Spin–orbit effects in pentavalent iridates: models and materials

Spin-orbit effects in heavy 5$d$ transition metal oxides, particular, iridates, have received enormous current interest due to the prediction as well realization of a plethora exotic and unconventional magnetic properties. While bulk these works are based on tetravalent iridates ($d^5$), where counter-intuitive insulating state rather extended orbitals explained by invoking strong spin-orbit coupling, recent quest iridate research has shifted other valencies Ir, which pentavalent constitute notable representative. In contrast entangled electrons $d^4$ systems expected be confined $J = 0$ singlet without any resultant moment or response. However, it been recently predicted that, magnetism may occur via condensation excitations across spin-orbit-coupled states. reality, Ir$^{5+}$ often quite debatable both from theoretical experimental point view. Here we provide comprehensive overview coupled model its implications studied iridates. review here understanding double perovskite ($A_2B$YIrO$_6$, $A =$ Sr, Ba, $B =$Y, Sc, Gd), 6H-perovskite (Ba$_3M$Ir$_2$O$_9$, $M Zn, Mg, Ca), post-perovskite (NaIrO$_3$), Hexagonal (Sr$_3$MIrO$_6$) along with number open questions that require future investigation.