Magnetic resonance of exchange-coupled copper complexes in perovskite-structure crystals: The potassium tantalate and cuprate superconductors

This paper reports on parallel EPR studies of high-temperature superconduct ors based on the cuprate perovskites RBa2Cu3O6 + x (R = Y, Gd, Nd) and of K TaO3 : Cu, which also has a perovskite structure. EPR measurements performe d on copper-doped KTaO3 crystals revealed Cu2+-Cu2+ copper pair centers. Th e copper ions making up pairs are assumed to occupy adjacent tantalum sites . The pair centers are chains consisting of two equivalent Cu2+ ions and th ree oxygen vacancies aligned in the [100] direction. The crucial point in t he model proposed is the presence of an oxygen vacancy sandwiched between t wo Cu2+ ions, whereas the outer vacancies do not necessarily occupy neighbo ring sites. In this structure, complete charge compensation is achieved. Fe rromagnetic exchange coupling takes place between the two copper ions. An i nvestigation of the exchange and superhyperfine interactions of copper cent ers in crystalline potassium tantalate has permitted the estimation of the respective interactions in crystals of the cuprate superconductors which ex hibit magnetic resonance signals due to exchange-coupled copper clusters in the case of oxygen deficiency. (C) 2001 MAIK "Nauka/Interperiodica".