A neutron spectroscopic comparison of the crystalline electric field in tetragonal HoBa 2Cu 3O 6.2 and orthorhombic HoBa 2Cu 3O 6.8

Inelastic neutron scattering has been employed to study the crystalline electric field (CEF) interaction in the tetragonal perovskite-type compound HoBa ₂Cu ₃O _6.2. The observed energy spectra exhibit a large number of well-resolved CEF transitions between 1 and 72 meV, so that we have been able to assign the energies of at least seven excited CEF levels and, based on the observed intensities, to unambiguously determine all five CEF parameters required for tetragonal symmetry. The latter turn out to be very similar to the leading CEF parameters of orthorhombic HoBa ₂Cu ₃O _6.8 as expected. The asymmetric peak shapes of some CEF transitions observed at low energies are explained on the basis of a model which describes the effect of the oxygen nonstoichiometry δ in terms of charge redistributions in the CuO ₂ planes.