Epitaxial hypothesis

The physical stmcture of mixed-layer minerals is open to question. In the traditional view, the MacEwan crystallite is a combination of 1.0 nm (10 E) non-expandable units (iUite) that forms as an epitaxial growth on 1.7 nm expandable units (smectite) that yield a coherent diffraction pattern (37). This view is challenged by the fundamental particle hypothesis which is based on the existence of fundamental particles of different thickness (160—162). 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

The catalytic reactivity in the range of 1 to 3 ML Pd on Au(llO) is surprisingly low taking into account the noticeable amount of Pd measured by LEIS on the surface (Fig. 17). This low reactivity could be due either to a low number of active sites (two adjacent Pd atoms) resulting from chemical order or to the tensile stress applied to Pd atoms in pseudomorphic epitaxy with Au substrate. Such a hypothesis would be in agreement with the theoretical predictions of Pallassama and Naurock [21] who propose that Pd stressed in... 

Epitaxy may reasonably be mentioned for the VOx/anatase system. But there are other cases where epitaxy is doubtful. There is no detectable evidence of Sb204 decorating epitaxially FeSb204. If the picture of oppositely oriented flat pyramids as active sites for butane oxidation is correct, the hypothesis attributing a crucial role to an epitaxy between various vanadium phosphates may lose credibility. 

Finally, it should be emphasized that the tendency of polyimides containing aliphatic spacers to form stable layer structures of various dimensions and degrees of order can be utilized for an even wider variety of studies. A recent example is a study of the epitaxial growth of polyethylene on smectic crystallites of PEI [140]. Most likely smectic crystallites adopt a lamellar form with the large surface covered by loops of the aliphatic spacers. This hypothesis still need detailed studies and confirmation. Anyway, smectic crystalline polymers are interesting substrates for studies of epitaxial crystallizations. Furthermore, layer structures derived from long aliphatic spacers (alkanes or oligoethers) may play... 

All vapor phase epitaxial growth processes involve the interaction of the vapor with the surface of the solid phase, thereby demanding the inclusion of heterogeneous kinetics into the overall rate discussion. The typical description of surfaces is a model based on the hypothesis that surfaces are composed of a fixed number of sites on which... 

Other chapters of this book cover modem high-yield MgC supported ZN catalysts [11, 38, 39] here 1 will only elaborate on their structural relationships with violet TiCls. As a matter of fact, it is plausible to imagine that the chemisorption of TiCLi on lateral terminations of MgCl2 stmctural layers, followed by alkylation and reduction of the adsorbates by an Al-alkyl, results into local environments mimicking the edges of authentic violet TiCla stmctural layers. In other words, according to this hypothesis, MgCl2 would act as a template for the epitaxial adsorption of TiCl species (rt=4 or 3) [6]. 

Epitaxial crystallization and crystalline polymorphism of PLAs have been studied by Cartier et al. [37]. A c-axis projection of the unit cell that is assumed as a regular IO7 helix conformation is shown in Figure 6.7. This hypothesis... 

In the same case where an epitaxial layer is not detected, its existence is proposed as the basic hypothesis. 

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