We report on infrared reflectivity and transmission spectra of ${\mathrm{Sr}}_2\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ samples with $\sim 70\%$ of $\mathrm{Fe}∕\mathrm{Mo}$ cationic ordering. In addition to an overdamped Drude component, localized secondary features are assigned to itinerant carriers interacting strongly with infrared active longitudinal optical modes of quasipolar insulating patches. Their origin is traced to antisite imperfections, and particularly, to antiphase boundary interactions. This implies that carriers are always prone to localization beyond the oxides standard scenario recreating electron localization and small polaron conductivity. Our finding is also supported by a remarkable well-defined second order spectra. The comparison between two samples of ${\mathrm{Sr}}_2\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ with $\sim 70\%$ and $\sim 92\%$ of $\mathrm{Fe}∕\mathrm{Mo}$ cationic ordering shows that by reducing the number the defects, there is an increment in the phonon coherent length yielding a cleaner picture in the most perfectly ordered sample. In ${\mathrm{Sr}}_2\mathrm{Fe}{\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_6$ solid solutions that scenario is maintained up to $x⩽0.5$ where breathing modes of Fe, Mo, and W octahedra sustain the strongest electron-phonon interaction. The reflectivity of ${\mathrm{Sr}}_2\mathrm{Fe}{\mathrm{Mo}}_{0.2}{\mathrm{W}}_{0.8}{\mathrm{O}}_6$ has well-defined phonon bands, a residual Drude contribution, and a distinctive low temperature small polaron localization. We also found that ${\mathrm{Sr}}_2\mathrm{Fe}\mathrm{W}{\mathrm{O}}_6$ is an insulator structurally stable from $20\text{to}700\mathrm{K}$ with a gap about $750{\mathrm{cm}}^{-1}(\sim 95\mathrm{meV})$. We conclude that the main clue for understanding low-field magnetoresistance resides in those interacting carriers, their confinement, and the important polaronic effects.

• Received 6 October 2004

DOI:https://doi.org/10.1103/PhysRevB.72.214303