Mixed crystal formation and structural studies in the mullite-type system Bi2Fe4O9-Bi2Mn4O10

► Mixed crystals of the mullite-type Bi2Fe4−xMnxO10−δ were synthesized. ► Two single-phase regions and an intermediate two-phase region were found. ► Site substitution was examined via X-ray diffraction and Mössbauer spectroscopy. ► Charge compensation models were examined for intermediate compositions.