Observation of a first-order metal-to-nonmetal phase transition in fluid iron

Abstract

Direct measurements of resistivity and caloric equation of state have been performed for fluid iron at pressures of 2 to 12 GPa in a wide density range. We found that the isochoric temperature coefficient of resistivity becomes negative, and this is considered as an indication of the metal-to-nonmetal transition, when density decreased by a factor of 3 to 4 compared to the normal solid density. We detected also that isentropes plotted in the pressure -specific volume plane have well-defined kinks localized on a convex curve with a maximum at about 5 GPa. Such behavior of isentropes evidences about a first order phase transition with a critical pressure one order of magnitude higher than the predicted pressure of the liquid-vapor critical point. Arguments are presented that the observed phase transition is most likely the liquid-vapor transition rather than an extra first order transition in the fluid state. We show that the gaseous nonmetallic phase represents dense plasma in the 1-2-th state of ionization so that it is a plasma phase transition as well.

The Thermophysical Properties of Iron Plasma

“…I. e. U → U − U 0 , where U 0 = −H sub [19,26]. The latter value is the enthalpy of sublimation (7.4 kJ/g www.cpp-journal.org at 300 K [10]). The internal energy U obtained within our model was corrected by this value to be compared with the measurement data.…”

“…The experimental data mentioned above were obtained along several isotherms [6][7][8] and several isochores [9,10]. We have chosen for the comparison the isotherm T = 10 kK and the isochore ρ = ρ n /6 = 1.312 g/cm 3 , where ρ n = 7.874 g/cm 3 is the Iron density under ambient conditions.…”

“…So let us consider the results for these values. The pressure and internal energy (per mass) were measured in [9,10]. More exactly there were no the measurements of the absolute internal energy, but its difference between the final (plasma) and the initial (crystal) states.…”

“…It is fulfilled for T ≥ 10 kK and densities ρ ≤ 1 g/cm 3 , which corresponds to the area studied in [6][7][8]. The densities and temperatures reached in [9,10] are corresponds to E ≈1. Nevertheless, as we will see below, our results on the pressure and internal energy are in good agreement with these measurements.…”

“…The experimental data for the electrical conductivity of Fe plasma are presented in [6][7][8][9][10]. In [9,10] the pressure and internal energy were measured too. We should note that the temperature can not be measured directly in these experiments.…”