Structure, dynamics, and electronic properties of lithium disilicate melt and glass

Ab initio molecular dynamics simulations within the framework of density functional theory have been performed to study the structural, dynamic, and electronic properties of lithium disilicate melt and the glass derived from quenching the melt. It is found that lithium ions have a much higher diffusion coefficient and show different diffusion mechanisms than the network forming silicon and oxygen ions in the melt. The simulated lithium disilicate glass structure has 100% four coordinated silicon, close to theoretical nonbridging oxygen to bridging oxygen ratio (2:3), and Q(n) distributions of 20.8%, 58.4%, and 20.8% for n=2,3,4, respectively. In the melt there are considerable amounts (10%-15%) of silicon coordination defects; however, the average silicon coordination number remains about 4, similar to that in the glass. The lithium ion coordination number increases from 3.7 in the glass to 4.4 in the melt mainly due to the increase of bridging oxygen in the first coordination shell. The bond length and bond angle distributions, vibrational density of states, and static structure factors of the simulated glass were determined where the latter was found to be in good agreement with experimental measurement. Atomic charges were obtained based on Bader and Hirshfeld population analyses [Atoms in Molecule: A Quantum Theory (Oxford University Press, Oxford, 1990); Theor. Chim. Acta 44, 129 (1977)]. The average Bader charges found in lithium disilicate glass were -1.729, 3.419, and 0.915 for oxygen, silicon, and lithium, respectively. The corresponding Hirshfeld charges were -0.307, 0.550, and 0.229. The electronic densities of states of the melt and glass were calculated and compared with those of crystalline lithium disilicate.