Lithium intercalation into monoclinic Cr4TiSe8:: Synthesis, structural phase transition, and properties of LixCr4TiSe8 (x=0.1-2.8)

The intercalation reaction of Li into the complex ternary selenide Cr4TiSe8 was investigated as function of time and temperature. The treatment of the host compound with a solution of butyl lithium in n-hexane (BuLi) leads to Li insertion up to the composition Li2.8Cr4TiSe8. The intercalation reaction is accompanied by a structural phase transition of the host material. The monoclinic symmetry of pristine Cr4TiSe8 is changed to trigonal when x in LixCr4TiSe8 exceeds approximately 0.4. With increasing Li content the axes of the new trigonal unit cell are significantly enlarged. The structural phase transition was characterized by Rietveld refinement of X-ray (XRD) powder patterns for different values of x. The phase transition is electronically driven rather than being due to geometric reasons. Immediately after removal of the excess BuLi of partially intercalated phases, the product is not in equilibrium and undergoes a structural relaxation. When the lithiated products are treated with water, the intercalated Li is removed while the symmetry does not switch back from trigonal to monoclinic. The final composition of the deintercalated samples was always Li approximate to 0.4Cr4TiSe8. The strain of the samples introduced by the Li insertion and the phase transition was investigated with line profile analysis (LPA) of the XRD patterns. The intercalation reaction is accompanied by a strong increase of the strain. Static Li-7 NMR measurements of the fully intercalated sample Li2.8Cr4TiSe8 show a broad line due to the strong interaction of the Li nuclei with the Cr ions. Li-7 magic angle spinning (MAS) NMR studies reveal the presence of two nonequivalent Li sites.