LAUSR

Abstract A stoichiometric CaGe2 sample was synthesized using high-purity Ca and Ge pieces, and its crystal structure was characterized by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray diffraction. HAADF-STEM clearly indicated the presence of three different polymorphs of CaGe2, viz., 3R-, 4H-, and 6R-CaGe2. In particular, the 4H structure appeared in Zintl phases. 3R-CaGe2 has a trigonal rhombohedral crystal structure, which belongs to the R 3 ¯ m space group and consists of an ABC stacking sequence. 4H-CaGe2 has a hexagonal crystal structure with the P 3 ¯ m1 space group, in which Ge layers follow an A’ABC stacking sequence. Density functional theory calculations suggest that the CaGe2 polymorphs have similar electronic structures, with a covalent interaction between the Ca d and Ge p states, and with an ionic interaction between the Ca s, p states and Ge s, p states. Thermodynamic calculations indicate that the most stable phases are 3R-CaGe2 and 6R-CaGe2, below and above 450 K, respectively.