Ca₂Ga₄Ge₆ and Ca₃Ga₄Ge₆: Synthesis, Structure, and Electronic Properties

Abstract

Dedicated to Mercouri Kanatzidis in celebration of his 65 th birthday. During the search for transition metal-free alkyne hydrogenation catalysts, two new ternary CaÀ GaÀ Ge phases, Ca 2 Ga 4 Ge 6 (Cmc2 1 , a = 4.1600(10) Å, b = 23.283(5) Å, c = 10.789(3) Å) and Ca 3 Ga 4 Ge 6 (C2/m, a = 24.063(2) Å, b = 4.1987(4) Å, c = 10.9794(9) Å, β = 91.409( 4)°), were discovered. These compounds are isostructural to the previously established Yb 2 Ga 4 Ge 6 and Yb 3 Ga 4 Ge 6 analogues, and according to Zintl-Klemm counting rules, consist of anionic [Ga 4 Ge 6 ] 4À and [Ga 4 Ge 6 ] 6À frameworks in which every Ga and Ge atom would have a formal octet with no GaÀ Ga or GaÀ Ge π-bonding. These compounds are metallic, based on temperature dependent electrical resistivity and thermopower measurements for Ca 3 Ga 4 Ge 6 , along with density functional theory calculations for both phases. Unlike the highly active 13-layer trigonal CaGaGe phase, these new compounds exhibit minimal activity in the semi/full alkyne hydrogenation of phenylacetylene, which is consistent with previous observations that the lack of a formal octet for framework atoms is essential for catalysis in these Zintl-Klemm compounds.