Interactions of yttrium and lanthanum fluorides with other fluorides

Abstract

Both yttrium and lanthanum fluorides are amphoteric compounds according to the Lewis acid and base theory, and this factor defines YF3 and LaF3 chemical behavior. In the present paper, we discuss interactions of yttrium and lanthanum fluoride with other metal fluorides (binary systems YF3-MFn and LaF3- MFn), using our own experimental data along with literature publications and suggesting the “cation cumulative moment M versus cation electronegativity X” diagram (M is the ratio of the cation charge to its ionic radius). Two areas of compound formation have been suggested in the said diagram for both groups of systems. These areas include well-known compounds in YF3-MFn and LaF3- MFn systems that are formed with fluorides of alkaline elements on the one hand, and compounds with numerous fluorides of tetravalent (Zr, Hf, Mo, Sn, Pt) and pentavalent (As, Sb) elements on the other hand. During the transition from yttrium to lanthanum, the number of compounds decreases. In particular, compounds disappear in systems with difluorides Ca, Sr, Ba and Pb. This is explained both by the weakening of the acidity of lanthanum fluoride compared to yttrium fluoride and by an increase in the ionic radius of the cation, which leads to a change in the preferred coordination numbers in crystal structures. We predict the existence of numerous compounds of yttrium fluoride with fluorides of pentavalent elements and suggest carefully checking formation of compounds in the YF3 systems with small trivalent cation fluorides.