Liquid–liquid critical point in supercooled silicon

Abstract

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 ∼ 1,120 ± 12 K, P c ∼ −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][4][5][6][7][8][9][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.The possibility of a phase transition between two forms of the liquid phase in some pure substances has attracted considerable interest and research activity in recent years [1][2][3][4][5][6][7][8][9][10] . Among the substances investigated are water 6,8,9 , silica 10 , silicon 1,11-17 , germanium, carbon and hydrogen-these substances together form a very significant component of our natural world, living organisms and technology. A phenomenon common to these is therefore of wide general interest. Furthermore, as illustrated in ref. 18, liquid-liquid transitions offer an avenue for interesting applications that exploit the different properties of distinct liquid phases.Although the liquid-liquid phase transition (LLPT) had been discussed in the context of silicon 11 earlier, the considerable current interest stems from various proposals for understanding the anomalies of water 2,4,[6][7][8]19,20 . These scenarios have alternately invoked the approach to a spinodal 4 , a liquid-liquid critical point 6,8 , general thermodynamic constraints without the presence of any singular behaviour 7 , and the presence of a transition without a critical point 2 , in rationalizing experimentally observed behaviour. In spite of substantial investigations, a general consensus has still to be reached on the interpretation of the observed behaviour 2,19 . In particular, recent experiments on confined water 20 and issues surrounding their interpretation 2 indicate the need to ascertain the existence of a critical point even when sufficient evidence exists for a liquid-liquid transition.The possibility of a transition in supercooled silicon was suggested 12 on the basis of estimates of the excess Gibbs free energies of amorphous and liquid silicon, implying an 'amorphous-liquid' phase transition near 1,450 K (below the freezing point of the liquid, 1,685 K...

Understanding tetrahedral liquids through patchy colloids

“…We thank colleagues Vishwas Vashist and Srikanth Sastry for providing us with quenched configurations of the SW model with 10000 particles at T = 1196 K and P = −1.88GPa. The state point is above the liquid-liquid critical temperature for that model, but below the line of compressibility maxima and in the tetrahedral network regime [45]. We are also grateful to colleagues Dr. Flavio Romano and Dr. John Russo for providing us, in the course of their current study of the model, with quenched configurations of 686 mW water molecules at P = 0 and T = 171.3 K, a state point below the transition temperature to the low density liquid (LDL) as presented in [19], but at which crystallization also occurs.…”

“…The fraction of defects is simply the fraction of node particles that do not have four neighbors. Generally, mW and SW show a relatively large number of defects, and are also both known to crystallize spontaneously in simulation without great difficulty [19,45].…”

Liquid–liquid transition in a strong bulk metallic glass-forming liquid

“…The LLT of two distinct forms of liquid water has been intensely studied and debated [1][2][3][4][5][6][7] . Not limited to water, experimental and numerical support for existence of a polyamorphism or LLT has also been found in other systems, for example, in triphenyl phosphate 1 9 and Ce-Al 10,11 .…”

“…T here is a growing interest in density-or entropy-driven liquid-liquid phase transitions (LLTs) [1][2][3][4][5] . The LLT of two distinct forms of liquid water has been intensely studied and debated [1][2][3][4][5][6][7] .…”

Phase diagram of the ST2 model of water

“…It is now clear that a LLCP, while common in tetrahedral network-forming liquids [14][15][16][17][18][19], can also be observed in complex one-component fluids when the (spherically symmetric) interaction potential generates two competing length scales [20][21][22][23]. In the last few years the interest has shifted towards the interplay between the liquid-liquid critical point and crystal nucleation [18,[24][25][26][27].…”