Spin-orbit tuned metal-insulator transitions in single-crystal Sr ₂Ir_1-xRh_xO₄ (0 ≤ x ≤ 0.16)

Sr₂IrO₄ is a magnetic insulator driven by spin-orbit interaction (SOI) whereas the isoelectronic and isostructural Sr ₂RhO₄ is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. Our investigation of structural, transport, magnetic, and thermal properties reveals that substituting 4d Rh4⁺ (4d5) ions for 5d Ir4⁺ (5d5) ions in Sr₂IrO₄ directly reduces the SOI and rebalances the competing energies so profoundly that it generates a rich phase diagram for Sr₂Ir_1-xRh_xO₄ featuring two major effects: (1) Light Rh doping (0 ≤ x ≤ 0.16) prompts a simultaneous and precipitous drop in both the electrical resistivity and the magnetic ordering temperature T_C, which is suppressed to zero at x = 0.16 from 240 K at x = 0. (2) However, with heavier Rh doping [0.24 < x < 0.85 (±0.05)] disorder scattering leads to localized states and a return to an insulating state with spin frustration and exotic magnetic behavior that only disappears near x = 1. The intricacy of Sr₂Ir _1-xRh_xO₄ is further highlighted by comparison with Sr₂Ir_1-xRu_xO₄ where Ru4 ⁺ (4d4) drives a direct crossover from the insulating to metallic states.