Figure 1. Orbital engineering to realize threefold degenerate p orbitals in layered CaAl₂Si₂-type Zintl compounds.

(a) Crystal structure and electronic bands of CaAl₂Si₂-type Zintl compounds. Nondegenerate band Γ(p_z) and doubly degenerate band Γ(p_x,y) are mainly composed of p_z and p_x,y orbitals from anions, respectively. Δ is the crystal field splitting energy between p_x,y and p_z orbitals at the Γ point. (b) Orbital-projected band structure of representative CaAl₂Si₂-type Zintl compound Mg₃Sb₂ with negative Δ value. p_x, p_y and p_z orbitals of Sb anions are projected on the band structure. Curve width indicates the relative weight of the component. Two representative compounds Mg₃Sb₂ (Δ<0) and SrZn₂Sb₂ (Δ>0, Supplementary Fig. 2) are used to demonstrate the p orbital characteristics of CaAl₂Si₂-type Zintl materials.