Measurements of electrical resistivity and caloric equation of state have been performed for fluid iron to investigate the metal-to-nonmetal transition induced by thermal expansion. The resistivity results published earlier [V. N. Korobenko and A. D. Rakhel, JETP, 112, 649 (2011)] have revealed the transition occurring at a density 4–5 times lower than ambient solid since in this density range the isochoric temperature coefficient of resistivity becomes negative when the magnitude of resistivity exceeds the minimum metallic conductivity range. The equation of state results reported here provide strong evidence for the existence of a first-order phase transition with a critical point located near the metal-to-nonmetal transition threshold. In particular, the isentropes plotted in the pressure-specific volume plane demonstrate pronounced kinks located on a convex line with a maximum at about 5 GPa. This suggests the presence of a critical point with the pressure of about one order of magnitude higher than that expected for the liquid-vapor transition. Arguments are given that the phase diagram of iron has the structure predicted in the well-known work [Ya. B. Zel'dovich and L. D. Landau, Zh. Eksp. Teor. Fiz. 14, 32 (1944)] but has not been observed yet.

• Received 29 December 2011

DOI:https://doi.org/10.1103/PhysRevB.85.014208