Relationship between Lattice Parameters and Cation Radius in the Homologous Series of Rare-Earth Oxofluorides with the fluorite structure

The relationship between the lattice parameters a and the ionic radii rR 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. $$Fm\bar {3}m$$ ) was investigated. It was found that all ROF compounds obey the following equation: a [Å] = 2.365rR + 3.004. The special electronic structure of the Y3+ 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)) the unknown ionic radii of triply charged actinium cations Ac3+ (rAc = 1.24 Å) and actinides Pu3+ (rPu = 1.14 Å), Cm3+ (rCm = 1.10 Å), and Cf3+ (rCf = 1.08 Å) for the coordination number 8. Within the framework of the crystal chemical model, the values of the “effective” radii of the fluorine ion (rF = 1.249 ± 0.017), oxygen ion (rO = 1.373 ± 0.005), and the average radius of the anion (rO,F = 1.327 ± 0.009 Å) for fluoride, oxide, and oxofluoride compounds with the fluorite structure were refined. The values of rF, rO, and rO,F can be used in calculations of the crystallochemical properties of fluorite oxofluoride solid solutions.