CRYSTAL-CHEMICAL ANOMALIES IN NONSUPERCONDUCTING PRBA2CU3O7

Studies of the crystal chemistry of nonsuperconducting PrBa2Cu3O7 indi cate that t is compound is strictly isostructural with its superconduc ting RBa2Cu3O7 (R = Y, rare earth) analogs. Crystallographically, Pr i s present in the trivalent state according to the structural trends ex hibited by the RBa2Cu3O7 series as a function of R3+ ionic radius. The sole structural anomaly attributable to the presence of Pr3+ in the Y Ba2Cu3O7 structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of t his anomaly with the nonmetallic/nonsuperconducting properties of PrBa 2Cu3O7 supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa2Cu3O7 struc ture plays a critical role in mediating the appearance of superconduct ivity. The mechanism by which the f-electrons in Pr3+ (f2) interact wi th the Cu-O manifold to produce the nonmetallic behavior of PrBa2Cu3O7 remains unknown; however, superconductivity is ''turned back on'' for Nd3+ (f3), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa2Cu3O7 and nonmetallic PrBa 2Cu3O7 are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides.