CRYSTAL STRUCTURE OF A NEW Li-Ge-S COMPOUND, Li₄GeS₄. Yoshitaka Matsushita, Mercouri G. Kanatzidis, Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 USA

The new Li-Ge-S compound, Li₄GeS₄, has been successfully grown as the main product in the Li₂S-La₂S₃-Ge-S system at temperature under 973K. The clear white to light yellow moisture-sensitive crystals are grown by heating the above reagents in 2:1:2:6 proportions at 973K for 200hrs and slowly cooling to 773K. The reagents were mixed and put in a carbon tube which was sealed inside an evacuated quartz tube (~10^-5 Torr). The well- developed single crystals had pillar-shaped habits.

An X-ray single crystal intensity data collection has been carried out for Li₄GeS₄ using a four-circle RIGAKU AFC6S diffractometer with MoK[[alpha]] radiation at room temperature. The crystal was mounted inside of a capillary with mineral oil. The space group was found to be Pnma, and the cell dimensions are a=14.107(6), b=7.770(3) and c=6.162(2)Å estimated from 18 well-centered reflections in the range of 55deg. < 2[[theta]] < 60deg. using MoK[[alpha]]1. The intensity data were collected up to 60deg. in 2[[theta]]. The crystal structure including the lithium-sites has been solved by direct-methods using SHELXS-86 and successive Difference-Fourier synthesis. The final R and wR-values are 1.85 and 1.65% for 866 observed reflections, respectively.

The Li₄GeS₄ structure has three lithium-sites and one germanium-site. The lithium sites have two different coordination types. The Li1-site is coordinated to five sulfurs in a square pyramidal geometry and can be viewed as a remarkably distorted NaCl-like framework. The other lithium sites, the Li2 and Li3 have distorted tetrahedral coordination types connected in a three dimensional framework. The Germanium atom is also tetrahedrally coordinated by four sulfur atoms and is found as an isolated GeS₄^4- unit. The crystal structure of Li₄GeS₄ is similar to the K₄SnSe₄ [1] structure type, but has slight differences caused by the differences in ionic radii of the alkali metals and the chalcogenides. The Li₄GeS₄ structure exhibits disorder at the Li1-site which is not found for K1 in K₄SnSe₄.

[1] K. O. Klepp; Z. Naturforsch. 47b (1992) 411