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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.

Direct measurements of the functional dependencies of the electric resistivity and the molar volume on enthalpy and pressure have been performed for graphite and liquid carbon. It has been found that for graphite at the pressures P  ⩽  1 GPa the isochoric temperature coefficient of resistance is positive, while for liquid carbon it is negative over the entire pressure range investigated where P  =  0.5-3.5 GPa. These observations probably indicate that graphite is a metal whereas liquid carbon is not a metal, so that the melting of graphite under such pressures coincides with a metal-to-nonmetal transition.

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