X-ray study of the atomic charge densities in MgO, CaO, SrO and BaO

Accurate X-ray structure factors of CaO, SrO and BaO were measured at 293 K from single crystals and analysed together with the data of Sanger [Acta Cryst. (1969), A25, 694-702] on MgO. The oxygen ion is seen to suffer an average compression and a fourth-order cubic harmonic type deformation, as compared to the atomic superposition model, yielding a lower average charge density in the surrounding region and a charge transfer from [111] to the [100] direction. These features strengthen with increasing atomic number of the cation. Mg and Ca show a weak broadening. About nine electrons are localized around the oxygen position. The tenth electron of O^2- is more widely distributed. The electron counts of the cation correspond most closely to Mg⁺, Ca²⁺, Sr²⁺ and Ba²⁺. The results confirm the dominance of ionic character and increase of ionicity with increasing atomic number of the cation.