Abstract
We have synthesized and studied single-crystal Ba5AlIr2O11 that features dimer chains of two inequivalent octahedra occupied by tetravalent Ir4+(5d(5)) and pentavalent Ir5+(5d(4)) ions, respectively. Ba5AlIr2O11 is a Mott insulator that undergoes a subtle structural phase transition near T-S = 210 K and a magnetic transition at T-M = 4.5 K; the latter transition is surprisingly resistant to applied magnetic fields mu H-o <= 12 T but more sensitive to modest applied pressure (dT(M)/dp approximate to + 0.61 K/GPa). All results indicate that the phase transition at T-S signals an enhanced charge order that induces electrical dipoles and strong dielectric response near T-S. It is clear that the strong covalency and spin-orbit interaction (SOI) suppress double exchange in Ir dimers and stabilize a novel magnetic state that is neither S = 3/2 nor J = 1/2, but rather lies in an "intermediate" regime between these two states. The novel behavior of Ba5AlIr2O11 therefore provides unique insights into the physics of SOI along with strong covalency in competition with double-exchange interactions of comparable strength.
