Crystal structure prediction at finite temperatures

TL;DR

This paper introduces a new computational method for predicting crystal structures at finite temperatures, enabling more accurate phase diagram predictions by incorporating entropy effects through molecular dynamics and thermodynamic corrections.

Contribution

The authors develop an efficient approach combining molecular dynamics with force fields and thermodynamic perturbation theory to accurately predict crystal structures at finite temperatures.

Findings

Wider stability field of hcp-phase of aluminum than previously thought

Transition temperature for WB's $eta$-phase at 2789 K

Clapeyron slope of MgSiO3 phase transition is 5.88 MPa/K

Abstract

Crystal structure prediction is a central problem of theoretical crystallography and materials science, which until mid-2000s was considered intractable. Several methods, based on either energy landscape exploration$^{1,2}$ or, more commonly, global optimization$^{3-8}$, largely solved this problem and enabled fully non-empirical computational materials discovery$^{9,10}$. A major shortcoming is that, to avoid expensive calculations of the entropy, crystal structure prediction was done at zero Kelvin and searched for the global minimum of the enthalpy, rather than free energy. As a consequence, high-temperature phases (especially those which are not quenchable to zero temperature) could be missed. Here we develop an accurate and affordable solution, enabling crystal structure prediction at finite temperatures. Structure relaxation and fully anharmonic free energy calculations are done by molecular dynamics with a force field (which can be anything from a parametric force field for simpler cases to a trained on-the-fly machine learning interatomic potential), the errors of which are corrected using thermodynamic perturbation theory to yield accurate ab initio results. We test the accuracy of this method on metals (probing the P-T phase diagram of Al and Fe), a refractory intermetallide (WB), and a significantly ionic ceramic compound (Earth-forming silicate MgSiO3 at pressures and temperatures of the Earth's lower mantle). We find that the hcp-phase of aluminum has a wider stability field than previously thought, and the temperature-induced transition $\alpha$-$\beta$ in WB occurs at 2789 K. It is also found that iron has hcp structure at conditions of the Earth's inner core, and the much debated (and important for constraining Earth's thermal structure) Clapeyron slope of the post-perovskite phase transition in MgSiO3 is 5.88 MPa/K.