Structural Study of Li1+x-yNb1-x-3yTix+4yO3 Solid Solutions

The structures of Li1+x-yNb1-x-3yTix+4yO3 solid solutions within the so-called M-phase field in the Li2O–Nb2O5–TiO2 system were investigated using high-resolution transmission electron, microscope (HRTEM) and single-crystal X-ray diffraction. The results demonstrated that the phase field is not a solid solution but rather a homologous series of commensurate intergrowth structures with LiNbO3-type (LN) slabs separated by single [Ti2O3]2+ corundum-type layers. The thickness of the LN slab decreases with increasing Ti-content from ~55 to 3 atomic layers in the metastable H-Li2Ti3O7 end-member. The LN slabs accommodate a wide range of Ti4+/Nb5+ substitution, and for a given homolog the distribution of Ti and Nb is not uniform across the slab. A single-crystal X-ray diffraction study of a structure composed of nine-layer LN slabs revealed preferential segregation of Ti to the slab surfaces which apparently provides partial compensation for the charge on the adjacent [Ti2O3]2+ corundum layers. The extra cations in phases with x>0 are accommodated through the formation of Li-rich Li2MO3-type layers in the middle of the LN slabs. The fraction of layers with extra cations increases with increasing Ti-content in the structure.