\emph{Ab initio} calculations of structural stability, thermodynamic and elastic properties of Ni, Pd, Rh, and Ir at high pressures

TL;DR

This study uses first-principles calculations to analyze the structural, thermodynamic, and elastic properties of Ni, Pd, Rh, and Ir under high pressures, revealing phase stability and melting points relevant to Earth's core conditions.

Contribution

It provides new insights into the high-pressure phase stability and thermodynamic properties of Ni, Pd, Rh, and Ir using ab initio methods, including phase diagrams and melting estimations.

Findings

Ni and Pd favor bcc structure above 0.14 TPa and 4 kK.

Rh and Ir show no structural change up to 1 TPa.

Estimated melting points under shock compression.

Abstract

The paper presents results of a comprehensive study from first principles into the properties of Ni, Pd, Rh, and Ir crystals under pressure. We calculated elastic constants, phonon spectra, isotherms, Hugoniots, sound velocities, relative structural stability, and phase diagrams. It is shown that in nickel and palladium under high pressures ($>$0.14 TPa) and temperatures ($>$4 kK), the body-centered cubic structure is thermodynamically most stable instead of the face-centered cubic one. Calculated results suggest that nickel under Earth-core conditions ($P$$\sim$0.3 TPa, $T$$\sim$6 kK) have a bcc structure. No structural changes were found to occur in Rh and Ir under pressures to 1 TPa at least. The paper also provides estimations for the pressure and temperature at which the metals of interest begin to melt under shock compression.