We report infrared reflectivity measurements on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{SmBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, in the tetragonal semiconducting phase. Ten of the eleven symmetry allowed modes are seen, and the eleventh is probably observed in the Sm material. When Sm replaces Y, two of the phonons at ≊200 ${\mathrm{cm}}^{\mathrm{-}1}$ shift to lower energy by 22 and 27 ${\mathrm{cm}}^{\mathrm{-}1}$. These modes, and the two at ≊110 ${\mathrm{cm}}^{\mathrm{-}1}$, which undoubtedly involve BA motions, are very sharp. From Sm-atom substitution, we can surmise which of the two-high-frequency Cu-against-O modes is the planar $E_u$ mode, and which is the chain $A_{2u}$ mode. Surprisingly, with Sm replacing Y, the planar mode shifts down in frequency by 25 ${\mathrm{cm}}^{\mathrm{-}1}$ (and the chain mode is hardly affected). Other aspects of the data are discussed.

• Received 20 November 1987

DOI:https://doi.org/10.1103/PhysRevB.37.5171