For the oxygen-annealed weak-ferromagnetic superconductor system RuSr2RCu2O8 (R = rare earths), superconducting transition temperature Tsc decreases steadily from maximum 56 K for smaller rare earth Gd3+ (ionic radius r = 0.105 nm), to 54 K for (Eu0.5Gd0.5)3+, 36 K for Eu3+, 8 K for (Sm0.5Eu0.5)3+, and metallic but not superconducting for larger Sm3+ (r = 0.108 nm), with a metal-insulator transition for even larger rare earth ions Nd3+ (r = 0.112 nm) and Pr3+ (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 Ru5+-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 RuO6 octahedron rotation. The metal-insulator transition as well as the variation of superconducting Tsc 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.
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