Recent experiments on bulk Zintl CaAl2Si2 reveal the presence of nontrivial topological states. However, the large family of two-dimensional (2D) Zintl materials remains unexplored. Using first-principles calculations, we discuss the stability and topological electronic structures of 12 Zintl single-quintuple-layer (1-QL) AM2X2 compounds in the CaAl2Si2-structure (A = Ca, Sr, or Ba; M = Zn or Cd; and X = Sb or Bi). Considering various layer-stackings, we show that the M-X-A-X-M stacking, where the transition metal M is exposed, is energetically most favorable. Phonon dispersion computations support the thermodynamic stability of all the investigated compounds. Nontrivial topological properties are ascertained through the calculation of Z2 invariants and edge states using the hybrid functional. Insulating topological phases driven by a band inversion at the Γ-point involving Bi-(px + py) orbitals are found in CaZn2Bi2, SrZn2Bi2, BaZn2Bi2, CaCd2Bi2, SrCd2Bi2, and BaCd2Bi2 with bandgaps (eV) of 0.571, 0.500, 0.025, 0.774, 0.650, and 0.655, respectively. Interestingly, van Hove singularities are found in CaCd2Bi2 and BaCd2Bi2, implying the possibility of coexisting insulating and superconducting topological phases. We discuss how topological 1-QL Zintl compounds could be synthesized through atomic substitutions resulting in Janus materials (1-QL AM2XY). In particular, the thermodynamically stable Janus BaCd2SbBi film is shown to exhibit both an insulating topological state and the Rashba effect. Our study identifies a new family of materials for developing 2D topological materials platforms and paves the way for the discovery of 2D topological superconductors.

A family of two-dimensional (2D) materials, particularly the complex-structured Zintl phase compounds, has attracted tremendous research attention because of tunable material properties and exceptional applications. Utilizing first-principles computations under the hybrid functional approach, we performed a systematic study on A2MX2 (A = Ca, Sr, or Yb; M = Zn or Cd; X = P, As, Sb, or Bi). Among the three surface terminations considered, the metal element XM-termination (T2) was found to be the most stable structural phase with the lowest total ground state energies. Thermodynamic stability was further confirmed through phonon dispersion and formation energy calculations. Surprisingly, the T2 monolayer Sr2CdBi2 was found to host the topological insulating phase with a sizable bandgap of 556 meV, as well as a Z2 topological invariance of 1. The corresponding topologically protected edge states link the conduction and valence bands. Moreover, the topological phase is driven by the spin–orbit coupling, which causes the inverted bands at Γ point close to the Fermi level concerning the Cd- s + Bi2- s and Bi2- px + py orbitals. Moreover, Rashba and chiral spin-splittings were also observed. The computed Rashba strengths along Γ-M (αR Γ‑M) are 1.970, 0.676, and 0.669 eV·Å, whereas the computed values along Γ-K (αR Γ‑K) are 1.701, 0.731, and 0.646 eV·Å, for Ca2ZnAs2, Sr2CdBi2, and Yb2ZnAs2, respectively. Our study provides fundamental knowledge to further experimental investigations and synthesis, which may lead to electronic applications of A2MX2 compounds in quantum computing or spintronics.