EPR spectra of (i) CuO annealed in air at different temperatures up to 900 °C and (ii) CuO reacted with BaO in the ratio 3:2 at different temperatures up to 900 °C were studied. In (i), evidence for the formation of monomers, dimers, and tetramers of copper ions was found. In (ii), in addition to those spectra obtained in (i), aggregates of four copper ions coupled through cyclic exchange with resultant electronic spin S=2 and affected by the immediate barium neighbors were obtained. It is found that these copper aggregates giving the EPR signals are magnetically isolated from the main CuO cluster.

• Received 9 August 1993

DOI:https://doi.org/10.1103/PhysRevB.49.1346