Simple view of waterlike anomalies of atomic liquids with directional bonding

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We propose that the shape of the equilibrium T-P phase diagram of a liquid is strongly correlated with the thermodynamic anomalies and the ease of vitrification. The T-P phase diagram of water is characterized by its V shape. By using this specific shape of the phase diagram as a fingerprint, we classify five elements Si, Ge, Sb, Bi, and Ga into water-type atomic liquids. Similarly, some group III-V (e.g., InSb, GaAS, and GaP) and II-VI compounds (e.g., HgTe, CdTe, and CdSe) are also classified into water-type liquids. We demonstrate that the phase behaviors and the thermodynamic and kinetic behaviors of these liquids can be described by the two-order-parameter model that deals with not only density ordering but also bond ordering. In these liquids, the former represents the metallic-bond nature favoring the isotropic symmetry and tries to increase the density upon ordering, while the latter represents the covalent-bond nature favoring the tetrahedral symmetry and tries to decrease it upon ordering. Thus, they are intrinsically competing. This model well explains the shape of the phase diagram, the temperature dependence of the structure factor, and the thermodynamic anomalies of density and heat capacity of liquid Si in a coherent manner. It also predicts that these water-type liquids will be very poor glass formers at ambient pressure, but their glass-forming ability should increase with increasing pressure. This increase in the glass-forming ability is not only due to the thermodynamic reasons, but also due to the kinetic ones.