zn)O solid solution with wurtzite type structure.