Preparation and Crystal Structure of the Clathrate-I Cs8-xGe44+y□2-y

The clathrate‐I phase Cs8–xGe44+y□2–y (space group Pm$\bar{3}$n) was prepared by high‐pressure high‐temperature reactions of Cs4Ge4 and α‐Ge. Different reaction conditions were found to have a strong influence on the lattice parameter of the clathrate‐I phase ranging from 10.8070(2) Å to 10.8493(3) Å. A single crystal with composition Cs8Ge44.40(2)□1.60(2) was obtained from a sample with a = 10.8238(2) Å (niobium ampoule, p = 3.4 GPa, Tmax = 1400 °C). Structure analysis based on X‐ray single crystal data shows unambiguously an excess of germanium atoms with respect to the electron balanced composition Cs8Ge44□2 on basis of the Zintl concept.