Band topology and phase stability of half-Heusler semiconductors with d0 alkaline earth atoms

Abstract

In the previous studies, the topological insulators in the half-Heusler ternary structure are assumed to be in the γ-phase having the transition metal atom at the unique middle site. In contrast, the minimum-energy half-Heusler phase can well be the β-phase with the highly electronegative anion positioned in between the electropositive metallic ions. For this subtle reason in the following a systematic study of the phase stability and the band topology of XAuZ compounds containing d0 alkaline earth atoms (X = Ca, Sr, Ba; Z = As, Sb) in their respective minimum-energy half-Heusler phases is presented. It is thus found that all the above compounds are mechanically (or dynamically) stable and have formation energies that are within a reasonable distance from the thermodynamic hull. The band structures of all compounds obtained from the more accurate TB-mBJ exchange potential with the SOC included indicate a strong band inversion such that the s-like Γ6 band falls below the p-like (Γ7+Γ8) bands. Furthermore, due to the negative spin–orbit splitting in the β-phase compounds SrAuAs and BaAuAs, the Γ8 falls below the Γ7, which opens a significant bulk insulating gap at the Fermi level. The outputs of WannierTools, such as the Z2 topological quantum numbers, surface states spectra and surface Fermi loops, indicate that the β-phase compounds SrAuAs and BaAuAs are indeed strong topological insulators with a polar character due to the lack of inversion symmetry, while non-uniform strains are needed to turn the γ-phase compounds into Weyl semimetals (ca>1) and topological insulators (ca<1).