Thallium under extreme compression

TL;DR

This study combines experimental and theoretical approaches to investigate thallium's high-pressure phases, confirming known transitions and predicting stability of structures up to terapascal pressures.

Contribution

It provides new high-pressure experimental data and ab initio calculations that extend understanding of thallium's structural stability to extremely high pressures.

Findings

Confirmed hcp-fcc transition at 3.5 GPa

Fcc structure remains stable up to the highest pressures studied

Predicted fcc stability up to 4.3 TPa and a new orthorhombic phase above that

Abstract

We present a combined theoretical and experimental study of the high-pressure behavior of thallium. X-ray diffraction experiments have been carried out at room temperature up to 125 GPa using diamond-anvil cells, nearly doubling the pressure range of previous experiments. We have confirmed the hcp-fcc transition at 3.5 GPa and determined that the fcc structure remains stable up to the highest pressure attained in the experiments. In addition, HP-HT experiments have been performed up to 8 GPa and 700 K by using a combination of x-ray diffraction and a resistively heated diamond-anvil cell. Information on the phase boundaries is obtained, as well as crystallographic information on the HT bcc phase. The equation of state for different phases is reported. Ab initio calculations have also been carried out considering several potential high-pressure structures. They are consistent with the experimental results and predict that, among the structures considered in the calculations, the fcc structure of thallium is stable up to 4.3 TPa. Calculations also predict the post-fcc phase to have a close-packed orthorhombic structure above 4.3 TPa.