Pentavalent iridium pyrochlore Cd2Ir2O7 : A prototype material system for competing crystalline field and spin-orbit coupling

Abstract

A new pyrochlore oxide $\mathrm{C}{\mathrm{d}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ with an $\mathrm{I}{\mathrm{r}}^{5+}$ 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 $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ in the assumption of an undistorted $\mathrm{Ir}{\mathrm{O}}_{6}$ octahedral crystalline field, the strongly distorted one in the current $\mathrm{C}{\mathrm{d}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ exhibits a competing interaction with the SOC. Unexpected from a strong SOC limit, $\mathrm{C}{\mathrm{d}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ deviates from a nonmagnetic and insulating $J=0$ ground state. It displays short-range ferromagnetic correlations and metallic electrical transport properties. First-principles calculations well reproduce the experimental observation, revealing the large mixture between the ${j}_{\mathrm{eff}}=1/2$ and ${j}_{\mathrm{eff}}=3/2$ bands near the Fermi surface due to the significant distortion of $\mathrm{Ir}{\mathrm{O}}_{6}$ 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.