Abstract

An investigation of the local environment of rare-earth doped lead silicate glasses, using molecular dynamics simulation techniques and crystal-field theory, is presented. The primary focus of the research was to develop a more realistic potential model to better describe the environment surrounding the rare-earth ion. The configurations generated from the MD simulations were used in a point-charge crystal-field model, using C 1 symmetry, to generate the emission spectra of the rare-earth ions. By refining both the aforementioned methods, we have developed a model that can isolate the individual geometrical RE 3+ environments and calculate the individual emission spectra corresponding to each of the different geometrical arrangements. This in turn has allowed us to obtain a more complete description of the local structure of the RE 3+ ion with respect to the overall emission spectrum. Molecular dynamics techniques were employed to simulate undoped and rare-earth doped lead silicate glasses, using both two- and three-body potential models. Structural features of the simulations of undoped PbO·SiO 2 glass were found to be in excellent agreement with published experimental results, and the three-body potential model produced a marked improvement over the two-body potential model. The simulations using the three-body potential showed the presence of two networks in the glass; a silicate network and a lead network. In order to obtain a better description of the effect of the lead ions in silicate glasses, a concentration study on undoped lead silicate glass was performed. At low modifier concentrations (22 mol% PbO) the lead ion was found to behave as a typical modifier. As the concentration of PbO was increased to 70 mol%, the lead ions behaved more as network formers, and an increase in PbO coordination was observed due to the lead ions sharing neighbouring oxygens. The presence of lead rich regions predominated in the glass and the two networks (silicon and lead) were found to be connected via edge-sharing oxygens. Simulations of the Eu 3+ -lead silicate glass, using the three-body potential model, showed that the Eu 3+ ions were found primarily in the lead network, with only a modest presence in the silicate network, showing that the two cations (Pb 2+ , Eu 3+ ) share similar environments. The average coordination number of Eu 3+ was found to be 6.5. Furthermore, two main geometrical arrangements were found to exist for Eu 3+ , a six-coordinated distorted octahedral and a seven-coordinated distorted pentagonal bipyramid. Since the spectroscopic properties of the dopant ions are dependent on the local environment, an in depth investigation on first coordination sphere of three different rare-earth ions (Eu 3+ -, Er 3+ - and Yb 3+ ) doped in lead silicate was performed using the three-body potential model. Although the differences in ionic radii, interionic distance and average coordination number between the three rare-earths was quite small, the three-body potential model successfully reproduced these differences in the bulk structural features, and the results were found to be in excellent agreement with experimental data.