Equation of State and Second Critical Point of Highly Compressed Nitrogen

First attempts to explain unusual behavior of Hugoniot in shock-compression experiments on liquid nitrogen [1] have been made many years ago [2-4]. They were based on an idea of polymerization reaction when triple chemical bonds in nitrogen diatomics broke and a spatial set of single chemical bonds appear [3]. According to predictions based on a simple model of sticky hard spheres [4] the molecular-to-polymer chemical reaction in nitrogen fluid leads to a first-order fluid-fluid transition with negative slope of P(T) curve and a critical point with TC > 10 000 K [5]. Recent ab initio simulations [6] reveal qualitatively the same behavior of the liquid-liquid phase boundary but predict much lower critical temperature between 4000 and 5000 K. A new equation of state for highly compressed nitrogen based on a more sophisticated than the earlier equation [4] model of polymerization [7] is developed and applied to prediction of the fluid-fluid coexistence line in strongly compressed nitrogen fluid. The position of the second critical point in nitrogen was estimated and compared with ab initio calculations [3] and our previous predictions [6]. We discuss effects of ‘shock cooling’ and negative thermal expansion in dense polymerized systems.