Generic mechanism for generating a liquid-liquid phase transition

Recent experimental results(1) indicate that phosphorus-a single component system-can have a high-density liquid (HDL) and a low-density liquid (LDL) phase. A first-order transition between two liquids of different densities(2) is consistent with experimental data for a variety of materials(3,4), including single-component systems such as water(5-8), silica(9) and carbon(10). Molecular dynamics simulations of very specific models for supercooled water(2,11), liquid carbon(12) and supercooled silica(13) predict a LDL-HDL critical point, but a coherent and general interpretation of the LDL-HDL transition is lacking. Here we show that the presence of a LDL and a HDL can be directly related to an interaction potential with an attractive part and two characteristic short-range repulsive distances. This kind of interaction is common to other single-component materials in the liquid state (in particular, liquid metals(2,14-21)), and such potentials are often used to describe systems that exhibit a density anomaly(2). However, our results show that the LDL and HDL phases can occur in systems with no density anomaly. Our results therefore present an experimental challenge to uncover a liquid-liquid transition in systems like liquid metals, regardless of the presence of a density anomaly.