Ruddlesden-Popper type phases

Several A2B04-stmctured materials of the RP type have been proposed as SOFC cathodes, including those with A = La, Sr, Ba, Pr, Nd and M = Ni, Cu, Co, Fe [37-40]. These Ruddlesden-Popper (RP) phases have been proven to have good electrochemical and transport properties, but chemical stability over the temperature range of 600-1000 °C and oxygen chemical potentials necessary for SOFC applications remains an issue [41 3]. Initial investigation of the RP phases... 

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. 

Ruddlesden-Popper structures, large numbers of cation combinations can give rise to phases of this type. Frequently the A and A ions are in a (+1/+3) pairing, combined with in oxides with n=2, for example, KLaNbjO, or a (+1/+2) pairing combined with in oxides with n=3, for example, RbCa2Ta30jg. 

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 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). 

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. 

To support this mechanism the consequence of reaction (2)-(4) have been performed. We can say that this is the usual way of formation of the Ruddlesden-Popper phases through the more simple perovskite-types structure if they stable at temperature of synthesis. This meehanism works for the beginning of lanthanide raw La-Sm. The structure-chemical reaction of the last stage of Ln2SrAl207 formation which determines the rate of overall proeess is presented in Figure 2. 

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.

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