The true crystal structure of Li17M4 (M=Ge, Sn, Pb)–revised from Li22M5

Single crystal studies of the compounds Li17Ge4, Li17Sn4 and Li17Pb4, previously thought to be of stoichiometry Li22M5 (M=Ge, Sn, Pb) are presented. Their space group is F4̄3m, rather than the F23 previously reported for Sn and Pb, with a=18.756(2) Å, 19.690(2) Å and 19.842(1) Å for the Ge, Sn and Pb compounds, respectively, with very good refinements, particularly for the tin structure (R(F≥4σ(F))=1.85% for Ge, 1.91% for Sn and 2.3% for Pb). Comparison with the related structure of Li21Si5 reveals that as one progresses down Group IV, there is a variation in the occupation of the lithium tetrahedra about the tetrahedral Wickoff sites based on the space apportioned by the surrounding metal atoms. Partial or complete occupation of some tetrahedral sites increases from Si to Pb resulting in centred tetrahedral Li5 groups replacing Li4 units, and hence stoichiometries that can be effectively described as Li21Si5 (‘Li16.8Si4’); Li(21+3/16)Ge5 (‘Li17Ge4’); Li(21+5/16)Sn5 (‘Li17Sn4’); and Li(21+1/4)Pb5 (or Li17Pb4).