We have synthesized and studied single-crystal Ba5AlIr2O11 that features dimer chains of two inequivalent octahedra occupied by tetravalent Ir4+(5d5) and pentavalent Ir5+(5d4) ions, respectively. Ba5AlIr2O11 is a Mott insulator that undergoes a subtle structural phase transition near TS=210K and a magnetic transition at TM=4.5K; the latter transition is surprisingly resistant to applied magnetic fields μoH≤12T but more sensitive to modest applied pressure (dTM/dp≈+0.61K/GPa). All results indicate that the phase transition at TS signals an enhanced charge order that induces electrical dipoles and strong dielectric response near TS. 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.

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Published in Physical Review B: Condensed Matter and Materials Physics, v. 91, no. 23, article 235147, p. 1-6.

©2015 American Physical Society

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This work was supported by the National Science Foundation (USA) via Grant No. DMR-1265162, Russian Foundation of Basic Research via Grant No. 13-02-00374, and Civil Research and Development Foundation via program FSCX-14-61025-0. Work at ORNL was supported by DOE BES Office of Scientific User Facilities (F.Y.), and China Scholarship Council (J.C.W.).

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