The concentrations of two of the three types of B-site cations were systematically varied within the B6TFMO Series 1, 2 and 3, while the third cation concentration was kept constant. XRD analysis (Fig. (a) to (c)) of Series 1, 2 and 3 was overall consistent with an m = 5 Aurivillius phase structure. In this section we will examine what factors influence deviations from this.