Investigation of ion-exchange ‘stuffed’ glass structures by molecular dynamics simulation

Abstract

Molecular dynamics simulations were used to examine ion-exchange ‘stuffed’ glass structures by substitution of Na+ ions with K+ ions in xNa2O·(100−x)SiO2, x=9, 16, and 23mol%. After substitution, relaxation was performed under varied boundary conditions, including NVT and NPT ensembles. Structural features of stuffed glasses were compared with those of the host glass and of the as-formed, compositionally-equivalent potassium silicate glass. Stuffing K+ ions had oxygen coordination number near that of the as-formed potassium silicate. Ion-exchange stuffed glasses were topologically confined, showing little Qn distribution or ring size distribution change compared to the host glass. Intertetrahedral re-orientation occurred to bear some of the network strain. Resulting molar volume change of ion-exchange stuffed systems was 58–67% of the difference between the as-formed sodium silicate host and the as-formed end-member potassium silicate glass. This accounted for a portion of the reduced experimental surface compression observed during ion exchange relative to that expected from the linear elastic suppression of the difference of as-melted molar volumes.