Reflectivity, transmission, and photoinduced infrared spectra ofNdNiO3

We report temperature-dependent infrared spectra of ${\mathrm{NdNiO}}_3$ prepared by synthesis under high oxygen pressure. Its room-temperature reflectivity 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 strength of involving the Nd ion against the ${\mathrm{NiO}}_6$ octahedra at $183{\mathrm{cm}}^{\mathrm{-}1}$ is remarkable. This and all phonon groups show strong antiresonances near the longitudinal optical frequencies and these are unusually close to absorptions seen in transmission measurements. We interpret this as evidence of strong electron-phonon interactions with carrier localization in a temperature-dependent regime. Low-temperature photoinduced spectra, excited with argon laser lines, support this picture by showing weak absorptions at about those frequencies as well as a relatively very strong photoinduced band at $810{\mathrm{cm}}^{\mathrm{-}1}$ and one very broad and weak peak at $\sim 2300{\mathrm{cm}}^{\mathrm{-}1}$ and tail up to $4000{\mathrm{cm}}^{\mathrm{-}1}.$ They are assigned to electronic transitions involving localized defects. Our measurements suggest that the metal-insulator phase transition in ${\mathrm{NdNiO}}_3$ is triggered by electron localization in a polaronic environment. We conclude that a suitable order parameter for perovskites with a metal-insulator phase transition may be identified in the coupling between those electrons and local spins