Lanthanum and Lanthanide Trifluorides: Lanthanide Contraction and Volume of Fluorine Ion

Abstract

Lanthanide contraction of lanthanum and 14 lanthanides (Ln), leading to a decrease in R³⁺ ionic radii by ~15% (with respect to the smallest one) with an increase in the element atomic number Z, is a way to implement a close packing of F^1– ions in trifluorides of heavy 4f elements of the periodic table of elements. The efficiency of lanthanide (Ln) and actinide (An) contraction in the formation of inorganic fluoride crystals with a minimum unit-cell volume per fluorine ion (V_F) is estimated. For AnF₃ (An = Ac–Am), the V_F value decreases from ~20 to ~17.4 ų with an increase in Z from 89 to 95. The unavailability of heavy An makes this value final. The close packing of the fluorine ion in fluorides could not be obtained using the effects of lanthanide and actinide contraction and increase in the cation valence. Fluorides LnF₃ (structure type β-YF₃) from the end of the series from ErF₃ (15.86 ų) to LuF₃ (15.48 ų) are nearest to the anion close packing. The latter V_F value is minimum among all studied LnF₃ compounds and actinide fluorides (both simple and complex) and close to the saturation value. It corresponds to the estimated fluorine ionic radius of 1.246 Å.