Rare earth ion clustering behavior in europium doped silicate glasses: Simulation size and glass structure effect

Understanding the atomic structures, especially the local environment of rare earth ions and their distribution and clustering, is critical for rare earth containing glasses in photonic devices, as well as model systems for nuclear waste immobilization. In this paper, we've studied the structure of Eu2O3-doped silica and sodium silicate glasses using molecular dynamics simulations for systems ranging from around 1500 to around 24,000 atoms with the purpose to understand the simulation system size and the glass composition and structure effect on the local environment and clustering behavior of rare earth ions with low rare earth oxide concentration (similar to 1 mol%). In silica glasses, Eu-O distances and coordination numbers are found to be in the ranges of 2.19-2.22 angstrom and 4.6-4.8, respectively in the series. The existence of Eu clusters and short Eu-Eu distances in the range of 3.40-3.90 angstrom was observed. In the modified sodium silicate glass, on the other hand, europium ions were found to have higher coordination number of around 5.8, as well as a narrower coordination distribution, and longer Eu-O distance of around 2.32 angstrom. Eu ions were also found to be in much better dispersion and lower probability to form clusters. The clustering behavior of europium ions was investigated by comparing the probability of clustering in the simulated glass with those from a random distribution. It was found that clustering behavior is sensitive with the system size. For systems with over 12,000 atoms, consistent results were obtained in terms of probability of clustering as compared to a random distribution. But for smaller systems, the clustering behavior can vary from sample to sample. The results suggest that for low doping level it is critical to have sufficient number of rare earth ions in order to obtain statistical meaningful results on the local environment and clustering. (c) 2012 Elsevier B.V. All rights reserved.