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Bi 2  M  4 O 9  ( M  = Al 3+ , Ga 3+ , Fe 3+ ) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of  M  2 O 7  tetrahedral groups and by BiO polyhedra. The Bi 3+  cations in Bi 2  M  4 O 9  contain stereo-chemically active 6s 2  lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi 2 Mn 4 O 10  are similar to those of Bi 2  M  4 O 9 , Bi 2 Mn 4 O 10  contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s 2  LEPs of Bi 3+  in Bi 2 Mn 4 O 10  are not stereo-chemically active. Complete and continuous solid solutions exist for Bi 2 (Al 1  –   x  Fe  x  ) 4 O 9  and Bi 2 (Ga 1  –   x  Fe  x  ) 4 O 9  ( x  = 0–1). Things are more complex in the case of the Bi 2 (Fe 1  –   x  Mn  x  ) 4 O 9+   y   mixed crystals, where a miscibility gap occurs between  x  = 0.25–0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn 4+ , with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe 3+  favorably replaces Mn 3+  in the pyramids.

Crystal chemistry and properties of mullite-type Bi 2 M 4 O 9 : An overview