The characterization of reactivity and stabilization mechanisms in intermetallics using mu3-acidity and DFT-calibrated Huckel theory

Intermetallic compounds exhibit a wide diversity of structural chemistry, much of which remains unexplained. Using DFT calibrated-Huckel theory calculations as a basis, we have advanced towards understanding their atomic arrangements through a generalization of the Lewis acid-base concept. We accomplish this with the Method of Moments, a technique based on linking moments ( mun) of the density of states (DOS) distribution to geometrical features. We demonstrate that the third moment (mu3), which evaluates the balance of states above and below a pseudogap of a DOS curve, dictates an ideal occupancy for a given DOS curve. The difference between the ideal and calculated occupancy, which we call mu3-acidity, provides a measure of reactivity useful for understanding the formation of intermetallic phases from metals with the same language as the formation of adducts from Lewis acids and bases.;Our method designates systems that are electron-poor with respect to the ideal are mu3-acidic, and those that are electron-rich as mu3-basic. The formation of either an intermetallic phase or Lewis acid-base adduct causes a resulting change in bonding to neutralize the reacting metals or frontier orbitals, respectively. Calculating the mu3-acidity of the 3d transtion metals revealed that Sc, Ti, and V are mu3-acidic, Cr is mu 3-neutral, and Mn, Fe, Co, Ni, Cu are mu3-basic. When forming a transition metal binary phase, the relative starting strength and ratio of the mu3-acidic and mu 3-basic atoms is a strong determinant of phase stability, as we have observed for phases forming CsCl and Laves-type structures. When the pairing of acid-base strengths is less than optimal, phases can respond with structural modifications to tune the neutralization interaction. This is observed for TiCu and Ti21Mn25: instead of forming the simple CsCl type, these phases adopt more complex structures to promote mu3-neutralization. For TiCu, this takes the form or interface layer insertion into the CsCl type, while in Ti21Mn25 a large unit cell is built from domains of a simpler structure type. Both structural changes can be thought of as mechanisms to obtain mu 3-neutrality for the constituent atoms. Using mu 3-acidity, we have provided a new framework to evaluate chemical bonding in extended solids, particularly those formed between transition metals.