UNUSUAL ELECTRONIC-STRUCTURE OF PRBA2CU3O7

A model for the electronic structure of PrBa2Cu3O7 is proposed with three independent components: insulating CuO2 planes with oxidation state Cu(II), mixed valent Pr ions with almost equal numbers of Pr(III) and Pr(IV), and CuO3 chains described by a t-J model with filling almost-equal-to 1/2. The difference from the high-T(c) superconductors comes from an enhanced stability of the Pr(IV) state due to the hybridization with O neighbors, and involves a transfer of holes from primarily planar O 2p(sigma) to 2(pi) states. The insulating nature of PrBa2Cu3O7 is ascribed to the extreme sensitivity of the dc conductivity of the mixed valent Pr ions and the CuO3 chains to vacancies and impurities.