Abstract
For the oxygen-annealed weak-ferromagnetic superconductor system RuSr 2 RCu 2 O 8 (R = rare earths), superconducting transition temperature T sc decreases steadily from maximum 56 K for smaller rare earth Gd 3+ (ionic radius r = 0.105 nm), to 54 K for (Eu 0.5 Gd 0.5 ) 3+ , 36 K for Eu 3+ , 8 K for (Sm 0.5 Eu 0.5 ) 3+ , and metallic but not superconducting for larger Sm 3+ (r = 0.108 nm), with a metal-insulator transition for even larger rare earth ions Nd 3+ (r = 0.112 nm) and Pr 3+ (r = 0.113 nm). Powder X-ray diffraction Rietveld refinement study indicates that the insulating phase is stabilized in the undistorted tetragonal phase (space group P4/mmm) with the larger tetragonal lattice parameter a ∼ 0.390-392 nm, which gives a reasonable Ru 5+ -O bond length of d ∼ 0.197 nm. On the other hand, the metallic phase with smaller rare earth ions can be stabilized only in the distorted tetragonal phase (space group P4/mbm), with the smaller a/√2 ∼ 0.383-0.385 nm but still provide a reasonable Ru-O bond length through RuO 6 octahedron rotation. The metal-insulator transition as well as the variation of superconducting T sc is closely related to oxygen deficiency content δ which control the variation of mobile hole concentration and structural variation in this hole-doped superconductor system. © 2007 Elsevier B.V. All rights reserved.