Structural study of a group of magnetic multilayered oxides in the BaO-TiO2-Fe2O3 system I. High-resolution electron microscopy

In recent work on phase equilibria relations in the BaO-Fe2O3-TiO2 system, numerous ternary compounds were found. Most of the new compounds are comple x and exhibit new structural types. The structures of six of these compound s, the so-called E, M, K, N, L and J phases, were characterized by high-res olution electron microscopy. All compounds were shown to belong to a class of ordered intergrowth structures built from two types of alternating slabs related to the structures of hexagonal BaTiO3 (h-BaTiO3) and Ba12Fe28Ti15O 84 ('12 t 14 t 15'), which are in thermodynamic equilibrium with most of th e studied phases. All the phases exhibit close-packed structures built from [O, (Ba/O)] layers in different stacking sequences. The so-called P slab h as a structure related to h-BaTiO3, in which Ti occupies the octahedral cat ion sites. The second type of slab, denoted H, exhibits related but somewha t different structures in the E and M phases (H'), and four other phases (H "). The structure of the H' slab is probably related to the spinel block p ortion of the 12 t 14 t 15 compound, whereas the H " slab is related to the entire 12 t 14 t 15 structure. Structural modelling on this basis resulted in a good fit with experimental compositions. One-dimensional structural d isorder was observed for all the phases studied (with the exception of E). The disorder was shown to be related to the lower symmetry of the II-type s labs and reflects the poor spatial correlation between them. The structural analysis indicates that the II-type slabs are preferentially occupied by F e, which is accommodated in both octahedral and tetrahedral sites, and dilu te in Ba. The strongly heterogeneous distribution of Fe suggests that all s ix phases can be considered as self assembled magnetic multilayer structure s, with potentially intriguing physical properties.