Investigation of (001), (010), and (100) surface termination and surface energies of the Zintl Ca5Ga2Sb6

Abstract

Zintl Phases are a class of intermetallic compounds that combine ionic and covalent bonding to form charge-balanced structures, leading to semiconducting properties. The Ca5M2Sb6 family of Zintl compounds, (M =Al, Ga, In), have demonstrated promising thermoelectric efficiency, meaning they can be used to convert a temperature gradient into useful electrical energy. Ca5M2Sb6 compounds crystallize in a highly anisotropic structure comprised of chains of corner-linked MSb4 tetrahedra, in which each neighboring chain is joined via Sb-Sb covalent bonds, forming an infinite polyanionic “ladder”. In the present study, we used first principles calculations to study various surface terminations of the Ca5Ga2Sb6 structure as a mechanism to understand its crystal growth morphology. Our results indicate that the calcium stoichiometry of the crystal surface plays an important role in surface reconstruction, specifically leading to the formation or breaking of covalent bonds. Our results suggest that surface energy as a function of stoichiometry and crystallographic orientation is only one piece of the puzzle in understanding the growth habits of Ca5Ga2Sb6 crystals.