8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P8.61Liquid-liquid critical point in supercooled siliconVishwas Vasisht 11 JNCASR, Jakkur Campus, 560064, Bangalore, IndiaA novel liquid-liquid phase transition has been proposed and investigated in a wide variety of puresubstances recently, including water, silica, silicon, and others marked by energetically stabilizedtetrahedral local geometries. From computer simulations using a classical empirical potential (theStillinger-Weber potential), Sastry and Angell [1] demonstrated a first order liquid-liquid transitionin supercooled silicon (LLPT) at zero pressure, supported by further experimental and simulationstudies subsequently. Here we present evidence for a critical point to the liquid-liquid phase transitionat negative pressures, from computer simulations using the SW potential. Compressibilities,evaluated from the equation of state and fluctuations in constant pressure-temperature simulationsexhibit a growing maximum upon lowering temperature below 1500 K and isotherms exhibit densitydiscontinuities below 1120 K, at negative pressure. Below 1120 K, isotherms obtained fromconstant volume-temperature simulations exhibit non-monotonic, van der Waals-like behaviorsignaling a first order transition. We identify T c ≈ 1120 ± 12 K, P c ≈ −0.60 ± 0.15 GP a as thecritical temperature and pressure for the liquid-liquid critical point. We have constructed the phasediagram of supercooled silicon which reveal a strong interconnection between thermodynamicanomalies and the phase behaviour of the system. The structure of the liquid changes dramaticallyupon decreasing the temperature and pressure. Diffusivities vary over 4 orders of magnitude,and exhibit anomalous pressure dependence near the critical point. We also observe a strongrelationship between local geometry (quantified by the coordination number) and diffusivity, bothof which change dramatically with decreasing temperature and pressure.[1] Sastry and Angell, Nature Materials 2:739-743.[2] Vasisht, Saw and Sastry, Nature Physics (in press), arXiv:1103. 3473v2.61