Electron density studies of KCl and LiF by γ-ray diffractometry

Use of hard electromagnetic radiation (λ = 0.12 Å) in diffraction experiments, in conjunction with thin crystals in transmission geometry (Laue case), minimizes multiple scattering, often referred to as the 'extinction problem'. A large crystal slab, completely intercepting the incident beam, was used in this experiment. Several structure factors have been measured in LiF and KCl, with greater accuracy than ever before, using the γ-ray spectrometer of the University of Missouri at Columbia. In the case of LiF the results indicates some compression of the valence electron densities, similar to what has been found earlier for NaF. In the case of KCl all reflections agree well with the values calculated from free-atom Hartree-Fock wave functions except for the 004 and 006, which are appreciably weaker than corresponding reflections with similar or identical sin θ/λ values. This result indicates asphericity of the valence electron density. The 111 is also much weaker than the calculated value, indicating that the negative ions are slightly compressed and the positive ions slightly expanded with respect to the free-ion charge densities.