Synchrotron infrared reflectivity measurements of iron at high pressures.

The infrared reflectance of iron was studied using high-pressure synchrotron radiation methods up to 50 GPa at room temperature in a diamond anvil cell of 1000-8000 cm(-1) (1.25-10 microm). The magnitude of the reflectivity shows a weak pressure dependence up to the transition from the body centered cubic (alpha) to hexagonal close packed (epsilon) phase transition, where a discontinuous change in both the slope and magnitude of the reflectivity was observed. Reflectance spectra were corrected for diamond absorption and treated with a Kramers-Kronig analysis to extract the optical constants; the emissivity of iron was derived from Kirchoff's law. The pressure and wavelength dependence of the emissivity is characterized by an empirical function for 1.5-1.9 microm; this wavelength range is useful for spectroradiometric temperature measurements from 1000 K up to approximately 2500 K. Alpha-Fe is a nonideal emitter; however, epsilon-Fe behaves as an almost perfect greybody in the infrared up to the highest pressures of the measurements. Temperature measurements based on the spectroradiometry of iron samples should take into account the wavelength dependent emissivity below the alpha-epsilon phase transition at approximately 13 GPa.