Far-infrared spectroscopy of R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{x}}$ with variations in R and x

We report far-infrared reflectance measurements of R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$, where R=Y, Er, Nd, and 6<x<7 for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$. A phonon mode at 150 ${\mathrm{cm}}^{\mathrm{-}1}$ appears to be a sensitive probe of disorder in the Cu-O linear chains, while the mode at 191 ${\mathrm{cm}}^{\mathrm{-}1}$ is shown to be primarily an R vibration. The reflectance spectra vary smoothly as a function of oxygen stoichiometry with quite distinct features characteristic of low- and high-oxygen-content samples. The gap which has been reported previously in fully oxygenated samples does not behave as expected for a Bardeen-Cooper-Schrieffer superconducting energy gap when the oxygen content is varied.