REFLECTIVITY, TRANSMISSION, AND PHOTOINDUCED INFRARED-SPECTRA OF NDNIO3

We report temperature-dependent infrared spectra of NdNiO3 prepared by synthesis under high oxygen pressure. Its room-temperature reflectivi ty is characteristic of a metal oxide with a Drude tail asymptotically extending to higher frequencies. The sharp metal-insulator transition is detected at about 200 K as phonons become unscreened. While their activity is in the frequency range for a distorted perovskite, the str ength of involving the Nd ion against the NiO6 octahedra at 183 cm(-1) is remarkable. This and all phonon groups show strong antiresonances near the longitudinal optical frequencies and these are unusually clos e to absorptions seen in transmission measurements. We interpret this as evidence of strong electron-phonon interactions with carrier locali zation in a temperature-dependent regime. Low-temperature photoinduced spectra, excited with argon laser lines, support this picture by show ing weak absorptions at about those frequencies as well as a relativel y very strong photoinduced band at 810 cm(-1) and one very broad and w eak peak at similar to 2300 cm(-1) and tail up to 4000 cm(-1). They ar e assigned to electronic transitions involving localized defects. Our measurements suggest that the metal-insulator phase transition in NdNi O3 is triggered by electron localization in a polaronic environment. W e conclude that a suitable order parameter for perovskites with a meta l-insulator phase transition may be identified in the coupling between those electrons and local spins.