For Series 1 (Bi6Ti3Fe1.6-αMn0.4+αO18 (α = 0, 0.1, 0.2, 0.3 and 0.4)), the concentrations of Fe and Mn are varied, while Ti concentration remains constant. In other words, the Ti : Mn/Fe stoichiometric ratio of 3 : 2 does not change across Series 1. This was considered isovalent substitution since both Fe and Mn are nominally in the 3+ oxidation state (o.s.) in a neutral B6TFMO Aurivillius stoichiometry. They also have similar ionic radii as noted above, although it is acknowledged that the Jahn-Teller effect can tend to elongate Mn 3+ O6 octahedra. The XRD plots of sample Series 1 shown in Fig. show that the peak positions, heights and widths correlate well with a computed XRD diffraction pattern for a refinement of a reference m = 5 structure shown in Supplementary Information Section SI3.1, Fig. . The reference pattern is computed with an assumed 90% preferred crystallite (00l) orientation, a nominal crystal size of 30 nm and an isotropic strain of 0.1%, to give a line breadth similar to that observed in the m = 5 phase in Fig. . The 30 nm crystallite size inferred from XRD analysis is between 0.5 to 0.3 that of the observed crystallite thickness in cross section TEM as shown in Figures ) and (f), an observation that is probably indicative that the crystallite coherence due to structural disorder is significantly smaller than the film thickness. As the concentration of Mn increases and Fe decreases, there is only a slight shift (~0.4 °) in 2θ peak positions accompanied by peak broadening.