Molecular dynamics simulation of lead germanate glasses

In recent years, there has been a great deal of focus on the development of glass materials with low transmission loss in optical fibers. Although the glass forming ability and efficiency of lead germanate glasses, PbO.GeO 2 , for such applications have already been established, little is known about the structure of these glasses. Germanate glasses tend to exhibit anomalous properties as a function of their composition, referred to as 'germanate anomaly.' There is a controversy in the literature concerning the dominant coordination of germanium ions in these glasses at different lead oxide compositions and the structural mechanism by which 'germanate anomaly' occurs. In spite of numerous Raman, Infra-red (IR), neutron, and X-ray Absorption Fine Structure (EXAFS) studies of lead germanate glasses, a full understanding of short-range and medium-range order of these glasses is still lacking. Using Molecular Dynamics (MD) simulation with a two-body potential model, a composition study was performed on xPbO.(1-x)GeO 2 glasses with x=0.05-0.50 in order to investigate the structure of the lead germanate glasses at different lead oxide concentrations. The structural features of the germanate framework and the lead environment are calculated and represented by pair and cumulative distribution functions, ring statistics, bond angle distributions, and percentage of non-bridging oxygens (NBOs) at each lead oxide composition. The results of the MD simulations show no evidence of the germanate anomaly in the simulated lead germanate glasses and indicate that the germanate framework consists predominantly of GeO 4 units. Continuous formation of NBOs is observed with the addition of lead oxide. Through connectivity studies, the presence of a secondary lead framework is predicted. To further enhance the potential model, lattice dynamics simulation using shell-model and a combination of two- and three-body potential model was used to generate crystal properties of {460}-quartz-like GeO 2 (infrared frequencies, lattice energy, bulk modulus, elastic constants, static dielectric constants, high frequency dielectric constants, and heat capacity at constant volume). This study suggests that it is possible not only to model, but also to predict various crucial properties of crystals by the use of appropriate potential models and computer modeling codes. The potential was also capable of reproducing the infrared frequencies, and elastic constants of rutile-like GeO 2 crystal.