Neutron spectroscopic studies of the crystal field in ${\mathrm{HoBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ (6≤x≤7)

Inelastic neutron scattering has been employed to study in detail the full crystalline-electric-field (CEF) energy-level scheme in the ground-state J multiplet of ${\mathrm{Ho}}^{3+}$ in ${\mathrm{HoBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ as a function of the oxygen content x. We have been able to resolve ten ground-state transitions for a series of samples covering the superconducting phase as well as the semiconducting one. J mixing and intermediate coupling effects are shown to be necessary to determine unambiguously the nine and five independent CEF parameters of the orthorhombic and tetragonal symmetry, respectively. The large energy shifts and intensity changes of the observed CEF spectra as a function of the oxygen content x are shown to be related predominantly to a charge-transfer process from the chains to the planes, the actual charge transfer being in good agreement with results derived from both bond valence sum considerations for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ and similar CEF data for ${\mathrm{ErBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$.