Short-range Cation Order

EXAFS at the Co and Fe K edges has demonstrated [72] that the cations in LDHs of the type [Co2(FejAli )(OH)6]Cl nH20 (0 j 1) are ordered. In particular, in an ordered Co 2M lattice of the type shown in Fig. 37 each Co cation should have coordination shells with 3 Co and 3M ions at a, 6C0 at aV3 and 3Co and 3M at 2a. The data indicate that the contribution at the Co K edge from the shell at aVs does not vary when aluminum is replaced by iron, but the contribution from the shell at 2a does vary with y over the range from 0 to 1, which is consistent with the ordered arrangement of cations shown in Fig. 37. 

In related species of the type [(Coi jCuy)2Al(OH)6]ClnH20 (0 7 1) it was shown [284] by EXAFS that although the local order of m and Al cations was maintained as j increased, the layers consisted of mixtures of domains of C02AI and CU2AI. The distortion in Cu coordination polyhedra associ- 

EXAFS data at the Co and Al K edges for Co Al - Cl LDHs were consistent with an ordered array of cations for n = 2,3 and 4. In each case the Al has a second coordination shell of 6 Co ions and the Co has a second shell of 6/n Al and (6-6/ ) Co next nearest neighbors [46]. The peaks arising from the focusing effect are still observed in the Co K edge EXAFS of the material showing that the cobalt cations in the layers remain ahgned. 

The presence of local cation ordering in Mg2Ga and MgsGa - CO3 LDHs noted in Sect. 3.3.1 has been confirmed by means of both EXAFS and by calculation of the electron radial distribution function from the Fourier transform of the diffracted X-ray intensity. In each case the gallium was found to have six magnesium ions and no galhum ions as next-nearest neighbors [39]. 

Fe ) have been used in an attempt to understand the vibrational spectra of phyllosihcate clays [78,291,292] and these studies may also be useful in analyzing IR data for LDHs. 

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Dissolution of alkali metal cations such as Cs+ results in short-range liquid order in water as a primary solvation shell of about eight water molecules is established about the metal cation. Lithium, however, exerts a much greater polarising power and is capable of organising a first- and second-coordination sphere of about 12 water molecules about itself, resulting in a much larger hydrated radius for the ion and hence decreased ionic mobility. 

Anomalous X-ray diffraction or resonant scattering refers to the modification of its intensity due to absorption processes involving interactions between the X-ray beam and the atoms in the sample. This interaction combines the chemical and short-range order sensitivity of absorption with the long-range order sensitivity of diffraction. This results in a chemical selectivity, i.e. it is possible to differentiate elements with close atomic numbers or even cations with the same number of electrons like Rb+ and Sr2+... 

The structure of crystalline FeO belongs to the NaCl type. When iron(II) oxide is prepared under normal conditions, the composition of the product (wustite) is always Fei 50. In order to retain overall electric neutrality, part of the Fe2+ is oxidized to Fe3+, and the chemical formula becomes Fc Fe. O. Since the radius of Fe3+ is small, the Fe3+ cations tend to occupy the tetrahedral holes to form a short-range ordered Fe40io cluster, which is called the Koch cluster of Fei 0, as shown in Fig. 10.1.3. The Koch clusters are distributed randomly in the crystal structure. To satisfy charge neutrality, the formation of a Koch cluster must be accompanied by the presence of six Fe2+ vacancies, one of which is located at the center of the cluster, and the remaining five are distributed randomly at the centers of the edges of the cubic unit cell. 

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