Given that bismuth is used in excess during the synthesis, structural and compositional rearrangement from an m = 5 phase to an m = 6 phase is permitted based on the concentrations of cations available. To correlate the charge balancing requirements with the compositions where structural adaptation occurs, we tabulate the nominal B-site proportion of Ti, Fe and Mn for the B6TFMO samples from a spread of compositions with decreasing Ti concentration in Supplementary Table . Next, we assume that Ti has a +4 valence, Fe has a +3 valence and we let Mn have a valence of either +3 or +4. We then assume that the proportion of Mn 4+ present is proportional to the concentration necessary to maintain a charge balanced m = 5 Aurivillius structure (see Supplementary Information SI5). Note that the displayed oxidation states are theoretical values based on nominal, assumed phase-pure compositions and are not taken from experimental measurements. The table is arranged in order of decreasing Ti concentration, and we correlate the compositions with the observations (from XRD and TEM data) of whether the sample shows an overall m = 5, m = 6 or mixed m = 5/6 Aurivillius phase. Supplementary Table follows the same trend as Table and summarizes that the appearance of the m = 6 phase (as observed in the XRD and TEM analysis) follows a systematic trend with decreasing Ti concentration. More precisely, the structural rearrangement occurs when the combined concentration of Fe and Mn at the B-site of the perovskite units is above 46% (see Supplementary Information Table ). Furthermore, when the nominal concentration of Mn 4+ is greater than 6.6%, meaning that the theoretical average oxidation state of Mn in the m = 5 structure is larger than ≈ 3.4, the appearance of the m = 6 phase begins to occur. It appears that the cation radius disparity restricts the maximum concentration of Mn 4+ that can be accommodated by the m = 5 structure, with a threshold Mn 4+ solubility limit of between 6.6 to 8.0% (or 46 to 48% magnetic ion content at the B-site). We note that this is a similar average oxidation state threshold to that previously observed in other Aurivillius phases systems by Zurbuchen et al. (Mn 3.2+ ) and