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

Abstract

In recent work on phase equilibria relations in the BaO-Fe₂O₃-TiO₂ system, numerous ternary compounds were found. Most of the new compounds are complex and exhibit new structural types. The structures of six of these compounds, the so-called E, M, K, N, L and J phases, were characterized by high-resolution 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 Ba₁₂Fe₂₈Ti₁₅O₈₄ (“12:14:15”), which are in thermodynamic equilibrium with most of the studied phases. All the phases exhibit close-packed structures built from [O, (Ba/O)] layers in different stacking sequences. The so-called P slab has a structure related to h-BaTiO₃, in which Ti occupies the octahedral cation sites. The second type of slab, denoted H, exhibits related but somewhat 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 portion of the 12:14:15 compound, whereas the H″ slab is related to the entire 12:14:15 structure. Structural modelling on this basis resulted in a good fit with experimental compositions. One-dimensional structural disorder 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 H-type slabs and reflects the poor spatial correlation between them. The structural analysis indicates that the H-type slabs are preferentially occupied by Fe, which is accommodated in both octahedral and tetrahedral sites, and dilute in Ba. The strongly heterogeneous distribution of Fe suggests that all six phases can be considered as self assembled magnetic multilayer structures, with potentially intriguing physical properties.