Quantum spin Hall insulating phase and van Hove singularities in Zintl single-quintuple-layer AM₂X₂ (A = Ca, Sr, or Ba; M = Zn or Cd; X = Sb or Bi) family

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). Phonon dispersion computations support the thermodynamic stability of all the investigated compounds. Nontrivial topological properties are determined via 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-(p_x + p_y) orbitals are found in CaZn₂Bi₂, SrZn₂Bi₂, BaZn₂Bi₂, CaCd₂Bi₂, SrCd₂Bi₂, and BaCd₂Bi₂ with relatively large bandgaps. 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.