Electronic redistribution around oxygen atoms in silicate melts by ab initio molecular dynamics simulation

TL;DR

This study uses ab initio molecular dynamics and Wannier orbitals to analyze electronic density redistribution around oxygen in various silicate melts, revealing differences in covalent and ionic character and the weak dependence of polarization on melt composition.

Contribution

It introduces a method to quantify electronic density redistribution around oxygen in silicate melts using Wannier orbitals, providing new insights into bond covalency and polarization effects.

Findings

Al-O bonds are less covalent than Si-O bonds.

Covalent character of M-O bonds decreases from Mg to K.

Oxygen dipole moment distribution is weakly dependent on melt composition.

Abstract

The structure around oxygen atoms of four silicate liquids (silica, rhyolite, a model basalt and enstatite) is evaluated by ab initio molecular dynamics simulation. Thanks to the use of maximally localized Wannier orbitals to represent the electronic ground state of the simulated system, one is able to quantify the redistribution of electronic density around oxygen atoms as a function of the cationic environment and melt composition. It is shown that the structure of the melt in the immediate vicinity of the oxygen atoms modulates the distribution of the Wannier orbitals associated with oxygen atoms. In particular the evaluation of the distances between the oxygen-core and the orbital Wannier centers and their evolution with the nature of the cation indicates that the Al-O bond in silicate melts is certainly less covalent than the Si-O bond while for the series Mg-O, Ca-O, Na-O and K-O the covalent character of the M-O bond diminishes rapidly to the benefit of the ionic character. Furthermore it is found that the distribution of the oxygen dipole moment coming from the electronic polarization is only weakly dependent on the melt composition, a finding which could explain why some empirical force fields can exhibit a high degree of transferability with melt composition.