The 2a₀ x 2a₀ x 2a₀ Superstructure in the Clathrate-I K₈Li_xGe_44-x/4□_2-3x/4

In the crystal structure of the clathrate-I K8LixGe44-x/4 square(2-3x/4), the K atoms fill the polyhedral cages of a host framework formed by Li and Ge atoms. The phase has been obtained in two forms, at elevated temperatures with space group Pm (3) over bar n, and at low temperatures with a 2a(0) x 2a(0) x 2a(0) superstructure. In this work, we report on the direct preparation of the low-temperature form by thermal decomposition of the precursor phase K7LiGe8 at 400 degrees C. The obtained product K8Li2.48(7)Ge43.37(9)square(0.15(6)) [a = 21.5489(2)angstrom, Z = 8] was found to be virtually identical with the one prepared by annealing of the hightemperature form at this temperature. The superstructure was investigated by X-ray powder diffraction and transmission electron microscopy, revealing the space group Ia3 _ d. The low-temperature form of K8LixGe44-x/4 square(2-3x/4) represents one of the rare examples for a clathrate-I superstructure primarily caused by an ordered arrangement of heteroatoms. Two similar, yet inequivalent models for the description of the ordering of lithium atoms in the framework are discussed