Molecular Dynamics Simulation of Vacancy Cluster Formation in β- and α-$S{i}_3{N}_4$

Abstract

Molecular dynamics simulation is used to study vacancy cluster formation in ${\hat{I}}^2$- and $\hat{I}\pm$-$S{i}_3{N}_4$ with varying vacancy contents (0 - 25.6 at%). Vacancies are randomly created in supercells, which were subsequently heat-treated for 114 nanoseconds. The results show that both ${\hat{I}}^2$ and $\hat{I}\pm$ can tolerate vacancies up to 12.8 at% and form clusters, confirming previous experimental data indicating 8 at% vacancy in $\hat{I}\pm$-$S{i}_3{N}_4$. However, 25.6 at% vacancy in ${\hat{I}}^2$ results in complete amorphization, while the same amount in $\hat{I}\pm$ results in a transformation of a semi-amorphous $\hat{I}\pm$ phase to a defective ${\hat{I}}^2$ phase, leading to the removal of the clusters in newly formed ${\hat{I}}^2$. This clearly explains why cluster vacancies are not experimentally observed in ${\hat{I}}^2$, considering that ${\hat{I}}^2$-$S{i}_3{N}_4$ ceramics are produced from $\hat{I}\pm$. Furthermore, the lattice parameters of both modifications increase with increasing vacancy content, revealing the cause of different lattice constants that were previously reported for $\hat{I}\pm$-$S{i}_3{N}_4$.

6 pages, 4 figures, 1 table, journal article