Pentavalent Iridium Pyrochlore Cd2ir2o7: A Prototype Material System For Competing Crystalline Field And Spin-Orbit Coupling

A new pyrochlore oxide Cd2Ir2O7 with an Ir5+ charge state was prepared by high-pressure techniques. Although strong spin-orbit coupling (SOC) dominates the electronic states in most iridates so that a SOC-Mott state is proposed in Sr2IrO4 in the assumption of an undistorted IrO6 octahedral crystalline field, the strongly distorted one in the current Cd2Ir2O7 exhibits a competing interaction with the SOC. Unexpected from a strong SOC limit, Cd2Ir2O7 deviates from a nonmagnetic and insulating J = 0 ground state. It displays short-range ferromagnetic correlations andmetallic electrical transport properties. First-principles calculations well reproduce the experimental observation, revealing the large mixture between the j(eff) = 1/2 and j(eff) = 3/2 bands near the Fermi surface due to the significant distortion of IrO6 octahedra. This work sheds light on the critical role of a noncubic crystalline field in electronic properties which has been ignored in past studies of 5d-electron systems.