Synthesis, structure and electronic structure of a new polymorph of CaGe2

Reported are the flux synthesis, the crystal structure determination, the properties and the band structure calculations of a new polymorph of CaGe2, which crystallizes with the hexagonal space group P63mc (no. 186) with cell parameters of a=3.9966(9) and c=10.211(4) Å (Z=2; Pearson's code hP  6). The structure can be viewed as puckered layers of three-bonded germanium atoms, ∞2[Ge2]2-, which are stacked along the direction of the c-axis in an ABAB-fashion. The germanium polyanionic layers are separated by the Ca cations. As such, this structure is closely related to the structure of the other CaGe2 polymorph, which crystallizes with the rhombohedral CaSi2 type in the R3¯m   space group (No. 166), where the ∞2[Ge2]2- layers are arranged in an AA′BB′CC′-fashion, and are also interspaced by Ca2+ cations. LMTO calculations suggest that in spite of the formal closed-shell configuration for all atoms and the apparent adherence to the Zintl rules for electron counting, i.e., Ca2+[3b-Ge1−]2), the phase will be a poor metal due to a small Ca-3d–Ge-4p band overlap. Magnetic susceptibility measurements as a function of the temperature indicate that the new CaGe2 polymorph exhibits weak, temperature independent, Pauli-paramagnetism.

Graphical abstractReported are the synthesis, the structure and the band structure analysis of a new polymorph of CaGe2 (α-CaGe2). The similarities and the differences between this compound and the known rhombohedral β-CaGe2 are discussed in detail. LMTO calculations suggest that in spite of the apparent adherence to the Zintl rules for electron counting, α-CaGe2 is metallic due to a small cation–anion orbital mixing.Figure optionsDownload full-size imageDownload as PowerPoint slide