Results of charge-density studies of Cu20 have been unimpressive for years, and the agreement between different research groups has been marginal. In this work, the influence of data-reduction procedures, temperature-factor models and deformation models on the results is studied with the aim to identify robust features. Eight deformation-density refinements have been carried out with respect to data measured with Ag Ks radiation to (sin 0/A)max = 1.73/~-~. It is shown that standard Fourier methods lead to unsatisfactory results, since ~ of the structure factors are too small to be measured. Model maps reproducibly show a density deficit along the linear O-Cu-O bond axis. This feature corresponds to the only robust variable, the population coefficient of the Y4o-hexadecapolar function of Cu. Inclusion of anharmonic displacement parameters results in bonds with cylindrical symmetry. The maps and the interpretation in terms of an orbital-product formalism are in good agreement with modern theoretical work: Cu ~+ ions with electron configuration dS(ds) 2 in the field of 0 2-ions form a~b(3dzQ + fl~b(4s) hybrid orbitals, with 0.7 (2). The electric field gradient at the site of Cu cannot be computed from the X-ray data.The electric field gradient tensor at the site of Cu has axial symmetry and is thus completely described by one component, VEz~ =-(a2V/az 2) (z along 3, V = electrostatic potential). The quadrupole coupling

No abstract

No abstract

No abstract