Perovskites ruddlesden-popper phases

Fig. 1. (a) The ideal cubic perovskite structure and (b) the n = 1 Ruddlesden-Popper phase AO AMO3. 

It is not surprising that perovskite oxides continue to attract unusual levels of attention. In this category, we include the Ruddlesden Popper phases of compositions (A0)(AT03) where A is a large cation, either a divalent alkaline earth or a trivalent lanthanide, and T is a transition element. The index, n, enumerates the number of perovskite ATO3 units intergrown with rock salt AO layers. 

Idealizations of the crystal stmctures of both the Ruddlesden-Popper phases from n = 1, 2, 3, and 00 (the parent perovskite) are shown in Fignre 15. 

The Ruddlesden-Popper phase has the general formula A 2[A ,B 03 +i], where [A iB 03 +i] donates perovskite-like slabs of n octahedra in thickness, formed by slicing the perovskite structure along one of the cubic directions, and A indicates the interleaved cations. Sr2Ti04, Sr3Ti207, Sr4Ti30,o, and A 2[Ln2Ti30,o] (A = K, Rb Ln = lanthanide) are included in this series. 

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. 

If the pair of atoms at the boundaries of the perovskite-like sheets in the Ruddlesden-Popper phases are replaced with just one A atom, the series takes the formula AXA jB Oj j), and the materials are called Dion-Jacobson phases (Table 4.4). The perovskite slabs are cut parallel to the [100] planes of an ideal perovskite parent and have a composition (A As with the... 

Type 111 structures have a displacement of (ap+bp)/2 between the successive perovskite layers (Figure 4.4e and f), which is the same as in the Ruddlesden-Popper phases. These form for the smallest A cations, Li, Na and Ag, and are represented by LiCajTajOjg and NaCa TajOj. The ideal structures are tetragonal with a=b a, with the c-axis taken as perpendicular to the perovskite slabs. The oxygen coordination of the A cations is tetrahedral. 

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-... 

The phases related to Ca Nb O, as in the phases described previously, are built from slabs of the perovskite stmcture, this time cut into slabs parallel to ideal per-ovskite [llOJp planes. The first materials of this type were found in the system bounded by the end members Ca Nb O and perovskite NaNbOj. The structure of CUjNbjO is, in reality, composed of slabs four octahedra in thickness and for this reason is better written Ca Nb Oj (Figure 4.6a). The perovskite slabs are stacked along the c-axis of the idealised unit cell, and each slab is displaced from its neighbours by (ap+bp)/2, as in the Ruddlesden-Popper phases. The BOg octahedra have a crenellated appearance viewed down and comer-linked rows when viewed along bp (Figure 4.6b). 

A similar ordering is found within each of the perovskite layers in the reduced n=3 Ruddlesden-Popper phase SrjMn Og, derived from the fuUy oxidised Sr Mn O,. Under normal preparation methods the ordering is confined to the perovskite layers and does not extend to three dimensions in the macroscopic crystal, although electron microscopy suggests that microdomains of such three-dimensionally ordered stmctures do exist. These structures are similar to those of Mn-containing brownmiUerite-related compounds (Section 2.5.1). 

SrTiOs is an archetype of the cubic form. A layered structure is obtained for the composition Sr3Ti207, which is built upon blocks of double perovskite slabs shifted to make SrO layers in between [7]. Another structure, Sr4Ti30io, exists with three perovskite slabs [8]. In general, these Ruddlesden-Popper phases can be described with the general formula A2[A iB 03 +i], where n is the thickness of the perovskite slabs (Figure 8.4). The = 1 member also refers to the K2Nip4 structure. 

Ruddlesden-Popper-phases, with general composition An+iB 03 +i, are more basic than the normal perovskite end members ABO3 and in principle more attractive to protons, but only modest proton conductivities have been found, notably in Sr2Ti04 [35]. 

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. 

Thus, today Ruddlesden-Popper phases include complex oxides eontaining metals from groups 1,2, 13, 14 and 15 as well as transition (d- and 4f-) elements. The general formula of such oxides can be written as A0(AB03)n, where A means alkaline, alkali earth or rare earth element, while B is d-element, Al, Ga, In, Pb or Bi. This formula shows the main structural feature of this class of the layered perovskite-like eompoimds, that is the intergrowth of the perovskite blocks AMO3 (P) and rock salt blocks AO (RS) in a consequence -Pn-RS-Pn-RS-. In case of n = 00 the perovskite structure itself is obtained. Pigure 1 shows schematically the structure of the Ruddlesden-Popper phases (n=3) as eompared to the other types of the layered perovskite-like compounds. 

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