THE INVESTIGATION OF THE PECULIARITIES OF CU3+ STATES LOCALIZED BY AL3+ IONS IN YBCO SINGLE-CRYSTALS BY EPR TECHNIQUE

A new EPR spectrum in YBCO single crystals doped by Al3+ ions has been revealed. The spectrum was assigned to trivalent Cu3+ (S=1) ions in the Cu(1) position and the spin-hamiltonian parameters describing the observed spectrum have been determined on the basis of computer calculation of the EPR spectrum. The theoretical consideration and numerical estimations carried out showed that the ground state of Cu3+ ions in the YBCO system might be a triplet and, thus, the possibility of the observation of its EPR spectrum has been confirmed. The analysis of the experimentally determined g-factor values showed the important role of the hybridization of 3d-copper and 2p-oxygen states and, in particular, the significant contribution of the Cu2+-O- state in the ground one. The stabilization of Cu3+ states in YBCO single crystals by Al3+ ions is discussed. It is shown that the observed temperature dependence of the g(x)-value was due to the displacement of the apex oxygen O(4) in the direction of the CuO2 plane as T-->T(c).