Microstructural evolution in BaO-Y sub(2)O sub(3)-Co sub(3)O sub(4) mixtures during high-energy milling and its role in the formation of Y sub()xa sub(1-)xoO sub(3-) delta and YBaCo sub(4)O sub(7)

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-Co sub(3)O sub(4)-Y sub(2)O sub(3) 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 degree C. Samples TT at 600-700 degree C show three phases (Co sub(3)O sub(4), Y sub(2)O sub(3) and the new phase Y sub()xa sub(1-)xoO sub(3-) delta [MathML equation] 0.29(2)), while samples TT at 900 degree C leads to the formation of a single phase (YBaCo sub(4)O sub(7)). The crystal structure of Y sub()xa sub(1-)xoO sub(3-) delta is described and a possible mechanism of its formation is proposed.