Riccardo Freccero

Abstract:

Intermetallics constitute the largest group of inorganic compounds [1], often displaying intriguing and unprecedented structural features and physical properties. Among them, those containing rare-earth metals (RE) enabled relevant breakthroughs and are set to play a key role in future technologies, required to successfully face emerging challenges. The rational design of such materials is still hampered by the lack of understanding of the structure–bonding relationships, as classical valence rules cannot be effectively applied [2]. Thus, relevant achievements may only stem from systematic experimental and theoretical investigations. In this work, the preparation and structural characterization of several RE–M–Tt (M = s, p or transition metals; Tt = Si, Ge, Sn) with more than 50at% of Tt are presented. Interatomic distances were in all cases compatible with the formation of covalent Tt interactions, building up polyanionic dumbbells, chains, layers, and frameworks resulting in a Tt homo-connectivity ranging from 1 to 3; for some representative, as RE₂MgGe₆ and RE₂Pd₃Ge₅, the Zintl rules were still formally fulfilled. Aiming at overcoming the approximation that the interactions between the Tt-polyanions and the surrounding metals are of the ionic type, in-depth chemical bonding analyses were performed, mainly applying quantum chemical techniques in position space [2]. Particular attention was devoted to the nature of interactions between the RE and the M metals, often neglected in the literature. For these purposes new tools and techniques were introduced or extended, namely the Penultimate Shell Correction (PSC0), the Electron Localizability Indicator (ELI-D) fine structure, and the 8–N^eff rule [3,4]. Independently on the intermetallic stoichiometry, the following trends were described: RE metals turned out to be always involved in multi-atomic covalent bonds with the polyanions through the Tt “lone pairs”; RE are involved in polar covalent interactions with late transition metals, particularly those of the 4^th and 5^th period. These outcomes clearly suggest that the chemical role of the RE as large cations lying in the biggest cavities must be definitely abandoned. Recent results on some intermetallics only formed by elements to the left of the p-block, like LaAuMg₂ and RENi₅, surprisingly revealed RE–Au/Ni bonds analogous to those described for the tetrelides. Thus, the chemical behavior of LaNi₅ and CeNi₅ was investigated testing them as Sabatier’s catalysts in the reduction of CO₂ to CH₄. The knowledge of the chemical bonding in the bulk phases enabled the understanding of surface phenomena taking place during the catalytic experiments, particularly on the adsorption and dissociation of H₂ molecules.