<p>The structural response of Aurivillius Bi₅Ti₃FeO₁₅ (BTFO) to <i>A</i>-site substitution with tetravalent cations (Hf⁴⁺ and Te⁴⁺) was investigated as a strategy to increase the Fe concentration within the perovskite-like layers and enhance the magnetic response of the material. Samples with the nominal composition Bi_5−yM_yTi_3−yFe_1+yO₁₅ (M = Hf⁴⁺, Te⁴⁺) were synthesized via the sol–gel method and characterized by X-ray diffraction (XRD) and ⁵⁷Fe Mössbauer spectroscopy. Hf substitution did not stabilize the targeted m = 4 Aurivillius phase but instead promoted a higher-layer m = 6 analogue (Bi₇Ti₃Fe₃O₂₁), alongside Bi₂Ti₂O₇ pyrochlore and α-Fe₂O₃. In contrast, Te substitution fully suppressed Aurivillius phase formation, yielding Bi₂Ti₂O₇ pyrochlore and Bi₂TeO₅ fluorite, with all Fe³⁺ incorporated into these structures, without the formation of secondary iron oxides. These results demonstrate that <i>A</i>-site donor substitution with Hf⁴⁺ and Te⁴⁺ ions is not an effective strategy for increasing the Fe content within the Bi₅Ti₃FeO₁₅ lattice. Instead, the system minimizes its free energy and realizes charge compensation through phase segregation or structural decomposition.</p>