The predominance of SiO5 species in MgSiO3 melt at transition zone pressures from a dynamics point of view

Ab initio molecular dynamics (AIMD) simulations of melt and glass of MgSiO 3 and of other compositions at high pressure consistently observe the predominance of fivefold-coordinated silicon ( ) over and in the pressure range of the mantle transition zone. However, the common approach to compute the abundance of different coordination states is only based on a static ‘sequential-3D’ structural analysis. The absence of a time scale in the analysis has led to concern about the validity of its results. To validate AIMD results, here we investigate the coordination dynamics beyond the static limit, by carrying out a dynamical coordination species analysis in ‘4D’ on the basis of the short-term part of the intermittent coordination autocorrelation function described in an earlier study. We found that the predominance of SiO 5 species at pressures of the transition zone is persistent even in a dynamical analysis. However, towards lower pressures and most pronounced at ambient pressure, the majority of all SiO 5 detected by the common static sequential-3D analysis are so short-lived that they should be considered ‘spurious’ rather than chemical species.