Increased Defect Resistance and Ordering in MnBi₂(Se_1–xTe_x)₄via Accurate Diffusion Monte Carlo

Stabilizing materials and controlling defect formation remain key challenges in materials science, particularly for theory, where small energy differences must be resolved for accurate predictions. We applied state-of-the-art theoretical methods to topological materials, focusing on MnBi₂Te₄(MBT), which is a promising intrinsic magnetic topological insulator. Antisite defects in MBT alter its electronic structure and magnetism, degrading topological properties and causing experimental inconsistencies. Using diffusion Monte Carlo and density functional theory, we investigated the thermodynamic stability and defect formation in MBT, MnBi₂Se₄(MBS), and MnBi₂(Se_1–xTe_x)₄. We found that MnBi₂Se₂Te₂can be stable at finite temperatures, with higher defect formation energies due to stronger Mn–Se bonding and reduced internal strain. Se preferentially substitutes Te near Mn instead of Te in the outer layer, encouraging long-range ordering when incorporated. For MnBi₂(Se_1–xTe_x)₄, cluster expansion phase diagrams revealed solid solution behavior when x <0.5 and phase separation for larger x. MBT and MBS are topological insulators; therefore, the MnBi₂(Se_1–xTe_x)₄family could offer tunable topological behavior and improved stability.