Perovskites aurivillius phases

Another group of materials that has displayed high oxide ion conductivity is based upon a layered bismuth perovskite-based structure, first reported by Aurivillius in 1949 [90-92], The so-called Aurivillius phases are chemically expressed normally as Bi2A B 03 +3 [82], where A is a large 12-coordinated cation and B a small 6-coordinated cation. The structure is formed by n perovskite-like layers, (A 1B 03n+1)2, sandwiched between bismuth-oxygen fluorite-type sheets, (Bi/) 2 [93,94],... 

As described in Section 8.2.6, along with YSZ, mixed oxygen-ion, and electron-conducting oxides with a perovskite-type structure, the so-called Aurivillius phase and pyrochlore materials are fundamentally used for the production of a variety of high-temperature electrochemical devices [50-58],... 

There is little doubt that many materials that at present are described as containing ordered arrays of point or extended defects will be successfully described as notionally defect-free modulated structures. For example, the intergrowth Aurivillius phases, described as containing extended planar defects, have recently been described compactly as modulated structures. " The same formalism has been applied to hexagonal perovskite structures and superconducting copper oxides. Others will certainly follow. 

Compounds Containing Perovskite Layers. A second class of layered oxides have structures related to the three-dimensional perovskite lattice and include the Auriv-illius phases, the Ruddlesden-Popper phases and the Dion-Jacobson phases. The general composition can be written Ma[A iB 03 +i] where A is an alkaline or rare earth metal, and B is niobium or titanium. In the Aurivillius phases = Bi2 02 +, whereas M is an alkali metal cation in the ion-exchangeable Ruddlesden-Popper a = 2) and Dion-Jacobson a = 1) phases. The relationships between the three structure types is shown in Figure 14. The intercalation chemistry of the Dion - Jacobson phases was the first to be studied. 

The Aurivillius phases again contain slabs of perovskite sliced along the ideal [100] direction. They are formed by replacement of the interlayer A structures in the Ruddlesden-Popper phases and A in the Dion-Jacobson phases with a layer of composition Bi O - This gives the series a general formula (Bip XA jB j, ), sometimes written in an ionic form (Bip ) (A jBPj j) " (Table 4.5). As before, the perovskite slabs have the formula (A where A is a large cation nom-... 

From symmetry considerations, the ferroelectric layered perovskites, such as the Ruddlesden-Popper, Dion-Jacobson and Aurivillius phases in which the perovskite layers are an even number of octahedra in thickness, tend to have the spontaneous polarisation vector lying parallel to the perovskite sheets. In phases with an odd number of octahedra in thickness, the spontaneous polarisation can lie perpendicular to the sheets or at least have a component in this direction. Thus the n=2 Dion-Jacobson phase RbBiNbjO has the spontaneous polarisation vector lying parallel to the perovskite layers. This polarisation arises from the tilted NbOg octahedra and, more importantly, from the displaced lone pair Bi -" cations that are found between the perovskite layers. 

In the layered Ruddlesden-Popper and Dion-Jacobson phases, improper ferroelectricity can also arise in a form termed hybrid improper ferroelectricity. Here, the ferroelectricity arises when two non-polar rotations of BO octahedra combine to produce a polar stmcture. The same effect may well also occur in the Aurivillius phases although this has not yet been proven. The creation of improper ferroelectric polarisation due to magnetic interactions in multiferroic perovskites is described in Section 7.10. 

Multiferroic behaviour is not confined to simple perovskite structures. Thin films of the more complex layered ferroelectric perovskite-derived Aurivillius phase... 

Perovskite-type layered compounds are the intergrowth of perovskite l ers (P) ABO3 and slabs of the differ type of structure (rock salt, calcium fluorite type, cations of metals). Depending on the nature of slabs between perovskite blocks, layered compounds belong to three big groups Ruddlesden-Popper phases, Aurivillius phases, Dion-Jacobson phases. 

Figure 25. The reaction lines for the formation of different types of the layered perovskite-like oxides, a - double-layered Ruddlesden-Popper aluminates b - double-layered Ruddlesden-Popper manganites c - double-layered Ruddlesden-Popper ferrites d - double-layered Aurivillius phases e - single- and triple-layered Ruddlesden-Popper titanates.

The family of bismuth oxides of the general formula Bi B -,Oj (B=Ti, Nb and W), first described by Aurivillius, may be regarded as B-cation deficient perovskites. Typical members of this family are Bi2WOfi, BiaTlNbOg and Bi4Ti30i2- These phases adopt layer structures consisting of PbO-like (BiaOj) layers which alternate in the orthorhombic... 

Figurell.1 Example of the main families Sr3Ru207 (n = 2) [22]/Ruddlesden-Popper with layered perovskite structure (a) KLaNb207 phases (c) Bi3TINb09 (n = 2) [23]/Aurivillius (n = 2) [19] and Ca2CsNb30,o (n = 3) [20]/Dion- phase (d) Sr5Nb40,5 (n = 5) [24]/hexagonal Jacobson phases (b) Sr2Ti04 (n = 1) [21] and layered perovskite.

---