Ionic interactions in lanthanide halides

We determine a model of the ionic interactions in RX3 compounds (where R is a metal in the rare-earth series from La to Lu and X = Cl, Br or I) by an analysis of data on the static and dynamic structure of their molecular mon omers. The potential energy function that we adopt is patterned after earli er work on Aluminium trichloride [Z. Akdeniz and M. P. Tosi, Z. Naturforsch . 54a, 180 (1999)], but includes as an essential element the electric polar izability of the trivalent metal ion to account for a pyramidal shape of RX 3 molecules. From data referring mostly to trihalides of elements at the en ds and in the middle of the rare-earth series (i. e. LaX3, GdX3 and LuX3), we propose systematic variations for the effective valence, ionic radius an d electric polarizability of the metal ions across the series. As a first a pplication of our results we predict the structure of the Dy(2)CI6 and Dy2B r6 molecular dimers and demonstrate by comparison with electron diffraction data that lanthanide-ion polarizability plays a quantitative role also in this state of tetrahedral-like coordination.