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A model of the crystal field generated by a periodic array of charged tapes is developed to analyse the crystal-field interaction in RBa2Cu3Ox (R = Er, Ho) high-Tc copper oxides observed by the inelastic neutron scattering technique. The explicit calculation of the parameters of the Stevens Hamiltonian describing crystalline electric field effects in solids is performed for a specific charge density distribution uniformly extended in a certain direction of the crystal lattice. The model accounts for the x-dependence of the crystal-field parameters and allows us to determine the hole concentration in the CuO2 planes as a function of oxygen stoichiometry. The model of the periodic array of charged tapes suggests a charge order induced in the CuO2 planes by doping.

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The inelastic neutron scattering technique was employed to study the transitions between the low-lying levels of the ground-state J -multiplet of the Ho 3+ ions in Ho 1−y Ca y Ba 2 Cu 3 O x (x ≈ 6.3, 7.0; y ≈ 0.1, 0.2) and Ho 1−y Th y Ba 2 Cu 3 O x (x ≈ 6.3; y ≈ 0.07, 0.13) split by the crystalline electric field. For oxygen-rich Ca-doped samples, the observed energy spectra exhibit a fine structure, which reflects the behaviour of the local hole density near the doping centre. For the first time this experimental observation unambiguously confirms that the charge inhomogeneity, which was found to be a characteristic feature of the underdoped cuprates, still exists in the deeply overdoped regime with T c = 56 K. On the other hand, the modification of the crystal-field spectra observed for the oxygendeficient samples with both Ca 2+ and Th 4+ substitutions for Ho 3+ is found to result from distortions of the copper-oxygen polyhedron around the Ho 3+ ion in the direct neighbourhood of the implanted ions, rather than from the dopinginduced variation of the carrier concentration in the CuO 2 planes.

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