Thermal expansivity of HCP iron at very high pressure and temperature

We determine the coefficient of thermal expansion, alpha, for hexagonal close-packed (HCP) iron at simultaneous extreme pressure, P, and temperature, T, conditions. Two approaches are used; both are based on reported properties of HCP iron at high pressure and 300 K. The first approach constrains alpha by high-temperature isotherms of P(V) (V is volume) determined from a thermal-pressure equation of state. Second, thermodynamic approximations are used without explicit appeal to the equation of state in extending the 300 K alpha(P) to high T. The two approaches give results nearly consistent with each other over 0-360 GPa and 300-7000 K. Our results for alpha for HCP iron are in near agreement at high P and all T with two theoretical studies. At low P, however, there is some discrepancy between the theoretical results, which complicates detailed comparisons, even though our results tend to be higher. Nevertheless, these new results for alpha, as well as those from theory, are higher than reported on the shock Hugoniot at 202 GPa and 5200 K. (C) 2002 Elsevier Science B.V. All rights reserved.