K2NiF4-type oxides

The most often found types of mixed oxides are perovskites (RBO3), K2NiF4-type oxides (R2BO4), R 1 B C>2 +1 ( = 2 or 3), lamellar perovskites, pyrochlores, spinels, and oxide solid solutions. Perovskite oxides are, by far, the most commonly used oxides. Therefore, this chapter will reflect this situation by putting more emphasis on this type of materials. Within this section the structure, preparation methods and general characteristics of the mixed oxides will be discussed. Note that R stands for rare-earth elements while A includes all types of elements. 

Diffusion coefficient and surface exchange coefficient measurements have been reported for the K2MF4 type oxide materials by a number of authors [4-8, 13-19] and have been complemented by electrochemical permeation measurements [20-27] all of which demonstrate the fast oxide ion conduction of hyperstoichiometric K2NiF4 type oxides. Early reports also demonstrate the relatively poor oxide ion mobility in those materials found to be hypostoichiometric [28,29]. Initial reports of the fast oxide ion conduction in La2Ni04+s [4, 6-8] have generated a number of further studies [13-19] regarding the optimization of composition and determination of the effects of anisotropy on the conduction properties of these materials. Each of these features will be discussed in more detail below. 

Wei, Y, Liao, Q., Li, Z. and Wang, H. (2013) Enhancement of oxygen permeation through U-shaped K2NiF4-type oxide hollow fiber membranes by surface modifications. Separation and Purification Technology, 110,74-80. 

The transport properties of perovskite-type oxides are dependent mainly on the B-site cations. Among them, Mn-based perovskites and Co/Fe-based perovskites are most frequently used for high-temperature and intermediate-temperature SOFCs, respectively. Recently, Ni-based K2NiF4-type oxides are also being investigated [5]. Their composition and microstructure are still to be optimized based on the defect chemistry, electrochemistry, and thermodynamics. 

Perovskite-type oxides based on Mn, Co, Fe, or K2NiF4-type oxide with Ni are studied as cathode materials for SOFCs. For high-temperature SOFCs, LaMnOs-based materials are mainly used because of the high compatibility... 

It was further difficult to derive the antiferromagnetic state in the case of transition metal oxides like Cu oxides. One of typical Cu oxides is La2Cu04, in the K2NiF4-type crystal structure, we displayed in Figure 5.10. The Cu oxide of... 

Let us review first the state of the art concerning nitric oxide decomposition. In the last 10 years the search for NO decomposition catalysts sharply increased in an effort to comply with more severe regulations which reduce the allowed emission level of this contaminant. Particularly during the 1990s quite a few papers have been published concerning the use of different rare-earth-containing perovskites and K2NiF4-type mixed oxides. Ni, Cu and Co are the most commonly used transition metals. 

Sayers R, Liu J, Rustumji B, Skinner SJ (2008) Novel K2NiF4-type materials for solid oxide fuel cells compatibility with electrolytes in the intermediate temperature range. Fuel Cells 8(5) 338-343... 

Structural Disorder and Diffusion Path of Oxide Ions in a Doped Pr2Ni04"Based Mixed Ionic-Electronic Condnctor (Pro.9Lao.i)2(Nio.74Cno.2iGao.os)04+5 with a K2NiF4-Type Structure [15]... 

This type usually called K2NiF4-structure 15) was investigated for the first time by Winkler and Brehler 337) at (KaMgF4). During the last years many oxides and fluorides of this type have been reported. Meanwhile isostructural transition metal fluorides containing the larger A-ions K, Rb, Cs, NH4 and T1 are known. 

The first-discovered oxide superconductor is La2Cu04 and it has the K2NiF4 structure type, as shown in Fig. 10.4.3(a). This structure can be derived from one B-type unit cell of the perovskite stmcture in combination with two A-type unit cells, each with one layer removed. In this manner, a tetragonal unit cell with c 3a is generated. The resulting structure belongs to space group — I4/mmm, in which the Cu atom has octahedral coordination and the La atom has nine-coordination. The CuC>6 octahedra share vertices and form infinite layers. 

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