Probing localization effects in Li0.9Mo6O17 purple bronze:: An optical-properties investigation -: art. no. 085120

We report the polarized reflectance and optical conductivity of the quasi-one-dimensional conductor Li0.9Mo6O17 as a function of temperature. The compound displays an unusual (non-Drude-type) mobile carrier response at low-energy, with partially screened vibrational features along the highly conducting b axis. In addition, we observe Mo d-->d transitions near 0.42, 0.57, and 1.3 eV, and an O p-->Mo d charge-transfer band near 4 eV. Perpendicular to the b axis, Li0.9Mo6O17 exhibits semiconducting behavior with an optical gap of 0.4 eV and electronic structure similar to that of the b axis at higher energies. The substantial temperature dependence of the vibrational modes in this direction reveals that the lattice of Li0.9Mo6O17 is not rigid. However, no noticeable change in the lattice through the 25 K metal-insulator transition is observed. Comparing x-ray and infrared data for several model materials, we establish an upper bound on the size of any lattice distortion in Li0.9Mo6O17. Based upon these combined results, we argue that localization effects dominate the bulk and microscopic properties of this material.