ESR spectra of high Tc superconducting oxides treated under various atmospheres

When completely free from parasitic phases, superconducting oxides are characterized by a flat ESR signal and by a strong zero-field microwave absorption. The presence of traces of non-superconducting impurities which often occur at the grain boundaries can be easily detected by ESR, which makes this technique a very powerful tool for the characterization of the quality of superconducting oxides. The influence on the ESR response of the thermal treatments undergone on YBa 2 Cu 3 O 7− δ and Bi 4 (Sr, Ca) 6 Cu 4 O 16+ δ samples under vacuum or under reactive atmospheres (oxygen, fluorine) has been investigated. The progressive transformation of Cu 2+ in Cu 3+ with increasing fluorination temperatures has been followed by ESR. The results have been compared with oxide and fluoride standards containing Cu 2+ or Cu 3+ species. The superconductivity has been systematically controlled by the zero-field microwave absorption.