We report results of far-infrared spectroscopic measurements on a series of isotopically ${(}^{18}$O) exchanged and isomorphically substituted materials in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ families. Based upon these measurements, we propose assignments for phonons in both the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$ semiconducting phase and the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ superconducting phase. In ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3^{18}$${\mathrm{O}}_7$, the lowest-frequency phonon at 155 ${\mathrm{cm}}^{\mathrm{-}1}$ is seen to shift ∼2% to lower frequency when compared to the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ material, demonstrating the role of oxygen in this vibration (which we have previously assigned as a c-axis-directed Cu-O chain bending mode of $B_{1u}$ symmetry). This transverse-optical branch may be important for phonon-mediated superconductivity in this material, as indicated in a recent theoretical treatment by Weber and Matthiess [Phys. Rev. B 37, 599 (1988)].

• Received 8 August 1988

DOI:https://doi.org/10.1103/PhysRevB.38.11382