Octahedral sublattice

In montmorillonite some of the Al3 + in the octahedral sublattice are replaced by Mg2+ ions, and in hectorite some of the Mg2 + in the octahedral sublattice are replaced by Li+ ions. With beidellite and saponite, however, the isomorphous substitution takes place in the tetrahedral sublattice with Al3 + replacing some of the Si4+ ions. The residual negative charges in the layers on montmorillonite, hectorite, beidellite, and saponite are counterbalanced in the natural state by coexisting interlamellar, hydrated cations, usually Na+,... 

Additionally, it was proposed that reaction 19 occurred only in the outermost 3-4 atomic layers of the magnetite crystallites. The MOssbauer spectrum of the catalyst in the reduced form agreed with this substitution. The spectral parameters of the tetrahedral cations were unaffected by the substitution, whereas the isomer shift and magnetic hyperfine field of the octahedral cations decreased. Also, the line width of the octahedral cations increased relative to an unsubstituted catalyst. Finally, the spectral area ratio of the iron cations in the tetrahedral to octahedral sublattices decreased. 

Fig. 10.14 Transmission spectrum of polycrystalline Mg Fc204 in a longitudinal field of 55 kG using a single-line source. The tetrahedral and octahedral sublattices are shown. The top bar diagram is the emission spectrum of the a-iron source, and the bottom diagram is the absorption spectrum for the a-iron absorber. Both are polarised. [Ref. 70, Fig. 2]...

In a subsequent paper, the same authors [40], by means of the deposition of ultrathin Fe304(00l) layers (0.4-2.4 nm) on Fe3 O4 (001) layers 20 nm thick, observed that the electronic structure of the surface layers of magnetite is strongly modified as compared to the bulk with a redistribution of the electronic char ge in the two or three outermost atomic layers. In particular, they observed that the intensity of the surface component associated with the Fe + octahedral ions was lower than the expected value either because the surface is rich in Fe + ions (due to vacancies in the octahedral sublattice) or because the electron hopping freezes at the outermost atomic layers. 

Figure 2.36 (Left) The corner-sharing octahedral sublattice of the B-site ions (BOA in the pyrochlore structure. The A ions are shown as light spheres. (Right) The A-site hexagonal bipyramidal polyhedra (AOs) in the pyrochlore structure. The B-site ions are shown as light spheres...

Partial molal entropies of cerium hydride as a function of r as calculated from the data of Lundin (1966), Streck and Dialer (1%0), and Hardcastle and Warf (1966) are shown in fig. 26.8. The minimum in the curve at about H/Ce = 2 is probably due to a maximum in the configurational entropy at this composition since hydrogen atoms enter the octahedral interstices in the rare earth dihydrides before all the tetrahedral sites are occupied (see section 2.2). Therefore, there is disorder in both the tetrahedral and octahedral sublattices near the stoichiometric dihydride composition. 

Figure 9.10 F6304— 7-Fe203 iransforinaiions induced by adsorption of various species. Dashed circles represent the intial size of the colloid in each transformation. In each case, adsorption is assisted by outward electron transfers within the particle, which renew the reactive chemical species on the surface (Fe ions). The surface may be oxidized in situ (steps I to 3) or desorbed (steps 4 and 5) depending on the physicochemical characteristics of the medium. In all steps, electron transfers are associated with migration of Fe from the octahedral sublattice towards the surface of the particle, causing formation of cation vacancies. Step 5 is apparently reversible. Adsorption of Fe and HO" (step 6) is also assisted by electronic and ionic (H ) inward transfers, but without appreciable Fe migration. This results in the epitaxial growth of an Fc304 layer...

Other structural analogues with two octahedral sublattices are also known. For example, for charge combination of B-cations (1+6). The structure of the compositions... 

---