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Infrared reflectivity of the solid solutions LaN1-xFexO3 (0.000?Tx?T1.00)

Article

Authorship:

N. E. Massa ; FALCON, HORACIO ; H Salva ; R Carbonio

Date:

1997

Publishing House and Editing Place:

AMER PHYSICAL SOC

Magazine:

PHYSICAL REVIEW B, vol. 56 (pp. 10178-10191) AMER PHYSICAL SOC

Summary *

We report temperature-dependent far and midinfrared reflectivity spectra of LaNi1-xFexO3 (0.00x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 that 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 and absorption dip at 650cm-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 LaNiO3 that have a direct correspondence to longitudinal-optical mode frequencies of the insulating phases of our series. We infer that strong electron-phonon 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 and Heese [H. G. Reik and D. Heese, J. Phys. Chem. Solids 28, 581 (1967)]. We find our spectral analysis relevant toward understanding the infrared reflectivity of conducting oxides in general. Since LaNiO3 is a three-dimensional compond we avoid the argument of misinterpreting spectral features as due to band leakages of unscreened phonons active in insulating crystal directions.[S0163-1829(97)05240-5] Information provided by the agent in SIGEVA

Key Words

electronic transitions reflectivity spectra solid solutionselectron-phonon interactions