NEW CATION DEFICIENT PEROVSKITE-LIKE OXIDES IN THE SYSTEM LA4TI3O12-LATIO3

A number of new hexagonal perovskite-like oxides have been obtained by reacting La4Ti3O12 and LaTiO3. Their crystal structure is based on cl ose-packed stacking of LaO3 layers and can also be described as consis ting of blocks of comers-sharing (TiO6) octahedra perpendicular to the c-axis, separated by layers of vacant octahedra. The first phase, La5 Ti4O15, corresponds to the 10H polytypoid with a (hccchhccch) sequence along the c-axis. The second phase, La6Ti5O18, is isostructural with Ca2La4Ti5O18 and corresponds to the 18R polytypoid with a (hcccch)3 st acking of the LaO3 layers. La9Ti7O27 belongs to the 54R polytypoid wit h a (hccchhcch)6 layers stacking. The structure consists of an alterna ting sequence of La5Ti4O15 and La4Ti3O12 blocks along the c-axis and t hus can be considered as a regular intergrowth between these two phase s. The main feature of these new phases is that in each structural uni t one of the possible octahedral positions remains unoccupied. These r esults have been confirmed by refinement of powder XRD data as well as by electron microscopy (SAED, HRTEM) methods.