  We have synthesized and studied single-crystal Ba5AlIr2O11 that features
dimer chains of two inequivalent octahedra occupied by tetravalent and
pentavalent ions, respectively. Ba5AlIr2O11 is a Mott insulator that undergoes
a subtle structural phase transition near 210 K and a magnetic transition at
4.5 K; the latter transition is surprisingly resistant to applied magnetic
fields up to 12 T, but sensitive to modest applied pressure. All results
indicate that the phase transition at 210 K signals an enhanced charge order
that induces electrical dipoles and strong dielectric response near 210 K. It
is clear that the strong covalency and spin-orbit interaction (SOI) suppress
double exchange in Ir dimers and stabilize a novel magnetic state. The 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.
