Newnham observed that "substantial substitution" of B-site metals "is obtained for only a narrow size range of octahedral ions": 0.58 A° to 0.65 A°. It is therefore expected that there will be a limit to how much Mn 4+ (octahedral site ionic radius 0.53 Å) can be accepted in the m = 5 B6TFMO structure. While the precipitation of trace levels (2.7 vol.%) of secondary phases has been observed for the Bi6Ti2.3Fe1.95Mn0.75O18 composition and may assist with compensating some charge/size effects (Supplementary Section SI4.3), overall, XRD analysis (Fig. and Fig. ) and TEM imaging (Fig. ) clearly demonstrates that the m = 5 Aurivillius structure adapts to a higher m = 6 layered homologue to accommodate increased levels (>46%) of magnetic cations.