The relationship between the lattice parameters and the ionic radii r of cations in the homologous series of rare earth oxofluorides with the general formula ROF (12R = La–Er, Y) and the fluorite structure (sp. gr. Fm3m) was investigated. It was found that for all ROF compounds the equation (Å) is satisfied: a = 2.365r + 3.004. The special electronic structure of the Y cation does not lead to deviation from the correlation equation. The obtained equation makes it possible to determine from the structural data for fluorite oxofluorides AnOF (An = Ac, Pu, Cm, Cf) unknown ionic radii of triply charged actinium cations Ac (r = 1.24 Å) and actinides Pu (r = 1.14 Å), Cm (r = 1.10 Å), Cf (r = 1.08 Å) for the coordination number 8. Within the framework of the crystallochemical model, the values of the effective radii of the fluorine ion (r = 1.249 ± 0.017), oxygen ion (r = 1.373 ± 0.005) and the average radius of the anion (r = 1.327 ± 0.009 Å) for fluoride, oxide and oxofluoride compounds with the fluorite structure were refined. The values of r, r and r can be used in calculations of the crystallochemical properties of fluorite oxofluoride solid solutions.
Н.И. Сорокин (Wed,) studied this question.