Interlayer charge

Influence of interlayer charge density and interlayer hydration... 

It is important to note that the clay minerals which have an interlayer charge between 1.0 and 0.7 contain almost exclusively potassium as an interlayer ion. In minerals with a lower charge 0.6-0.25 a large charge is less, no alkali or alkaline earth elements are present. 

The pronounced anisotropy of the electrical conductivity in layered compounds (8,15) suggests that the charge carriers move, on their way to the surface, predominantly within the layers, i.e. parallel to the main surface as shown in Fig. 14 (in which the relative path of the charge carriers within the layers is compressed). The random character of interlayer charge transport due to extrinsic conduction leads to a variety of possible paths, two of which are represented in Fig. 14. 

Figure 14. Schematic of trajectories of minority carriers for an ideally smooth surface in contact with an electrolyte. The random character of the interlayer charge transport is also indicated (small arrows). The average minority carrier motion is given by the large arrows.

Layered double hydroxides (LDHs), also known as anionic clays and hydro-talcite-like materials, are layered solids that are of increasing interest [5-8]. They consist of stacks of positively charged hydroxide layers with interlayer, charge-balancing, anions and are available as naturally occurring minerals [9] and as synthetic materials. They were prepared in the laboratory in 1942 when Feitknecht reacted dilute aqueous metal salt solutions with base [10, 11], although the first detailed structural analyses of LDHs were not performed until the late 1960s by Allmann and Taylor and their co-workers [12-15]. 

This situation, clear in the case of more ionic structures, is less stringent in graphite intercalates where, presumably, there is electron transfer to (in the case of alkali-metal intercalates) or from (in the case of metal halide intercalates) the half-filled conduction bands of the graphite layers (produced by overlap of the 7t orbitals). Similarly, the periodicity requirements are less stringent for the alternating composite layers of layer silicates with complex intralayer and interlayer charge balance. 

Surface interlayer contraction creates the low-energy component of the CLS [8]. For example, a 12 % contraction of the first-layer spacing Nb(OOl) shifts the 3d3/2 by 0.50 eV [4] a (10 3) % contraction of the first-layer spacing of Ta(OOl) offsets the 4fs/2(7/2) by 0.75 eV [8]. The surface bond contraction enhanced interlayer charge density, and resonant diffraction of the incident light is supposed to be responsible for the positive energy shift [4, 8]. 

Although each of the proposed reactions is based on ample experimental evidence, the role of the different interdependent mechanisms can hardly be elucidated in the individual case, because with the known analytical methods, a determination of the definite structural coordination of protons in hydrated micas seems to be impossible. Nourish [1972] pointed out that, for natural minerals of the mica-vermiculite sequence, charge reduction can, in most cases, be sufiiciently explained by iron oxidation. This was concluded from the fact that in the vast majority of these minerals, the sum of interlayer charge and octahedral ferric iron per structural unit exceeds a value of 2. 

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