S. Wefing, Modeling of continuous random networks: a case study for vitreous GeO2. II. Topological analysis and refinement, J NON-CRYST, 244(2-3), 1999, pp. 112-127
In this paper we present a new method to compare the topological structure
of two continuous random network (CRN) models in a quantitative manner. The
comparison serves to assess the sampling properties of the molecular model
ing program TUMBLEWEED. This program generates spherical CRN clusters and h
as been introduced in paper I of this series. The structural diversity of t
he models constructed with this program is found to be satisfactory. Furthe
r, the models are very different from the alpha-cristobalite and alpha-quar
tz structures of GeO2. In addition, we investigate three different techniqu
es for the refinement of the clusters presented in paper I: the conjugate g
radient (CG) method, the Monte Carlo (MC) method, and the Reverse Monte Car
lo (RMC) method. The former two procedures minimize the potential energy, V
, whereas the latter minimizes the deviation, R-x, of the model neutron tot
al correlation function, T(r), from corresponding experimental data. It is
found that neither technique gives satisfactory results by itself, since th
e minimization of R-x leads to an increase of V (including the violation of
geometric constraints) and vice versa. By combining the MC and RMC methods
, i.e. by simultaneous minimization of V and R-x, we obtain models with rea
listic geometric properties and an improved fit to the experimental data (R
-x = 0.018 +/- 0.003 in the range 0-1 nn). This further supports the validi
ty of the CRN approach to the structure of glassy GeO2. New models with a n
on-uniform torsion angle distribution have been constructed. It turns out t
hat such models cannot be excluded as possible structures (final R-x = 0.02
0 +/- 0.006). (C) 1999 Elsevier Science B.V. All rights reserved.