Infrared properties of ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The infrared reflectance spectra of ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pr}}_{\mathit{x}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (x=0, 0.1, 0.2, 0.4, 0.5, 0.6, and 1.0) have been measured in the temperature range from 4.2 to 300 K. It is found that near the Ba mode there is an additional peak whose relative strength varies with increasing x, and whose position—unlike its strength—at 194 ${\mathrm{cm}}^{\mathrm{-}1}$ does not depend on the concentration of Pr. We discuss these experimental phenomena and make tentative assignments of these modes.