Tricritical Points and Liquid-Solid Critical Lines

TL;DR

This paper discusses tricritical points as a tool to understand liquid-solid phase transitions, enabling predictions of Earth's core conditions and effects of impurities on transition temperatures.

Contribution

It introduces a method using tricritical points to extrapolate liquid-solid phase boundaries to extreme pressures and temperatures, including Earth's core conditions.

Findings

Predicted Earth's core liquid-solid boundary temperature at 329 GPa.

Impurities reduce the core boundary temperature by approximately 600 K.

Validated phase transition models with high-pressure experimental data.

Abstract

Tricritical points separate continuous and discontinuous symmetry breaking transitions. They occur in a variety of physical systems and their mathematical models. A tricritical point is used to determine a liquid-solid phase transition line in the pressure-temperature plane [Aitta, J. Stat. Mech., 2006]. Excellent experimental agreement has been obtained for iron, the material having the most high pressure data. This allows extrapolation to much higher pressures and temperatures than available experimentally. One can predict the temperature at the liquid-solid boundary in the core of the Earth where the pressure is 329 GPa. Light matter, present as impurities in the core fluid, is found to generate about a 600 K reduction of this temperature.