Crystal chemistry and properties of mullite-type Bi2 M 4O9: An overview

Abstract

Abstract Bi2 M 4O9 (M = Al3+, Ga3+, Fe3+) 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 2O7 tetrahedral groups and by BiO polyhedra. The Bi3+ cations in Bi2 M 4O9 contain stereo-chemically active 6s2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi2Mn4O10 are similar to those of Bi2 M 4O9, Bi2Mn4O10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s2 LEPs of Bi3+ in Bi2Mn4O10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi2(Al1 – x Fe x )4O9 and Bi2(Ga1 – x Fe x )4O9 (x = 0–1). Things are more complex in the case of the Bi2(Fe1 – x Mn x )4O9+ 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 Mn4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe3+ favorably replaces Mn3+ in the pyramids. The crystal structure of Bi2 M 4O9 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 Bi3+ is substituted by Sr2+ in Bi2Al4O9 corresponding to a composition of (Bi0.94Sr0.06)2Al4O8.94. Sr2+ for Bi3+ 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 Bi2Al4O9, especially in coexistence with liquid Bi2O3. Therefore, Bi2Al4O9 single crystals and polycrystalline ceramics both with significant amounts of M 2+ doping (M = Ca2+, Sr2+) have not been produced yet. Thus the question whether or not M 2+-doped Bi2 M 4O9 is an oxygen conducting material is still open.