Map of the Zintl AM₂Pn₂ Compounds: Influence of Chemistry on Stability and Electronic Structure

The AM₂Pn₂ (A= Ca, Sr, Ba, Yb, Mg; M = Zn, Cd, Mg; and Pn = N, P, As, Sb, Bi) family of Zintl phases has been known as thermoelectric materials and has recently gained much attention for highly promising materials for solar absorbers in single-junction and tandem solar cells. In this paper, we will, from first principles, explore the entire family of AM₂Pn₂ compounds in terms of their ground-state structure, thermodynamic stability, and electronic structure. We also perform photoluminescence spectroscopy on bulk powder and thin film samples to verify our results, including the first measurements of the band gaps of SrCd₂P₂ and CaCd₂P₂. The AM₂Pn₂ compounds exhibit broad stability, are mostly isostructural to CaAl₂Si₂ (P3̅m1), and cover a wide range of band gaps from 0 to beyond 3 eV. This could make them useful for a variety of purposes, for which we propose several candidates, such as CaZn₂N₂ for tandem top cell solar absorbers and SrCd₂Sb₂ and CaZn₂Sb₂ for infrared detectors. By examining the band structures of the AM₂Pn₂, we find that Mg₃Sb₂ has the most promise as a thermoelectric material due to several off-Γ valence band pockets, which are unique to it among the compositions studied here.