  We have prepared polycrystalline samples of the trimer Ir oxide BaIrO3 with
face-shared Ir3O12 trimers, and have investigated the origin of the phase
transition at 182 K by measuring resistivity, thermopower, magnetization and
synchrotron x-ray diffraction. We propose a possible electronic model and
transition mechanism, starting from a localized electron picture on the basis
of the Rietveld refinement. Within this model, BaIrO3 can be basically regarded
as a Mott insulator, when the Ir3O12 trimer is identified to one pseudo-atom or
one lattice site. The transition can be viewed as a transition from the Mott
insulator phase to a kind of charge ordered insulator phase.
