LAUSR

We have identified linear correlations between the molecular refractions Rmol (cm3/mol) of crystals and the ionic radii ri (Å) of the cations in the RF3 rare-earth trifluoride homologous series: Rmol = 8.33ri + 0.102 for R = La–Nd and Rmol = 13.03ri–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 (4f6–145d0–16s2) in that it has no 4f electrons.