Group connectivity in binary silicate glasses: A quasi-chemical approach and molecular dynamics simulation

The medium-range order of molecular dynamics (MD) simulated binary alkali silicate glass with 5, 10, 15, 20, 25, and 33.3 mol% of Na2O is studied in terms of the connectivity groups Qn,a1,…,an, where n stands for the number of bridging oxygen atoms, and a1, …, an indicate the type of the connected Qn units. MD structure significantly deviates from the random connectivity model. The deviations were quantified by means of the quasi-chemical approach. The interchange energy between Q3 and Q4 units was found to be negative, indicating preferential formation of Q3-Q4 connections. This means that no tendency to phase separation was observed. Although alkali cations were apparently non-homogeneously distributed in space, regions abounding in Q3-Q3 and Q4-Q4 connections were not formed. It was suggested that grouping of alkali cations can be also mediated by the Q4, 4433 groups.