occurs in A+A′3+B4+O4: A = Na, A′ = La → Lu, B = Ti; Na and the lanthanides occupy different layers in the c direction. Anion‐deficient and anion‐excess K2NiF4 structures exist. For example, in Sr2XO3: X = Cu, Pd, one‐quarter of the oxygens are removed, giving square planar coordination for X and seven‐coordinate Sr, whereas in La2NiO4+x , interstitial oxygens are present.
The Ruddlesden–Popper series of compounds have the K2NiF4 structure as their simplest family member; other family members exist with more layers in the perovskite blocks. Thus, with strontium titanates, a family of phases exists with general formula Sr n + 1Ti n O3n + 1 and consist of nSrTiO3 perovskite blocks interleaved with SrO rock salt layers. Two members of this family are Sr3Ti2O7 (n = 2) and Sr4Ti3O10 (n = 3). The n = ∞ member corresponds to SrTiO3 perovskite. These phases retain a tetragonal unit cell with similar a dimensions but have much longer c axes. A similar family of phases forms for La n + 1Ni n O3n + 1 : n = 1, 2, 3, ∞. In most cases, only one or two members of the Ruddlesden–Popper series are known, with examples such as
A large and diverse family of Cu‐based perovskite‐rock salt intergrowth phases contains many examples of the high T c cuprate superconductors. The key element for the superconductivity is Cu and it shows enormous structural diversity in the cuprates with coordination numbers ranging from 2 (linear) to 4 (square planar), 5 (pyramidal) and 6 (octahedral), as discussed in Section 8.3.6. In addition to YBa2Cu3O7 and related materials which are basically oxygen‐deficient perovskites, Bi‐ and Tl‐based cuprates are perovskite–rock salt intergrowth phases with considerable compositional and structural complexity. The BiSCCO superconductors consist of three phases that are labelled according to their Bi:Sr:Ca:Cu ratios: 2201, 2212 and 2223. The 2212 phase (80 K superconductor), Bi2Sr2CaCu2O8 has rock salt‐like layers, Bi2O2, that separate double perovskite layers containing Cu in octahedral sites, whereas the 2223 phase (110 K superconductor) has triple perovskite layers. In these structures, SrO layers form a coherent interface between rock salt and perovskite components.
Figure 1.50 (a) The K2NiF4 structure. (b) The Bi2O2 layers that form part of the intergrowth structure of Aurivillius phases.
Table 1.26 Some compounds with the K2NiF4 structure
| Compounda | a/Å | c/Å | z (M+ ion) | z (anion) | |
|---|---|---|---|---|---|
| K2NiF4 | 4.006 | 13.076 | 0.352 | 0.151 | |
| K2CuF4 | 4.155 | 12.74 | 0.356 | 0.153 | |
| Ba2SnO4 | 4.140 | 13.295 | 0.355 | 0.155 | |
| Ba2PbO4 | 4.305 | 13.273 | 0.355 | 0.155 | |
| Sr2SnO4 | 4.037 | 12.53 | 0.353 | 0.153 | |
| Sr2TiO4 | 3.884 | 12.60 | 0.355 | 0.152 | |
| La2NiO4 | 3.855 | 12.652 | 0.360 | 0.170 | |
| K2MgF4 | 3.955 | 13.706 | 0.35 | 0.15 | |
| Other examples b | |||||
| M2Y6+O4: M = K, Rb, Cs; Y = U, Np | |||||
| Ln2YO4: Ln = La → Nd; Y = Ni, Cu | |||||
| CaLnAlO4: Ln = La → Er, Y | |||||
| SrLnFeO4: Ln = La → Tb | |||||
| SrLnCrO4: Ln = La → Dy | |||||
| BaLnFeO4:Ln = La → Eu | |||||
|
|
|||||
| A2BF4: A = K, Rb, Tl; B = Mg, Ni, Zn, Co, Fe | |||||
| A2BCl4: A = Rb, Cs; B = Cr, Mn, Cd | |||||
| Sr2BO4: B = Ti, Sn, Zr, Hf, Mo, Tc, Ir, Ru, Rh, Mn | |||||
a R. W. G. Wyckoff, Crystal Structures, Vols 1 to 6, Wiley (1971).
b O. Muller and R. Roy, The Major Ternary Structural Families, Springer‐Verlag (1974).
The Aurivillius
