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

Abstract

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.

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

“…On the other hand, CaZn 2 Bi 2 in CaAl 2 Si 2 -type exhibits imaginary phonon frequencies and is thermodynamically unstable, implying it may adopt another stable structure. Interestingly, the monolayer CaZn 2 Bi 2 is stable, as demonstrated in another study 56 . Focusing on CaCd 2 Bi 2 , Figure S5 shows that CaCd 2 Bi 2 is thermodynamically stable, implying possible experimental synthesis.…”

High Thermoelectric Performance in 2D Technetium Dichalcogenides TcX2 (X = S, Se, or Te)

“…In the past decades, with the development of theoretical simulation and synthesis methods, some 2D materials have been successfully studied both theoretically and experimentally . These materials are black phosphorus, transition-metal dichalcogenides (TMDs), − Janus TMDs, − group IVA–VA compounds, MXenes, , and Zintl compounds. − These 2D materials exhibit various outstanding properties. Thus, they have been successfully used in electronic, optoelectronic, and TE applications. , Also, these materials can be experimentally synthesized using various growth methods, such as epitaxial growth, chemical vapor deposition, mechanical exfoliation, and micromechanical cleavage .…”

The Zintl phase compounds AEIn2As2 (AE = Ca, Sr, Ba): topological phase transition under pressure

“…26 The AM 2 X 2 (A = Ca, Sr, or Ba; M = Zn or Cd; X = Sb or Bi) series of materials were confirmed as topological insulators with band inversion and the coexistence of insulating topological state and the Rashba effect in the BaCd 2 SbBi film was achieved. 27 Inspired by SrX 2 As 2 (X = Cd, Sn) non-magnetic materials, we propose the new idea that AEIn 2 As 2 (AE = Ca, Sr, Ba) topological insulators may also undergo topological quantum phase transitions or metal–insulator phase transitions. A recent report confirmed our conjecture, which shows that BaIn 2 As 2 is a typical degenerate p-type semiconductor, but it does not attain the electrical and transport properties of CaIn 2 As 2 and SrIn 2 As 2 .…”