Microstructural evolution in BaO-Y2O3-Co3O4 mixtures during high-energy milling and its role in the formation of YxBa1-xCoO3-δ and YBaCo4O7

X-ray diffraction study of the microstructural evolution induced by high-energy ball milling and thermal treatment (TT) of the quaternary Y-Ba-Co-O system is described. Mixtures of BaO-Co3O4-Y2O3 were milled for up to 39 h. No reaction was observed during the milling and the threshold behavior of the system yields to mixtures of amorphous and crystalline phases. During thermal treatment it was observed that the microstructure is temperature dependent with a critical temperature of transformation between 700 and 900 oC. Samples TT at 600-700 o C show three phases (Co3O4, Y2O3 and the new phase YxBa1-xCoO3-δ; x∼ 0.29(2)), while samples TT at 900 o C leads to the formation of a single phase (YBaCo4O7). The crystal structure of YxBa1-xCoO3-δ is described and a possible mechanism of its formation is proposed.