Periodic vs. molecular cluster approaches to resolving glass structure and properties: Anorthite a case study

Periodic and molecular cluster models are presented for anorthite (CaAl2Si2O8), a cement forming glass with desirable thermal and mechanical properties also tested in dental applications. Both the crystalline and amorphous structures were characterised by ab initio molecular dynamics and found to be in good agreement with experiment. Additional investigations of the elongation and fracture of the glass were also made. The recovery of material properties signaled the failure of the periodic method to generate acceptable fracture surfaces to model cement forming-sites. Isolated molecular cluster models of anorthite were therefore investigated with electronic structure methods and showed sound structural matches with the traditional periodic structures. The equilibrated glass clusters were used to develop cement models, through binding of short acid oligomers to selected Al-centres, simulating the glass-polymer interface. Overall, the anorthite glass structures emerging from periodic and cluster models were in close agreement. Results suggest that bare molecular cluster models represent an alternative avenue for accurately investigating amorphous systems, providing more realistic descriptions of edge and corner sites, as well as interfaces.