Crystal chemistry and properties of mullite-type Bi₂ <i>M</i> ₄O₉: An overview

Abstract

Abstract 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. The crystal structure of Bi 2 M 4 O 9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c -axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature-induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c -axis expansion appears as relatively high and does not display its lowest value parallel to c , as could be inferred. Maximally 6% of Bi 3+ is substituted by Sr 2+ in Bi 2 Al 4 O 9 corresponding to a composition of (Bi 0.94 Sr 0.06 ) 2 Al 4 O 8.94 . Sr 2+ for Bi 3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low ( σ = 7 · 10 − 2 S m − 1 at 1073 K, 800°C). An explanation could be the low thermal stability of Sr-doped Bi 2 Al 4 O 9 , especially in coexistence with liquid Bi 2 O 3 . Therefore, Bi 2 Al 4 O 9 single crystals and polycrystalline ceramics both with significant amounts of M 2+ doping ( M = Ca 2+ , Sr 2+ ) have not been produced yet. Thus the question whether or not M 2+ -doped Bi 2 M 4 O 9 is an oxygen conducting material is still open.