Prediction of topological Dirac semimetal in Ca-based Zintl layered compounds CaM₂X₂ (M = Zn or Cd; X = N, P, As, Sb, or Bi)

Topological Dirac materials are attracting a lot of attention because they offer exotic physical phenomena. An exhaustive search coupled with first-principles calculations was implemented to investigate 10 Zintl compounds with a chemical formula of CaM₂X₂ (M = Zn or Cd, X = N, P, As, Sb, or Bi) under three crystal structures: CaAl₂Si₂-, ThCr₂Si₂-, and BaCu₂S₂-type crystal phases. All of the materials were found to energetically prefer the CaAl₂Si₂-type structure based on total ground state energy calculations. Symmetry-based indicators are used to evaluate their topological properties. Interestingly, we found that CaM₂Bi₂ (M = Zn or Cd) are topological crystalline insulators. Further calculations under the hybrid functional approach and analysis using k · p model reveal that they exhibit topological Dirac semimetal (TDSM) states, where the four-fold degenerate Dirac points are located along the high symmetry line in-between Г to A points. These findings are verified through Green's function surface state calculations under HSE06. Finally, phonon spectra calculations revealed that CaCd₂Bi₂ is thermodynamically stable. The Zintl phase of AM₂X₂ compounds have not been identified in any topological material databases, thus can be a new playground in the search for new topological materials.