Structural and electronic properties of CaCu3Ti4O12 have been calculated using density-functional theory within the local spin-density approximation. After an analysis of structural stability, zonecenter optical phonon frequencies are evaluated using the frozen-phonon method, and mode effective charges are determined from computed Berry-phase polarizations. Excellent agreement between calculated and measured phonon frequencies is obtained; calculated mode effective charges are in poorer agreement with experiment, although they are of the correct order of magnitude; and the lattice contribution to the static dielectric constant is calculated to be ∼40. On the basis of these results, various mechanisms are considered for the enormous dielectric response reported in recent experiments. No direct evidence is found for intrinsic lattice or electronic mechanisms, suggesting that increased attention should be given to extrinsic effects.

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

We investigated temperature dependent optical spectra of the one-dimensional chain compound Sr2CuO3. The charge transfer transition polarized along the chain direction shows a strongly asymmetric line shape as expected in one-dimensional extended Hubbard model. At low temperature, the charge transfer peak shows a large blueshift and reveals additional sharp peaks at the gap. Even though many spectroscopic studies suggest that this material cannot have a bound exciton based on the one-dimensional extended Hubbard model, we attribute the additional sharp peaks to excitons, which come to exist due to the long-range Coulomb interaction.

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