Energetics of substituted polyhedral oligomeric silsesquioxanes: a DFT study

First principles density functional theory calculations were conducted to investigate the structures and energetics of polyhedral oligomeric silsesquioxane (POSS) molecules with varying aluminum and alkali (sodium or potassium) concentrations. Notable trends emerge from this study namely, (1) the thermodynamic stability of the substituted POSS molecules is critically dependent on the interplay between size and composition of the POSS structures, and (2) larger POSS structures provide lower central electron density and hence better accommodate the central alkali atom. These observations, when viewed in the context of aluminosilicate based geopolymers, provide fundamental insights into the relations that describe the structure composition interplay of their underlying monomers.