Probing localization effects in Li_0.9Mo₆O₁₇ purple bronze: An optical-properties investigation

We report the polarized reflectance and optical conductivity of the quasi-one-dimensional conductor Li_0.9Mo₆O₁₇ 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 dd transitions near 0.42, 0.57, and 1.3 eV, and an O pMod charge-transfer band near 4 eV. Perpendicular to the b axis, Li_0.9Mo₆O₁₇ 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 Li_0.9Mo₆O₁₇ 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 Li_0.9Mo₆O₁₇. Based upon these combined results, we argue that localization effects dominate the bulk and microscopic properties of this material.