Amorphous intermediate, direct formation

Stoichiometry has long been used by solid-state chemists to control the final products of a reaction. However, traditional synthetic techniques do not have the ability to control reaction intermediates and all stable phases will form as illustrated in Figure 2. For example, in the iron-silicon system, thin film diffusion couples have been used to determine the sequence of phase formation (79). FeSi was always found to nucleate first, followed by the crystallization of FeSi2 at the FeSi-Si interface and FeSi3 at the FeSi-Fe interface. The following paragraphs provide evidence that stoichiometry of the amorphous intermediates can be used to control nucleation to obtain the desired crystalline compounds directly. Thus, we use stoichiometry to control the mechanism of the reaction. 

A12(S04) decomposes directly to A1203 without the formation of an intermediate.3 However, different researchers have obtained A1203 in different states. Knutsen and Searcy found it to be amorphous,4 whereas Ingraham obtained y-Al203.5... 

The HRTEM study of diamond nucleation and growth on copper TEM grids in HFCVD by Singh provides direct evidence for the formation of a diamond-like amorphous carbon layer. The intermediate layer is 8-14 nm thick, in which small diamond microcrystallites approximately 2-5 nm across were embedded (Fig. 6a), and large diamond crystallites were observed to grow from these microcrystallites (Figs. 6b-c). 

Direct precipitation of pyrite without intermediate iron sulfide precursors was reported for salt marsh sediments, where pore waters were undersaturated with respect to amorphous FeS but oversaturated with respect to pyrite (Howarth 1979 Giblin and Howarth 1984). In these sediments, the oxidizing activity of the roots favored the formation of elemental sulfur and polysulfides which were thought to react directly with Fe +. The direct reaction pathway may proceed within hours, resulting in the formation of single small, euhedral pyrite crystals (Rickard 1975 Luther et al. 1982). Framboidal pyrite, apart from that formed by the mechanism presented by Rickard (1997), is formed slowly (over years) via intermediate iron sulfides (Sweeney and Kaplan 1973 Raiswell... 

Although other probes may be substantially easier to use, XAS is sometimes the only method that is sensitive to the structures of interest, particularly for solid-state samples and in situ studies of catalysts.For example, in a study of the reduction of NiO, time-resolved X-ray diffraction had shown that the catalyst went directly from NiO Ni without a well-ordered intermediate phase, but could not rule out the existence of an amorphous NiO phase, since the diffraction was not sensitive to disordered phases. However, the formation of an intermediate phase could be ruled out by time-resolved EXAFS. A similar situation exists for spectroscopically silent metals systems), which are difficult to probe with methods other than XAS. Examples that are important in bioinorganic chemistry include Cu and Zn. For carboxypeptidase, time-resolved XAS could be used to determine the rate of change of the native site, while conventional UV-visible methods could only be used on the Co " " substituted enzyme. The importance of XAS in both of these examples is as a tool for measuring rate constants. 

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