Some predictive aspects of rare earth solid state chemistry

Abstract

Rare earth chemistry is a particularly favorable field for illustrating the predictive character of solid state chemistry as a result of both size similarities and significant electronic configuration differences of the rare earth ions. Examples selected from various research fields will illustrate this concept along with the fact that chemical surroundings may sometimes be a decisive factor: 1. (i) The increasing localization of the e g electron of nickel(III) (3t 2g 6e g 1) in the LnNiO 3 perovskite-type related oxides with the smaller size of the Ln 3+ ion. 2. (ii) The raised L.S. → H.S. transition temperature of the homologous LnCoO 3 cobalt(III) compounds. 3. (iii) The competition between magnetic interactions in the rare earth oxynitrides or in the K 2NiF 4-type La 1− x Y x CaCrO 4 solid solution. 4. (iv) The influence of the chemical bonding of Eu 2+ on the monochromatic 6P 7 2 → 8S 7 2 emission in the fluorides or on the Ce 3+ → Tb 3+ energy transfer in the rare earth phosphates. 5. (v) The enhancement of the luminescence of the trivalent rare earths in sulfides obtained by UV excitation, either by 5d → 4f or by ligand → 4f emission with an energy smaller than that of the intrinsic band gap of the material. 6. (vi) The influence of chemical composition on the hydrogen storage properties of the LnT 5H x -type hydrides (where T is a 3d element, generally Ni). The catalytic role played by LaNi 5 on the hydriding process of magnesium will be cited in this context.