Extraordinary Cation-Replace-Cation Antisite Defects Predominate in α-Phase Bi2SeO5

Abstract As a newly identified single-crystalline van der Waals dielectric with a high dielectric constant, α-phase Bi2SeO5 (α-BSO) plays a pivotal role in advancing 2D electronic devices. In this work, we systematically investigate the defect properties of α-BSO using first-principles calculations based on a hybrid functional. Although α-BSO is a chemically ternary compound, each constituent element occupies several crystallographically nonequivalent sites, rendering its defect chemistry highly complex. Due to the anomalous +4 cationic valence state of Se, the defect formation energies of same main group anion antisite defects (SeO and OSe) are prohibitively high, and their concentrations can therefore be neglected. In contrast, the extraordinary cation–cation antisite defects BiSe and SeBi emerge as the dominant defects. The pronounced variability in the formation energies of the six types of VO defects demonstrates that identical defect types located on nonequivalent atomic sites can exhibit markedly different properties. Under O-rich and Se/Bi-poor conditions, α-BSO shows relatively good p-type behavior. Conversely, under O-poor and Se/Bi-rich conditions, or at intermediate O, Se, and Bi partial pressures, α-BSO behaves as an intrinsic semiconductor or displays very weak n-type conductivity due to strong donor–acceptor compensation. This study provides theoretical insights to guide the design and development of high-performance α-BSO-based electronic devices.