The magnetic, transport, optical, and structural properties of quasi-one-dimensional BaIrO 3 show evidence for the simultaneous onset of electronic density wave formation and ferromagnetism at T c3 175 K: Two additional features in the chain direction dc conductivity show a sudden change to metallic behavior below T c2 80 K and then a Mott-like transition at T c1 26 K: Highly non-linear dc conductivity, optical gap formation at Ϸ9k B T c3 , additional phonon modes, and emergent X-ray satellite structure support density wave formation. Even at very high (30 T) fields the saturation Ir moment is very small, Ϸ0.04m B /Ir. ᭧ 2000 Elsevier Science Ltd. All rights reserved. Transition metal oxides (TMO) with low crystalline symmetry are known to exhibit electronic density wave formation [1][2][3]. However, to our knowledge, density wave formation has not yet been observed accompanying the onset of ferromagnetic order. However, the ferromagnetism at T c3 175 K in BaIrO 3 [4] appears to be accompanied by and possibly driven by a collective electronic excitation or at least partial gapping of the Fermi surface. This demonstrates once again the strong coupling between spin and charge in the heavy (4d-and 5d-based) TMOs [5][6][7]. BaIrO 3 has a highly anisotropic quasi-one-dimensional structure [8][9][10] and this gives rise, in our single crystal samples, to large anisotropy of r(T), the electrical resistivity, with the quasi-one-dimensional axis, the c-axis, having much lower resistivity. This kind of low-dimensional structure is necessary for the formation of an insulating charge density wave (CDW) ground state, which is a collective electron mode normally incommensurate with the underlying lattice for partially filled bands [3].Evidence for density wave formation comes from: (1) A discontinuous increase in the slope of r (T) vs. T at T c3 T C ; the Curie temperature-an abrupt transition to a more insulating phase. (Two additional features of r (T) along the c-axis, at T c2 80 K and T c1 26 K; mark a sudden return to "metallic" behavior (possibly a crossover from partial toward full gapping of the Fermi surface) and a well-defined Mott-like metal-insulator transition, respectively). (2) An abrupt feature in the non-linear conductivity showing negative differential resistivity. (3) Gap formation at about 1200 cm Ϫ1 in the electron excitation spectrum and a splitting of a phonon mode at 350 cm Ϫ1 , which appear for T Ͻ T c3 (This was determined by optical reflectivity studies in the far and near infrared.). (4) Additional satellite formation for T Ͻ T C3 in the X-ray diffraction spectrum.The structure of BaIrO 3 is monoclinic and consists of Ir 3 O 12 trimers of face-sharing IrO 6 octahedra which are vertex-linked to other trimeric clusters forming columns roughly parallel to the c-axis. These clusters form channels accommodating Ba ions. The space group is C2/m and the