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

Abstract

Periodic and molecular cluster models are presented for anorthite (CaAl 2 Si 2 O 8), 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 (300 atoms) and hydrated anorthite were therefore investigated with electronic structure methods and showed sound structural matches with the traditional periodic structures. The equilibrated 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 as interfaces.

Anisotropic diffusion of water molecules in hydroxyapatite nanopores

“…The P-P RDF profile of the nanopore is very similar to that of the crystal, which indicates that HAP nanopores remain crystalline. The P-P RDF profile of the HAP surface shows some deviations compared with that of HAP nanopore, suggesting some restructuring of the surface but not to an extend to indicate amorphousization of the surface (Tian et al 2016). This agrees with the previous MD work by de Leeuw, which showed that HAP surfaces maintain its crystalline structure (de Leeuw 2004a, b).…”

Resolving nanoscopic structuring and interfacial THz dynamics in setting cements

“…The PBE method with Goedecker-Teter-Hutter pseudopotentials were employed for core-valence electron interactions, whilst all valence interactions were represented using a double-zeta valence polarized basis set. Additional investigations of the low-energy vibrations were completed with electronic structure calculations using a molecular cluster model 38 and the Gaussian 09 program package, 39 with focus on resolving the interfacial THz modes. Models were geometry-optimised and complemented by analytical frequencies determined at the B3LYP/DGDZVP level of theory.…”

Biomedical NiTi and β-Ti Alloys: From Composition, Microstructure and Thermo-Mechanics to Application

“…EDX determined composition for the NiTi alloys was deemed unreliable, firstly due to the unrealistically high carbon content; secondly, EDX only determines superficial composition due to the short penetration depth of electrons as well as the marked discrepancy between EDX-determined surface and bulk compositions [18,40,41]. Thus PGAA, one of the most accurate and efficient non-destructive techniques [42], was applied taking advantage of the deeper penetration depth.…”