Nanograincd (grain size 10 nm) ZnO films with various Co contents (0-52 at.percent Co) were synthesized by a novel liquid ceramics method. The solubility limit for Co was determined at 550 deg C. The lattice parameter c of the ZnO-based solid solution with wiirzite structure ceases to grow at 33 at.percent Co. The peaks of the second phase (Co_2O_3 or with cubic lattice) become visible in the X-ray diffraction spectra at 40 at percent Co. The same second phase appears in the bulk ZnO already at 2 at percent Co Bates CH, White WB, Roy R. J Inorg Nucl Chem 1966;28:397. A few years ago it was predicted theoretically that ZnO could become ferromagnetic at room temperature and above by doping with Co and other transition metals. Recently published papers on the structure and magnetic behaviour of Co-doped ZnO allowed us to obtain the size dependence of Co solubility in ZnO for the polycrystals and small single-crystalline particles. The overall Co solubility drastically increases with decreasing grain size. The quantitative estimation leads to the conclusion that, close to the bulk sol- limit, the thickness of a Co-enriched layer is several monolayers in grain boundaries and at least two monolayers in the free surfaces.
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