The structures of Li 1 > > x \ \ y Nb 1 \ \ x \ \ 3y Ti x > > 4y O 3 solid solutions within the so-called M-phase 5eld in the Li 2 O+Nb 2 O 5 +TiO 2 system were investigated using high-resolution transmission electron, microscope (HRTEM) and single-crystal X-ray di4raction. The results demonstrated that the phase 5eld is not a solid solution but rather a homologous series of commensurate intergrowth structures with LiNbO 3 -type (LN) slabs separated by single [Ti 2 O 3 ] 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-Li 2 Ti 3 O 7 end-member. The LN slabs accommodate a wide range of Ti 4 > >/Nb 5 > > substitution, and for a given homolog the distribution of Ti and Nb is not uniform across the slab. A single-crystal X-ray di4raction 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 [Ti 2 O 3 ] 2 > > corundum layers. The extra cations in phases with x'0 are accommodated through the formation of Li-rich Li 2 MO 3 -type layers in the middle of the LN slabs. The fraction of layers with extra cations increases with increasing Ti-content in the structure.
2002 Elsevier Science (USA)