Density minimum and liquid-liquid phase transition.pdf

a r X i v : c o n d - m a t / 0 5 0 4 5 7 4 v 1 [ c o n d - m a t .s t a t - m e c h ] 2 2 A p r 2 0 0 5 Density minimum and liquid-liquid phase transition Peter H. Poole,1 Ivan Saika-Voivod,2, 3 and Francesco Sciortino2 1Department of Physics, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada 2Dipartimento di Fisica and INFM-CRS-Soft, Universita’ di Roma La Sapienza, Piazzale Aldo Moro 2, I-00185, Roma, Italy 3Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5C9, Canada (Dated: April 21, 2005) We present a high-resolution computer simulation study of the equation of state of ST2 water, evaluating the liquid-state properties at 2718 state points, and precisely locating the liquid-liquid critical point (LLCP) occurring in this model. We are thereby able to reveal the interconnected set of density anomalies, spinodal instabilities and response function extrema that occur in the vicinity of a LLCP for the case of a realistic, off-lattice model of a liquid with local tetrahedral order. In particular, we unambiguously identify a density minimum in the liquid state, define its relationship to other anomalies, and show that it arises due to the approach of the liquid structure to a defect-free random tetrahedral network of hydrogen bonds. PACS numbers: 61.25.-f,64.30.+t,64.70.-p Water is not only the most abundant liquid on earth, but also a prototype of many network forming materials. As for water, important substances such as silicon and silica exhibit in their liquid phase a disordered network structure that arises due to highly directional tetrahe- dral bonding interactions. It has long been appreciated that the development of an open tetrahedral network on cooling is related to the occurrence of the density maxi- mum observed in these liquids [1]. In water the density maximum occurs at 277 K at ambient pressure P . For temperatures T above the density maximum, the isobaric expansivity αP is positive, as for