A large number of high-temperature superconductors (HTS) contain rare-earth
(R) ions with 4f-electron multiplets which are split by the crystal field
(CF) of the neighboring ligands and, in many cases, their energy difference
s are in the range of optical phonons. When these two excitations (phonons
and CF) have the same symmetry and are close in energy they can couple to e
ach other, leading to a doublet structure at low temperature. Since the R i
ons are located between the CuO2 superconducting planes, this crystal-field
-phonon coupling can be used as a probe to investigate superconductivity re
lated phenomena. In this paper, we discuss the coupling of phonons to CF ex
citations in (R)Ba2Cu3O7-delta and demonstrate that Raman and infrared spec
troscopy can be successfully used as a novel tool for CF studies in HTS, si
nce they are not limited to low energy transfer and can be applied to study
small single crystals.