Ruddlesden-Popper compounds

The superconducting oxides include both perovskites and Ruddlesden-Popper compounds which have an orthorhombic arrangement of cubic cells, alternatively of the perovskite and sodium chloride structures. The common feature of all of these is the presence of copper as a major component. The first ceramic superconductor was a lanthanum-strontium substituted cuprate (Lai Sr Cu04 z), which is a perovskite, but subsequently the inter-oxide compound Y203 2BaO 3CuO, commonly referred to as a 123 compound, was shown to have superior performance. The speculation concerning the conduction mechanism is that this involves either Cu3+-Cu2+ positive hole... 

BIMEVOX) and the Ruddlesden-Popper compounds with general formula A2(A iB 03 +i). 

K2La2Ti3Oio is a typical oxide in the Ruddlesden-Popper series exhibiting interlayer reactivity44) It forms a hydrate with about 1-2 H20 molecules per formula unit under ambient conditions.44,74) Fig. 16.9 shows schematic structures of anhydrous and hydrated oxides. Variation of the number of waters of hydration with humidity and exposure time has been reported for the analogous compound K2Nd2Ti3O10.70) A band gap of 3.5 eV has been estimated for K2La2Ti3Oi0 from reflectance measurements.102)... 

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. 

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). 

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 6.7 The A0(AB03) Ruddlesden-Popper phase, (a) T-tetragonal La2Cu04 n= 1) (b,c) Compounds with stoichiometry A3B2O7 n = 2) (b) and A4B30no = 3) (c).

Among the oxides with the Ruddlesden-Popper structure, the Ln2Ni04 (Ln = La, Pr, Nd) compound with the K2Nip4 structure is of great interest as electrode... 

Figure 13.14 The crystal structures of the n = 2andn = 3 (R,AU,Cu 02n+2+6 Ruddlesden-Popper type compounds...

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