The K2NiF4 structure

Many examples of Ruddleston-Popper phases have been synthesized. The structure of the first member of the series, corresponding to n = 1, is adopted by a number of compounds, including the important phase La2Cu04 (Section 4.3.3) and is often referred to as the K2NiF4 structure. In practice, synthesis of A +1B C)3 +i phases frequently results in disordered materials in which random or partly ordered regions of 100 faults occur, and particular efforts have to be made to produce perfectly ordered crystals. 

The incorporation of Cu ions in the perovskite structure is known for only a few examples since this particular structure is normally stabilized by or requires a B atom in a high formal oxidation state such as Ti4+ in BaTiOs, or Rhs+ in LaRhOs. Further, since Cu can not be readily stabilized in its Cu(m) state, and is unknown in the tetravalent state, the simple formation of ternary compounds such as LaCuOg or BaCuOs is not expected. Even in the K2NiF4 structure, the stabilization of Cu4+ as in Ba2Cu04 is not expected, but the formation of a stable Cu(II) state is a distinct possibility, as in La2Cu04. Copper(II), however, has been introduced in the doubled-or tripled-perovskite structure. Examples of these, which include structural distortions from cubic symmetry, are listed ... 

Figure 33 The polyhedral representation of the tripled perov-skite-type structure as compared to the K2NiF4 structure.

La2 xAExCu04 AE = Alkaline Earth Large crystals of the 30K superconductor, Ba016Laj 86Cu04, having the K2NiF4 structure... 

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. 

Both symmetry and unit cell constants are similar to the corresponding values of K2NiF4 type compounds, as is shown in Table II. Indeed, we have found that a reasonable structure can be derived for Sr2Pd03 which is closely related to the K2NiF4 structure. In this proposed structure, the space group D2h25-Zmmm is assumed and the atoms are placed as follows ... 

Compounds of the general formula A2BXa with the K2NiF4 structure (Fig. 1) may be considered to be built up of alternating layers of perovskite (ABX3) and rock-salt (AX) structures. There are no close-packed A 2X4 layers in A2BX4 similar to the close-packed AX3 layers (Fig. 2a) present in perovskites. In a close-packed A2X4 layer, there would be considerable electrostatic repulsion between the two A ions since they are forced to be adjacent to each other if the perovskite AX3 layers are to be retained within the structure (see Fig. 2b). 

Fig. 1. The K2NiF4 structure of A2B04 oxides showing Oi and Ou ions. B-Oi and A-Oic bonds are also shown. Shaded portion shows close-packed AO3 layers.

Stability of the K2NiF4 structure in oxides. Just as for perovskite oxides, a tolerance factor t may be defined for A2B04 oxides as... 

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