Crystal-chemical anomalies in nonsuperconducting PrBa[sub 2]Cu[sub 3]O[sub 7]

Studies of the crystal chemistry of nonsuperconducting PrBa[sub 2]Cu[sub 3]O[sub 7] indicate that this compound is strictly isostructural with its superconducting RBa[sub 2]Cu[sub 3]O[sub 7] (R - Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa[sub 2]Cu[sub 3]O[sub 7] series as a function of R[sup 3+] ionic radius. The sole structural anomaly attributable to the presence of Pr[sup 3+] in the YBa[sub 2]Cu[sub 3]O[sub 7] 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 this anomaly with the nonmetallic/nonsuperconducting properties of PrBa[sub 2]Cu[sub 3]O[sub 7] supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa[sub 2]Cu[sub 3]O[sub 7] structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr[sup 3+] (f[sup 2]) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa[sub 2]Cu[sub 3]O[sub 7] remains unknown: however, superconductivity is [open quotes]turned back on[close quotes] for Nd[sup 3+] (f[sup 3]), immediately next to Pr and just slightly smaller. more » 46 refs., 5 figs. « less