Revisiting the topological properties of XMg2Bi2 (X = Ca, Sr, Ba, Yb and Eu)
Density functional theory is known to underestimate band gaps in semiconductors and to over- estimate inverted band gaps, frequently exaggerating the predicted size of the topological phase diagram of materials. Employing hybrid functionals and calculating the topological invariants, we revisit the topological properties of compounds crystallizing in the CaAl2Si2-type and demonstrate that the overestimation of the inverted band gaps is particularly pronounced in compounds with this crystal structure. Among these, the class of XMg2Bi2 materials (X = Ca, Sr, Ba, Yb, and Eu) is topologically trivial for all considered cations. Our calculations show that these materi- als are narrow-gap semiconductors with direct band gaps of 0.24-0.34 eV, slightly decreasing with increasing the atomic weight of the element X. We confirm this by applying uniaxial strain and hydrostatic pressure, confirming these results. We emphasize that the experimental observation of surface states alone is insufficient to establish nontrivial topology, as trivial semiconductors may host surface states without a surface Dirac point. Consequently, since these materials are intrinsi- cally topologically trivial, any experimentally observed topological signatures should be attributed to extrinsic effects such as doping or surface reconstruction. Our results underscore the importance of accurately treating electronic correlations when assessing topological character, even in materials containing heavy elements with strong spin-orbit coupling, such as bismuth.