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Abstract:
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.
