Infrared reflectivity of the solid solutions LaNi1-xFexO3 (0.00<=x<=1.00)

We report temperature-dependent far- and midinfrared reflectivity spectra of LaNi(1-x)FexO3 (0.00 less than or equal to x less than or equal to 1.00) solid solutions that span the passage from LaFeO3, a room-temperature antiferromagnetic insulator, to LaNiO3, a known metal oxide. Light Ni doping creates defects that induce extra bands assigned to electronic transitions within the Insulating gap. An incipient Drude term emerges in the reflectivity spectrum of LaNi0.39Fe0.61O3 together with subbands that contribute to the electronic background, At these concentrations the dielectric response shows a picture in which the spectral weigh switches over toward far-infrared frequencies while phonon features develop strong antiresonances near longitudinal-optical modes. Further increment of carriers produces phonon screening and the development of a reflectivity tail chat extends beyond 1 eV. We assign extra-non-Drude terms in the 700-4000 cm(-1) frequency region to transitions due to intrinsic defects. While the increment in reflectivity at far-infrared frequencies is evident for Fe concentrations well above the insulator-metal transition (x-0.30), the spectral features of a metal oxide, with phonons mostly screened, are found for x=0.23. These metallic spectra show an absorption dip at similar to 650 cm(-1) that is traced to the perovskite symmetric stretching longitudinal mode, It is evidence that electron-phonon interactions are present in our solid solutions even when their numbers of effective carriers are those of a metal. This characterization is also supported by the observation of weak reflectivity dips in LaNiO, that have a direct correspondence to longitudinal-optical mode frequencies of the insulating phases of our series. We infer that strong electroionizing interactions play a role in the conductivity of those solid solutions and are likely related to polaron formation and carrier phonon-assisted hopping motion. This conclusion is supported by the quantitative agreement with experimental data achieved by calculation of optical conductivities using the small-polaron theory by Reik sind Heese [H. G. Reik and D. Heese, J. Phys. Chem. Solids 28, 581 (1967)]. We iind our spectral analysis relevant toward understanding the infrared reflectivity of conducting oxides in general. Since LaNiO3 is a three-dimensional compound we avoid the argument of misinterpreting spectral features as due to band leakages of unscreened phenons active in insulating crystal directions. [S0163-1829(97)05240-5].