Metastable liquid-liquid phase transition in a single-component system with only one crystal phase and no density anomaly

Abstract

We investigate the phase behavior of a single-component system in three dimensions with sphericallysymmetric, pairwise-additive, soft-core interactions with an attractive well at a long distance, a repulsive soft-core shoulder at an intermediate distance, and a hard-core repulsion at a short distance, similar to potentials used to describe liquid systems such as colloids, protein solutions, or liquid metals. We showed ͓Nature ͑London͒ 409, 692 ͑2001͔͒ that, even with no evidence of the density anomaly, the phase diagram has two first-order fluid-fluid phase transitions, one ending in a gas-low-density-liquid ͑LDL͒ critical point, and the other in a gas-high-density-liquid ͑HDL͒ critical point, with a LDL-HDL phase transition at low temperatures. Here we use integral equation calculations to explore the three-parameter space of the soft-core potential and perform molecular dynamics simulations in the interesting region of parameters. For the equilibrium phase diagram, we analyze the structure of the crystal phase and find that, within the considered range of densities, the structure is independent of the density. Then, we analyze in detail the fluid metastable phases and, by explicit thermodynamic calculation in the supercooled phase, we show the absence of the density anomaly. We suggest that this absence is related to the presence of only one stable crystal structure.

Thermodynamic and dynamic anomalies for a three-dimensional isotropic core-softened potential

“…In two dimensions, such potentials have density and diffusion anomalies and in some cases a second critical point [22,27,28,29]. In three dimensions, these potentials do not have dynamic and thermodynamic anomalies but possess a second [30] and sometimes a third [25] critical point, accessible by simulations in the region predicted by the hypernetted chain integral equation [23,26,31].…”

“…In the first case, the potential consists of a hard core, a square repulsive shoulder and, in some cases, an attractive square well [21,22,23,24,25,26,27,28,29,30,31,34]. In two dimensions, such potentials have density and diffusion anomalies and in some cases a second critical point [22,27,28,29].…”

“…They possess a repulsive core that exhibits a region of softening where the slope changes dramatically. This region can be a shoulder or a ramp [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36].…”

Diffusion anomaly in a three-dimensional lattice gas

“…They possess a repulsive core that exhibits a region of softening where the slope changes dramatically. This region can be a shoulder or a ramp [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. Unfortunately, these models, even when successful in showing density anomaly and two liquid phases, fail in providing the connection between the isotropic effective potential and the realistic potential of water.…”

Relation between occupation in the first coordination shells and Widom line in core-softened potentials

“…Even with this limitation, this hypothesis has been supported by indirect experimental results 18, 33,46 . The presence of two liquid phase and of second critical point is also observed in certain CS potentials [6][7][8][9][10][11][12][13][14][15][16][17][18][22][23][24][25][26][27] .…”