INFRARED REFLECTIVITY OF THE SOLID-SOLUTIONS LANI1-XFEXO3 ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1.00)

We report temperature-dependent far- and midinfrared reflectivity spec tra 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-te mperature antiferromagnetic insulator, to LaNiO3, a known metal oxide. Light Ni doping creates defects that induce extra bands assigned to e lectronic transitions within the Insulating gap. An incipient Drude te rm emerges in the reflectivity spectrum of LaNi0.39Fe0.61O3 together w ith subbands that contribute to the electronic background, At these co ncentrations the dielectric response shows a picture in which the spec tral 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 deve lopment of a reflectivity tail chat extends beyond 1 eV. We assign ext ra-non-Drude terms in the 700-4000 cm(-1) frequency region to transiti ons due to intrinsic defects. While the increment in reflectivity at f ar-infrared frequencies is evident for Fe concentrations well above th e insulator-metal transition (x-0.30), the spectral features of a meta l oxide, with phonons mostly screened, are found for x=0.23. These met allic 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 s olutions even when their numbers of effective carriers are those of a metal. This characterization is also supported by the observation of w eak reflectivity dips in LaNiO, that have a direct correspondence to l ongitudinal-optical mode frequencies of the insulating phases of our s eries. We infer that strong electroionizing interactions play a role i n the conductivity of those solid solutions and are likely related to polaron formation and carrier phonon-assisted hopping motion. This con clusion is supported by the quantitative agreement with experimental d ata achieved by calculation of optical conductivities using the small- polaron theory by Reik sind Heese [H. G. Reik and D. Heese, J. Phys. C hem. Solids 28, 581 (1967)]. We iind our spectral analysis relevant to ward understanding the infrared reflectivity of conducting oxides in g eneral. Since LaNiO3 is a three-dimensional compound we avoid the argu ment of misinterpreting spectral features as due to band leakages of u nscreened phenons active in insulating crystal directions. [S0163-1829 (97)05240-5].