Origin of metal-insulator transition in the weak-ferromagnetic superconductor system RuSr₂RCu₂O₈ (R = rare earths)

For the oxygen-annealed weak-ferromagnetic superconductor system RuSr₂RCu₂O₈ (R = rare earths), superconducting transition temperature T_sc decreases steadily from maximum 56 K for smaller rare earth Gd³⁺ (ionic radius r = 0.105 nm), to 54 K for (Eu_0.5Gd_0.5)³⁺, 36 K for Eu³⁺, 8 K for (Sm_0.5Eu_0.5)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 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.