Sequential variation of super periodic structures emerged in Bi-layered perovskite pillar-matrix epitaxial nanocomposite films with spinel ferrites

The phase stability of Aurivillius bismuth-layer structured Bi5Ti3FeO15 (BTFO15) has been investigated in an epitaxial pillar-matrix nanocomposite system with spinel ferrites. Depending on the growth temperature a variety of super periodic structures of BTFOs appeared, denoted by the general formula BTFO15·nBiFeO3 with almost continuous values of n between -0.5 and +1. In nanocomposites with CFO, n takes the positive values from n = 0, increasing up to n = 1 corresponding to Bi6Ti3Fe2O18 (BTFO18), as the growth temperature decreases. In contrast, in nanocomposites with NFO, n takes the negative values from n = 0, decreasing close to -0.5 corresponding to a well-known intergrowth structure of Bi4Ti3O12 (BIT)-BTFO15, as the growth temperature increases. The formation mechanism of such super periodic structures is discussed in terms of the ordering of Bi2O22+ layers in the perovskite blocks, driven by the Fe3+ transfer between BTFO15 and spinel ferrites as starting materials during phase separation, depending on the degree of excess or deficiency of the Bi amount.