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 Tsc decreases steadily from maximum 56K for smaller rare earth Gd3+ (ionic radius r=0.105nm), to 54K for (Eu0.5Gd0.5)3+, 36K for Eu3+, 8K for (Sm0.5Eu0.5)3+, and metallic but not superconducting for larger Sm3+ (r=0.108nm), with a metal–insulator transition for even larger rare earth ions Nd3+ (r=0.112nm) and Pr3+ (r=0.113nm). 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∼0.390–392nm, which gives a reasonable Ru5+–O bond length of d∼0.197nm. 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∼0.383–0.385nm 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.