Structural origin of the anomalous density maximum in silica and alkali silicate glasses

YT Shih and S Sundararaman and LP Huang, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 103, 3942-3953 (2020).

DOI: 10.1111/jace.16850

Silica, sharing the same tetrahedral order and many structural, thermodynamic and dynamic anomalies with water, has been speculated to have a density increase upon melting similar to water. In this work, an increase in density upon melting cristobalite silica and a shallow density maximum followed by a density minimum during cooling of silica liquid are observed in classical molecular dynamics simulations. The density maximum gradually diminishes with the increase in alkali size/content in alkali silicate glasses. The structural origin of the anomalous density maximum in silica is revealed by detailed structural analysis. During the cooling process, a range of rings with different sizes form in liquid silica, with 6-member rings being the most dominant, which cause the silica network to open up and compensate the regular volume shrinkage upon cooling. These two competing factors lead to a density maximum, but to a less extent than that observed in melting of cristobalite silica. With the increase in modifier size/content in the alkali silicate glasses, the connectivity of silica network gradually breaks down; the population of 6-member rings decrease with the increase in smaller or larger rings, therefore the density maximum becomes less obvious and eventually disappears.

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