Phase Stability of Dynamically Disordered Solids from First Principles

The study of phase stability in solid materials containing dynamic disorder, eg. solids with rotating molecular units, or superionic conductors, is a challenging problem in theoretical materials science. This is mainly due to the failure of the standard picture of atoms vibrating around fixed equilibrium positions, which makes theoretical phonon schemes inapplicable. Superionic conductors are dynamically disordered solid materials with exceptionally high rates of ionic conductivity, which makes them very promising solid electrolytes for fuel cells and solid-state batteries.

Here, we present a method to study the phase stability of dynamically disordered materials [1]. The method is based on a stress-strain thermodynamic integration on a deformation path that connects the dynamically disordered phase to a stable variant. We apply the method to study the phase stability of superionic Bi₂O₃. The phase transformation from the low temperature ground state α-phase to the heavily disordered superionic δ-phase is well reproduced, with the critical temperature and the (very large) transition enthalpy closely matching the experimental values.

[1] J. Klarbring and S.I. Simak, “Phase Stability of Dynamically Disordered Solids from First Principles” Phys. Rev. Lett. (2018), in press