Far infrared reflectivity and Raman scattering of Tl2-xBixMn2O7 (x=0.00, 0.10, 0.50)

We report temperature dependent infrared and Raman spectra of pyrochlores T l2-xBixMn2O7 (x = 0.00, 0.10, 0.50). When Tl is substituted by Bi the magne toresistance increases dramatically. This shows up in our reflectivity spec tra for x = 0.10 as a weaker Drude contribution implying a down-scaling of the number of active carriers at the insulator-metal phase transition (T-IM ) and lower temperatures. A system magnetically frustrated with no defined T-c is found for nominal x = 0.50. While in this last case the overall infr ared features are maintained there is a weakening and broadening in the ban ds centered at approximate to 250 cm(-1). A relative increase in intensity of the band at approximate to 900 cm(-1) assigned to lattice polarons, is u nderstood as indicative of stronger localization. Raman spectra are charact erized by phonon bands sensitive to the bismuth content and for a band at a pproximate to 816 cm(-1) that weakens when approaching T-IM We tentatively associate its origin to a bound state due to a polaron related octahedral d istortion.