An \emph{ab initio} study of structural phase transitions of crystalline aluminum under ultrahigh pressures based on ensemble theory

TL;DR

This study uses ensemble theory and ab initio calculations to accurately predict the pressure-induced phase transitions of crystalline aluminum up to 600 GPa at room temperature, revealing thermal effects on stability.

Contribution

It introduces a novel ensemble theory-based approach to determine phase transitions with ab initio precision, including thermal effects at room temperature.

Findings

Transition pressures at 194 GPa and 361 GPa for FCC to HCP and HCP to BCC phases.

Axial ratio of HCP structure is 1.62.

Volume changes of -0.67% and -0.90% at transitions, matching recent experiments.

Abstract

It is a long-time pursuit of computations with \emph{ab initio} precision of thermal contributions to phase behaviors of condensed matters under extreme conditions. In this work, the pressure induced structural phase transitions of crystalline aluminum up to $600$ GPa at room temperature are investigated based on the criterion of Gibbs free energy derived directly from the partition function that formulated in the ensemble theory with the interatomic interactions characterized by density functional theory computations. The transition pressures of the FCC$\rightarrow$HCP$\rightarrow$BCC phase transitions are determined at $194$ and $361$ GPa, the axial ratio of the stable HCP structure is found to be equal to $1.62$ and the discontinuities in the equations of states are confirmed to be associated with $-0.67\%$ and $-0.90\%$ volume changes, which are all in an excellent agreement with the measurements by one of the recent experiments but differ from other experimental observations. Compared with the results obtained by the criterion of enthalpy at $0$K, this work further shows the nontrivial thermal impacts on the structural stability of aluminum under ultrahigh-pressure circumstances even at room temperature.