Factors Affecting Ionicity in All-Silica Materials: A Density Functional Cluster Study

Atomic charge analysis is performed on the basis of electron densities obtained from systematic density functional (DF) cluster calculations on structural fragments from seven different all-siliceous zeolites and two all-silica minerals. Charges and ionicities for these fragments are calculated using three different charge analysis schemes: Mulliken, Hirshfeld, and Bader methods. The dependency of the ionicty of the Si−O bond on geometric parameters of the clusters is investigated and it is demonstrated that a correlation exists between the Si−O bond length and its ionicty. Ionicities derived from Bader atomic charges were shown to have a trend opposite to that of ionicities derived from Mulliken and Hirshfeld charges. A simple model is proposed to account for these differences, demonstrating that these opposing trends may be reconciled. The results also suggest a possible link between Si−O bond ionicity and the energetic stability of the corresponding bulk silica polymorph.