K2NiF4 structure

Ternary alkali-metal halide oxides are known and have the expected structures. Thus Na3C10 and the yellow K3BrO have the aqti-perovskite structure (p. 963) whereas Na4Br20, Na4l20 and K4Br20 have the tetragonal anti-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. 

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 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... 

Figure 1 Idealized crystal structures of (a) La2Cu04 tilted along [110] from [010] directions, (b) Oxygen defect K2NiF4 structure of La2 xSrxCu04 (x >0.05), in [010], with c-axis vertical, and is related to Figure la a = aorth / /2. ...

Following the discovery of HTSC in La2Cu04 doped with Ba in the lanthanum site (Bednorz and Mueller 1986), work describing HTSC with Sr substitution in the La site was reported. The parent La2Cu04 compound (abbreviated to 214 ) has a distorted K2NiF4 structure, with an orthorhombic unit cell (a =... 

For the tetragonal K2NiF4 structure, of the total 12 zone center phonon modes, 4 (2Alg + 2EB) are Raman active. Figure 2 shows room temperature... 

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. 

La,Ba)2Cu04, which has the layer perovskite K2NiF4 structure. The discovery won them a Nobel prize. 

Fluorides with a K2NiF4 Structure. Extensive studies have been carried out on the magnetic properties of compounds exhibiting a K2NiF4 structure. We will not list the results of all the measurements made, but only tabulate the relevant data of some compounds to facilitate their comparison. 

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 ... 

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