(K₄Li₄)Al₈Ge₈O ₃₂·8H₂O: An Li⁺-exchanged potassium aluminogermanate with the zeolite gismondine (GIS) topology

The title compound, lithium potassium dialuminium digermanium octaoxide dihydrate, (K,Li)-(Al,Ge)-GIS (GIS is gismondine), is the result of a 50% Li⁺ exchange into the K-(Al,Ge)-GIS structure. The (K,Li)-(Al,Ge) -GIS structure was determined from a 4 × 4 × 2 μm octahedral single crystal at the ESRF synchrotron X-ray source. The ion exchange results in a symmetry transformation from I2/a for K-(Al,Ge)-GIS to C2/c for (K,Li)-(Al,Ge)-GIS. The structural change is due to disordering of K ⁺ ions with Li⁺ ions along the [001] channel and ordering of water molecules in the [101] channels. The distance between sites partially occupied by K⁺ ions increases from 2.19 (3) Å in K-(Al,Ge)-GIS to 2.94 (3) Å in (K,Li)-(Al,Ge)-GIS. The Li⁺ ions occupy positions along the twofold axis at the intersection of the eight-membered-ring channels in a twofold coordination with water molecules. For the four closest framework O^2- anions, the Li⋯O distances are 3.87 (4) Å.