Thermodynamic origin of medium-entropy stabilization in multicomponent rock-salt oxides

Supriya Ghosal; Swapan Pati; Ashutosh Kumar

paper

Thermodynamic origin of medium-entropy stabilization in multicomponent rock-salt oxides

arXiv:2606.00753

Abstract

High entropy oxides are commonly associated with high configurational entropy ( $I^ S_{co n f} \geq$ 1.61R) corresponding to five equimolar cations occupying a crystallographic sublattice. However, recent experimental observations indicate that medium-entropy compositions may also exhibit entropy-stabilized rock-salt phases, raising an important question regarding the minimum entropy required for phase stabilization. In this work, we employ a first-principles thermodynamic framework to investigate the stability of rock-salt oxides containing two to five principal cations components analogous to (Ni $_{0.8}$ Cu $_{0.2}$ )O, (Ni $_{0.6}$ Cu $_{0.2}$ Zn $_{0.2}$ )O, (Ni $_{0.4}$ Cu $_{0.2}$ Zn $_{0.2}$ Co $_{0.2}$ )O, (Ni $_{0.2}$ Cu $_{0.2}$ Zn $_{0.2}$ Co $_{0.2}$ Mg $_{0.2}$ )O. Density functional theory, MCSQS-based structural modeling, and finite-temperature Gibbs free-energy analysis are combined to quantify the roles of enthalpy mixing ( $I^ H_{mi x}$ ), configurational ( $I^ S_{co n f}$ ), vibrational ( $I^ S_{v ib}$ ), and electronic contributions towards ( $I^ S_{e l ec}$ ) entropy change in governing phase stability. The results show that $I^ S_{co n f}$ alone is not a universal descriptor of phase stability. While the two-cation system is enthalpy-stabilized but three-, four- and five-cation systems become thermodynamically stable at high-temperature due to entropy-driven reduction of the Gibbs free energy. These findings demonstrate that single-phase rock-salt oxides are not restricted to the conventional high-entropy limit and that medium-entropy compositions can also be stabilized under suitable thermodynamic conditions.

10 Pages, 5 figures