  MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topological insulators,
offering a promising materials platform for realizing exotic topological
quantum states. However, high densities of intrinsic defects in these materials
not only cause bulk metallic conductivity, preventing the measurement of
quantum transport in surface states, but may also affect magnetism and
topological properties. In this paper, we show by density functional theory
calculations that the strain induced by the internal heterostructure promotes
the formation of large-size-mismatched antisite defect BiMn in MnBi2Te4; such
strain is further enhanced in MnBi4Te7, giving rise to even higher BiMn
density. The abundance of intrinsic BiMn donors results in degenerate n-type
conductivity under the Te-poor growth condition. Our calculations suggest that
growths in a Te-rich condition can lower the Fermi level, which is supported by
our transport measurements. We further show that the internal strain can also
enable efficient doping by large-size-mismatched substitutional NaMn acceptors,
which can compensate BiMn donors and lower the Fermi level. Na doping may pin
the Fermi level inside the bulk band gap even at the Te-poor limit in MnBi2Te4.
Furthermore, facile defect formation in MnSb2Te4 and its implication in Sb
doping in MnBi2Te4 as well as the defect segregation in MnBi4Te7 are discussed.
The defect engineering and doping strategies proposed in this paper will
stimulate further studies for improving synthesis and for manipulating magnetic
and topological properties in MnBi2Te4, MnBi4Te7, and related compounds.
