Dion-Jacobson phase

If the pair of A atoms at the boundaries of the perovskite-like sheets in the Ruddleston-Popper phases are replaced with just one A atom, the series of phases takes the formula A (A iB 03 +i), where A and A are large ions, typically a (+1/+3) pairing, and B is a medium-sized transition-metal ion, typically Nb5+. These materials are called Dion-Jacobson phases. The majority of examples synthesized to date are of the n = 2 phase, typified by KLaNb207, CsBiNb207, and so on (Fig. 4.29). A few examples of the n = 3 phase are also known, including CsCa2Nb3Oio-... 

Figure 4.29 Idealized structures of the Dion-Jacobson phases (a) A AB207, n = 2 and (b) A AB3O10, n = 3. The shaded squares represent B06 octahedra and the shaded circles A and A atoms.

Compounds Containing Perovsldte Layers. A second class of layered oxides have structures related to the three-dimensional perovsldte lattice and include the Auriv-iUius phases, the Ruddlesden Popper phases and the Dion-Jacobson phases. The general composition can be written M [A iB 03 +i] where A is an alkaline or rare earth metal, and B is niobium or titanium. In the AurivUhus phases M = Bi202 +, whereas M is an aUcah 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. 

Exfoliation of the perovskite related layer structures is more difficult than for the clays and acid phosphates discussed earlier but can be achieved by intercalation of large bulky amines. Treacy etal. reported that the layered perovskite HCa2Nb30io could be made to form unilamellar sheets by first intercalation of an amine polyether. Spontaneous exfoliation of the layers occurs on subsequent exposure of the intercalated phase to a suitable solvent. Exfoliation techniques have been extended to other systems using tetra(n-butyl)-ammonium hydroxide (TBAOH) by Mallouk and others. A number of examples of the protonated layered perovskite phases that intercalate bases have been exfoliated. The Dion-Jacobson phases typically exfoliate to form plates but others including Ruddlesden-Popper tantalates curl to form tubular scrolls . Part of the interest in these single layer dispersions arises from their use as building blocks in the layer-by-layer self-assembly of thin films. Single layers derived from exfoliated perovskites can be attached to or alternately stacked with polycationic layers to produce thin films. Tiled monolayer structures and multilayer perovskite heterostructures result from the self-assembly. 

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-... 

There are large numbers of these reactions described in the literature. The transformation of Dion-Jacobson phases into the corresponding Ruddlesden-Popper phases is often the first step in making new compounds. Ruddlesden-Popper phases can, in most cases, be made by heating the corresponding Dion-Jacobson solid in the vapour of an alkali metal, lithium being the exception. These Ruddlesden-Popper structures can then be transformed into modular structures separated by A and OH- or A and Cl- (Figure 4.16). 

Figure 4.15 Schematic flow chart for the formation of Dion-Jacobson phases with small cations and restacked structures starting from a large cation precursor such as RbCa Nb O g. The exact chemical steps are more complex than those indicated in this outline...

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. 

Upon reaction with a bulky organic base such as tetra-( -butyl)ammonium hydroxide (TBAX)H ), the proton forms of many-layered perovskites exfoliate into colloidal sheets. For example, in perovksite-related layer phases, HCa2Na 3Nb 03 , a surfactant molecule with an amine head group is first intercalated by protonation. The surfactant generally has a hydrophilic polyether tail, which enhances the intercalation of solvent molecules. Stable dispersions in water and other polar solvents have thus been obtained [35]. Triple-layer Dion-Jacobson phase HCa2Nb30,(, exfoliates into TBA Hj Ca2Nb30jj sheets upon reaction with TBA OH" (Fig. 10.2.3) [36]. 

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. 

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