Liquid-liquid critical point in supercooled silicon

A novel liquid-liquid phase transition has been investigated for a wide variety of pure substances, including water, silica and silicon. From computer simulations using the Stillinger-Weber (SW) classical empirical potential, Sastry and Angell(1) demonstrated a first order liquid-liquid transition in supercooled silicon at zero pressure, supported by subsequent experimental and simulation studies. Whether the line of such first order transitions will terminate at a critical point, expected to lie at negative pressures, is presently a matter of debate(2). Here we report evidence for a liquid-liquid critical point at negative pressures, from computer simulations using the SW potential. We identify T-c similar to 1,120 +/- 12 K, P-c similar to -0.60 +/- 0.15 GPa as the critical temperature and pressure. We construct the phase diagram of supercooled silicon, which reveals the interconnection between thermodynamic anomalies and the phase behaviour of the system as suggested in previous works(3-10). We also observe a strong relationship between local geometry (quantified by the coordination number) and diffusivity, both of which change dramatically with decreasing temperature and pressure.