Lanthanide Contraction in LnF3 (Ln = Ce-Lu) and Its Chemical and Structural Consequences: Part 2: Specialized Empirical System of R3+ (R = Y, La, and 14 Ln) and F1− Ionic Radii for RF3 Series

A specialized empirical (Spec-zd Emp) system of ionic radii (SIR) for R = Y3+, La3+, Ln3+, and F1− (R rare earth elements (REE)) was derived from the dependence of lanthanide contraction (LC) on the atomic number (Z) of lanthanides (Ln). LC decreased the radius of the cation with increasing Z. The structures of t-RF3 (LaF3-NdF3, “pseudo t-SmF3”) of the LaF3 type, 11 β-LnF3 (Ln = Sm-Lu), and β-YF3 of the β-YF3 type were studied. The empirical basis of the shortest (F-F)min and (R-F)min distances was calculated from the structural data for the RF3 complete series. The dependence of (F-F)min on Z reached saturation at Z = 67 (Ho). The base F1− radius r− = 1.2539(16) Å was calculated as the arithmetic mean of five (F-F)min in LnF3 with Ln = Ho-Lu. For the LnF3 series with Ln contributions up to 75 % wt., the dependence of (Ln-F)min on Z reflected the non-uniformity of the 4f orbital filling. SIR was calculated as the difference in the empirical constants of RF3 (ionic radii of (R,Ln)3+ (r+) and F1− (r−)), the change in which was continuous over the series and did not depend on the type of structure: r+ = (ZR-F)min − ½(F-F)min (Z = 57–71). The changes in LC in the LnF3 series were described by a third-degree polynomial. LC reduced r+ by 24% (percentage relative to less) from 1.1671(16) Å (La3+) to 0.9439(17) Å (Lu3+). In the Spec-zd Emp SIR, r+ were constants that did not require corrections for a coordination number (CN). A comparison of r+ in the Spec-zd Emp SIR with other SIRs was performed.