Prediction of the Pressure-Resistant Hardest AlFeO3

Perovskite iron-oxide AlFeO₃ is an environmentally friendly (lead-free) multiferroic material that simultaneously exhibits ferromagnetism and the potential magnetoelectric coupling, making it ideal for next-generation spintronic devices. Recently, high pressure has emerged as an effective approach in achieving superior physical properties of perovskite structures. Here, we systematically study the phase diagram and physical properties of AlFeO₃ under pressure. Using an ab initio evolutionary algorithm, we explore all of the potential stable phases of bulk AlFeO₃ between 0 and 50 GPa, and we successfully discover two new stable phases (P2₁/c and R32). We identify three pressure-induced phase transition sequences 𝑃𝑛𝑎21‐A‐AFM−→−3GPa𝑃21/𝑐‐G‐AFM−→−−15GPa𝑃21/𝑐‐FM$Pna{2}_1\mathrm{‐A‐AFM}\stackrel{3\mathrm{GPa}}{\to }P{2}_1/c\mathrm{‐G‐AFM}\stackrel{15\mathrm{GPa}}{\to }P{2}_1/c\mathrm{‐FM}$$Pna{2}_1\mathrm{‐A‐AFM}\stackrel{3\mathrm{GPa}}{\to }P{2}_1/c\mathrm{‐G‐AFM}\stackrel{15\mathrm{GPa}}{\to }P{2}_1/c\mathrm{‐FM}$ from 0 to 50 GPa, providing a comprehensive pressure-induced phase diagram. It is found that the coplanar Fe–O octahedron configuration and ferromagnetic configuration can effectively enhance the structural stability of AlFeO₃ under high pressure. Moreover, the stable new phase (P2₁/c-AlFeO₃) exhibits superior physical properties. It is the hardest AlFeO₃, with the largest shear modulus, bulk modulus, and Young’s modulus among all phases. Additionally, AFM and FM configurations of this phase could maintain stable semiconductor performance in their respective pressure ranges. This study not only resolves the longstanding issue of the pressure-induced phase transition sequence in the AlFeO₃ system, but also discovers the hardest P2₁/c-AlFeO₃ with a stable electronic structure. These findings offer critical scientific insights that may guide future applications of AlFeO₃ in high-performance materials and devices.