PNNL

Abstract

Ab initio molecular dynamics simulations within the framework of density functional theory (DFT) 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 mechanism 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 non-bridging oxygen (NBO) to bridging oxygen (BO) ratio (2:3), and a Qn distribution of 20.8%, 58.4% and 20.8% for n=2,3,4 respectively. In the melt there are considerable amount (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 experiment. Atomic charges were obtained based on Bader and Hirshfeld population analyses. 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 density of states of the melt and glass were calculated and compared with those of crystalline lithium disilicate. Battelle operates PNNL for the USDOE