Laser heating at megabar pressures: Melting temperatures of iron and other transition metals
The melting temperature of iron at high pressure is key to deriving the temperature in the Earth’s interior because theboundary between the solid inner core and the liquid outer core at about 3.3 Mbar is due to the melting (or freezing) of an iron-rich alloy. Data at core pressures measured in the laser-heated diamond cell (up to 200 GPa) have been reported over ten years ago (Boehler, 1993). In the last few years the experimental data obtained with this technique have converged, but there is still considerable disagreement between shock data, theory and diamond cell measurements. Thispaper will provide some of the latest dateon the phase diagram of iron and compare its melting curve with some other transition metals. This comparison is useful to understand systematic behaviour in transition metal melting.
What is the melting temperature of iron at 3.3 Mbar? This question has addressed by many researchers over the past 20 years and the answers reach from 2000 to 10000 K. Figure 1 represents the latest solutions obtained from shock experiments, diamond cell experiments and from theory, and clearly indicates the difficulties associated with the question.
The experimental techniques for generating high temperatures in the laser-heated diamond cell, pressure and temperature measurements are described in Chapter 9 in this volume (Boehler, 2005). Melting can be detected in situ for most materials by several methods: 1) by measuring discontinuous changes of the absorption of the laser radiation, 2) from changes in the reflectivity of the sample at a wavelength different from that of the heating laser, and 3) direct visual observation of melting on the sample surface.