Electronic-entropy-driven phase transitions in compressed iron oxides

Electronic entropy is usually treated as a secondary correction to structural stability, but under strong electronic excitation it can become a primary thermodynamic driving force. Here, we show that electronic entropy can drive both polymorphic and stoichiometric phase transformations in compressed iron oxides. Using finite-temperature density functional theory, we calculate the electronic-temperature-dependent Gibbs free energies of Fe2O, FeO, Fe4O5, Fe3O4, and multiple Fe2O3 polymorphs, including α-, ι-, ζ-, η-, and θ-Fe2O3, over the pressure range 60–260 GPa. At 60–140 GPa, electronic excitation mainly reorganizes the relative stability of Fe2O3 polymorphs, driving transitions from ι-Fe2O3 to η-Fe2O3. At 180 GPa, the free-energy landscape becomes strongly competitive as FeO is stabilized over an intermediate range of electronic temperature, while η-Fe2O3 becomes favorable at higher T. At 220–260 GPa, the lowest-free-energy phase at low T is the Fe-rich compound Fe2O, but increasing electronic temperature stabilizes FeO. These results demonstrate that electronic entropy can control not only the relative stability of crystal structures at fixed composition, but also the competition between different iron-oxide stoichiometries. The predicted electronic-entropy-driven phase boundaries provide a route to nonthermal structural transformations in ultrafast and high-energy-density experiments.