Modeling of continuous random networks: a case study for vitreous GeO 2. I. Model generation

Abstract

Here a molecular modeling program called TUMBLEWEED which generates and analyzes spherical models (clusters) for covalent amorphous solids of the continuous random network (CRN) type is introduced. The algorithm sequentially adds atoms to a core in accordance with topological and geometrical constraints. These constraints comprise the type and number of atomic neighbors, intervals for bond lengths and angles, and minimum distances for non-bonded atoms. The program includes a new mechanism to control the model density via selection from a pool of models, and a procedure to avoid infinite loops if atoms are repeatedly added to and removed from the model surface. The method is demonstrated for glassy GeO 2, for which CRN models are presented for the first time. Various distributions are derived from the models to assess their quality. It is found that the models are homogeneous and have a density of (3.60±0.03) g/cm 3 [experimental data: (3.65±0.01) g/cm 3]. The bond angle distributions are similar to those derived from synchrotron experiments. Neutron scattering data are simulated for the model structures and compared to experimental data. The normalized root mean square deviation, R χ , between the simulated and the measured total correlation functions was found to be as small as 0.027±0.003 over the interval 0–1 nm indicating that CRN models are well suited to describe the structure of vitreous germania.