Optical-phonon spectra were investigated for single crystals of the parent materials of various cuprate superconductors and related materials. Comparing the spectra, all the observed phonons were unambiguously assigned. It was found that the main phonon modes in the ${\mathrm{CuO}}_2$ plane, the Cu-O stretching and bending modes, exhibit completely different dependences of their frequency on the Cu-O bond distance a/2; that is, the former shows strong a dependence while the latter is relatively insensitive to a. From a quantitative analysis of the spectra, the highly covalent character of these layered cuprates was confirmed. The remarkable change of the stretching-mode frequency against a indicates a direct coupling of this phonon with the Cu-O charge-transfer electronic excitation.

• Received 7 December 1990

DOI:https://doi.org/10.1103/PhysRevB.43.10496