Correlation between the Molecular Refractions of Crystals and the Ionic Radii of the Cations in the Rare-Earth Trifluoride Homologous Series

Abstract

We have identified linear correlations between the molecular refractions R_mol (cm³/mol) of crystals and the ionic radii r_i (Å) of the cations in the RF₃ rare-earth trifluoride homologous series: R_mol = 8.33r_i + 0.102 for R = La–Nd and R_mol = 13.03r_i–4.42 for R = Sm–Lu. The fact that the RF₃ rare-earth trifluorides can be divided into two groups is related to the morphotropic transition from the LaF₃ (tysonite) structure to the β-YF₃ structure. The correlation found here for RF₃ crystals with the β-YF₃ structure because the electronic structure of Y (4d¹5s²) differs from that of Sm–Lu (4f^6–145d^0–16s²) in that it has no 4f electrons.