FIR-spectroscopy on iron-doped YBaCuO ceramics: energy gap, phonons and a band at 500 cm−1

We have measured the reflectance of superconducting iron-doped YBaCuO ceramics in the frequency range from 20 to 10000 cm−1 for various temperatures above and belowT c . In these spectra we could directly observe an energy gap. A Kramers-Kronig analysis was performed. We discuss the temperature dependence of two infrared-active phonons and an absorption band located around 500 cm−1.