Melting, thermal expansion, and phase transitions of iron at high pressures

Abstract

The melting curve of iron to 1.2 Mbar was remeasured in a laser‐heated diamond anvil cell. A defocused 35 W Nd∶YAG laser beam provided low‐temperature gradients which were measured within an area of 2 μm in diameter with a chromatically corrected optical geometry. Argon to 350 kbar and A1 2 O 3 to 1.2 Mbar were used as pressure media. Melting was detected by in situ measurements of optical reflectivity and electrical resistivity and by visual observation of the surface texture. At 1.2 Mbar, iron melts at 3000 ± 100 K. The same laser‐heating technique was used to measure volumes of fcc‐(γ)‐iron to 168 kbar and 2000 K using synchrotron radiation, leading to a decrease in the thermal expansion coefficient of γ‐iron represented by (∂ In α/∂ In V ) T = 6.5±0.5. The α‐ε transition of iron has been studied by X ray and by the change in resistivity of a fine wire using a variety of pressure media. The hysteresis of the transition changes systematically with the shear strength of the pressure medium with an equilibrium transition pressure at 130±10 kbar.