Differences and similarities in the chemical bonding of intermetallic phases in the Ca-Al-Pt system

Intermetallic compounds belong to an important class of materials, not only due to the sheer number of compounds known but also due to their application in everyday life. These compounds possess their very own peculiarities, especially when it comes to chemical bonding. To address this point, bonding analyses based on Crystal Orbital Bond Index (COBI) values, Löwdin charges, and - for the first time - ab initio oxidation numbers (ON ^ai ) were conducted, all extracted from delocalized plane-wave functions. From the integrated COBI values, to be understood as quantum-chemical bond orders, the differences and similarities in the bonding behavior of the elements, binary and ternary compounds in the Ca-Al-Pt system were analyzed. It became apparent that the Al-Pt interactions, almost regardless of the respective compounds, show significant covalency, while Ca-Al and Ca-Pt interactions are of ionic nature in most cases. Homoatomic Al-Al or Pt-Pt interactions, however, tend to be ambivalent, depending on the respective crystal structure of a given compound.