Tetrahedral order, pair correlation entropy, and waterlike liquid state anomalies: Comparison of GeO₂ with BeF₂, SiO₂, and H₂O

Abstract

Molecular dynamics simulations of the Oeffner-Elliot model of germania (GeO₂) are performed to identify nested regions of anomalous behavior in structural order, diffusivity, and pair entropy in the density-temperature plane, analogous to that seen in BeF₂, SiO₂, and H₂O. The decreasing constraint of local tetrahedrality in GeO₂, compared to SiO₂ and BeF₂, substantially lowers the onset temperatures for anomalous behavior relative to the experimental melting temperatures (T_m). Germania resembles water, more strongly than the ionic melts, in terms of temperatures for onset of anomalous behavior as well as in the order maps; for example, the structural anomaly sets in at 3.42T_m in BeF₂, 3.09T_m in SiO₂, 1.43T_m in GeO₂, and 1.21T_m in H₂O. The detailed shapes of the anomalous regimes vary for different systems but the relative temperatures of onset for different anomalies are very similar in the different systems. The pair correlation entropy is shown to be a crucial and experimentally accessible quantity for relating structure, entropy, and diffusivity that could be potentially useful for a large class of inorganic ionic liquids.