The tin-rich copper lithium stannides: Li3Cu6Sn4 and Li2CuSn2

The Sn rich ternary intermetallic compds. Li3Cu6Sn4 (CSD-427097) and Li2CuSn2 (CSD-427098) were synthesized from the pure elements by induction melting and annealing at 400°C. Structural investigations were performed by powder- and single-crystal XRD. Li3Cu6Sn4 crystallizes in space group P6/mmm; it is structurally related to but not isotypic with MgFe6Ge6 (a = 5.095(2) Å, c = 9.524(3) Å; wR2 = 0.059; 239 unique F2-values, 17 free variables). Li3Cu6Sn4 is characterized by two sites with a mixed Cu:Sn occupation. In contrast to all other Cu-Li-Sn compds. known so far, any mixed occupation was found for Cu-Li pairs only. In addn., one Li site is only half occupied. The second Sn rich phase is Li2CuSn2 (space group I41/amd, a = 4.4281(15) Å, c = 19.416(4) Å; wR2 = 0.033; 213 unique F2-values, 12 atom free variables); it is the only phase in the Cu-Li-Sn system which is noted for full ordering. Both crystal structures exhibit 3D-networks which host Li atoms in channels. They are important for understanding the lithiation mechanism in Cu-Sn electrodes for Li-ion batteries.