Crystal-Chemical Study of Layered [Al2Li(OH)6]+X− · nH2O

A layered double hydroxide with a chemical composition (A12Li(OH)6)+X �9 nH20, where X is an interlayer anion, has been synthesized hydrothermally at 130~ from aluminum-tri-(sec-butoxide) and lith- ium carbonate. Electron micrographs showed the product to have a platy morphology with distinct hex- agonal symmetry, which has been corroborated by selected area electron diffraction patterns corresponding to a projection of the structure on its (001) plane. Evidence for a superlattice with a = 5.32/~ was obtained, indicating cation ordering among octahedral sites. X-ray powder diffraction data also can be interpreted by reference to a hexagonal supercell with dimensions a = 5.32/~ and c = 15.24/~. The arrangement of the octahedral sites appears to be that of gibbsite, but with the vacancies filled with lithium cations. Anions must be present between the sheets to balance the charge. A complete assignment of the observed infrared lattice vibrations can be made for the anion (AlzLiOt) with the ideal D3d symmetry for motions within one octahedral sheet. The results show that (A12Li(OH)tl+X -- nH20 is a hydrotalcite-like compound with the octahedral cations largely ordered. The general formula for hydrotalcite-like compounds, (M2+i_xM3+x(OH)2)x+xm-x/m. nH20, should be extended to include the monovalent lithium cation.