Irremovable Mn-Bi Site Mixing in MnBi₂Te₄

MnBi2Te4, an antiferromagnetictopologicalinsulator, was theoretically predicted to have a gapped surface stateon its (111) surface. However, a much smaller gapped or even gaplesssurface state has been observed experimentally, which is thought tobe caused by the defects in MnBi2Te4. Here,we have theoretically identified the antisite Mn-Bi andBi(Mn) as dominant defects and revealed their evolution duringthe phase transition from MnTe/Bi2Te3 to MnBi2Te4. We found that the complete elimination ofMn(Bi) and Bi-Mn defects in MnBi2Te4 by simple annealing is almost impossible due to the highmigration barrier in kinetics. Moreover, the gap of the Dirac point-relatedbands in a MnBi2Te4 monolayer would be eliminatedwith an increasing concentration of Mn-Bi and Bi-Mn defects, which could explain the experimentally unobserved large-gapsurface state in MnBi2Te4. Our results providean insight into the theoretical understanding of the quality and theexperimentally measured topological properties of the synthesizedMnBi(2)Te(4).