Development of empirical potentials for amorphous silica
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Abstract
Development of empirical potentials for amorphous silica
Amorphous silica (SiO2) is of great importance in geoscience and mineralogy as well
as a raw material in glass industry. Its structure is characterized as a disordered continuous
network of SiO4 tetrahedra. Many efforts have been undertaken to understand
the microscopic properties of silica by classical molecular dynamics (MD) simulations.
nIn this method the interatomic interactions are modeled by an effective potential that
does not take explicitely into account the electronic degrees of freedom. In this work,
we propose a new methodology to parameterize such a potential for silica using ab
initio simulations, namely Car-Parrinello (CP) method [Phys. Rev. Lett. 55, 2471
(1985)]. The new potential proposed is compared to the BKS potential [Phys. Rev.
Lett. 64, 1955 (1990)] that is considered as the benchmark potential for silica.
First, CP simulations have been performed on a liquid silica sample at 3600 K.
The structural features so obtained have been compared to the ones predicted by the
classical BKS potential. Regarding the bond lengths the BKS tends to underestimate
the Si-O bond whereas the Si-Si bond is overestimated. The inter-tetrahedral angular
distribution functions are also not well described by the BKS potential. The corresponding
mean value of theSiOSi angle is found to be ≃ 147◦, while the CP
yields to
aSiOSi angle centered around 135◦.
Our aim is to fit a classical Born-Mayer/Coulomb pair potential using ab initio
calculations. To this end, we use the force-matching method proposed by Ercolessi
and Adams [Europhys. Lett. 26, 583 (1994)]. The CP configurations and their corresponding
interatomic forces have been considered for a least square fitting procedure.
The classical MD simulations with the resulting potential have lead to a structure that
is very different from the CP one.
Therefore, a different fitting criterion based on the CP partial pair correlation
functions was applied. Using this approach the resulting potential shows a better
agreement with the CP data than the BKS ones: pair correlation functions, angular
distribution functions, structure factors, density of states and pressure/density were
improved. At low temperature, the diffusion coefficients appear to be three times
higher than those predicted by the BKS model, however showing a similar
temperature
dependence.
Calculations have also been carried out on crystalline samples in order to check
the transferability of the potential. The equilibrium geometry as well as the elastic
constants of α-quartz at 0 K are well described by our new potential although the
crystalline phases have not been considered for the parameterization.
We have developed a new potential for silica which represents an improvement over
the pair potentials class proposed so far. Furthermore, the fitting methodology that
has been developed in this work can be applied to other network forming systems such
as germania as well as mixtures of SiO2 with other oxides (e.g. Al2O3, K2O, Na2O).