Molecular level investigation of the dynamic structure model in molten and solid alkali glasses

• Published in: Structural chemistry 2012-12, Vol.23 (6), p.1729-1738
• Main Authors: Bakk, István P., Deme, István, Szieberth, Dénes, Nyulászi, László
• Format: Article
• Language: English
• Subjects: Chemistry; Chemistry and Materials Science; Computer Applications in Chemistry; Original Research; Physical Chemistry; Theoretical and Computational Chemistry
• ISSN: 1040-0400; 1572-9001
• DOI: 10.1007/s11224-012-9969-2

The structure and bonding of alkali glasses has been investigated using a cluster approach and density functional calculations. The clusters used in the study were selected to account for all the important interactions found on small model compounds. Interaction between alkali ions (M + ) and the silica host matrix was investigated in terms of complexation energies and bond distances for bridging (O b ) and non-bridging oxygens (O nb ) as well as for systems containing pentacoordinate silicon. The structures with pentacoordinated silicon are only slightly less stable than the structures with O nb , the relative stability depends on the ionic character of M + , potassium exhibiting the largest propensity for the formation of pentacoordinated structure. The effect of ring size and interaction between geminal oxygens was also considered, to select the suitable clusters. Pentacoordinated silicon structures (in the vicinity of alkali ions) can open up with a low barrier resulting in O nb -s at different positions, allowing for the migration of the corresponding alkali ions. Differences in the binding energies and bond distances for the different cations can explain their different mobility. The mismatch of preferred sites corresponding to different M + ions were shown by investigating the spatial distribution of O b and O nb of the siloxane network, as well as the energy barriers for diffusion in pore-like structures.