Cations, silicon-containing formation

Different crystallographic planes of a semiconductor electrode usually exhibit different reaction kinetics. It was found in III-V compounds in indifferent electrolytes that the (lll)B face terminated with the anion plane (P, As) etched faster than the (lll)A face containing the cations (Ga, In) [47]. The planes composed entirely of metal atoms react more slowly than any other crystal plane because of the stable metal oxide layer, which can be formed on such planes. Consequently on these planes termed etch stop planes, provision of reactants (diffusion control) is not rate-limiting. In Si, the (100) planes are known to etch faster than (111) planes in alkaline solutions. This property is at the origin of various apphcations, such as texturization of silicon surface [formation of pyramids on (100) planes], which allows reduction of reflectivity of the front surface of solar cells and Si micromachining [48]. The semiconductor surface may be shaped during the anodic dissolution... 

Over the past years a number of small silicon-containing ions have been structurally characterized by collisional-activation mass spectrometiy. These ions have been used as precursors for the generation of their neutral counterparts via Neutralization-Reionization Mass Spectrometry (NRMS) . To a large extent the interest in small elusive silicon-containing molecules is due to the role which these species have been postulated to play as intermediates and building blocks in the genesis of interstellar matter" . In particular, the different conceivable pathways which lead from atomic silicon cations to the formation of neutral silicon-containing molecules have been studied intensely" A second... 

Some of the diversity that characterizes the properties and compositions of the silicate minerals stems from the ability of the aluminum ion (Al ) to substitute for silicon in the tetrahedral unit. When silicate tetrahedra in a mineral are replaced by aluminum-containing tetrahedra, concomitant changes occur in the size of the tetrahedron (usual Si—O bond length = 0.160 nm. A1—O bond length = 0.178 nm) and in the cations or protons that balance the tetrahedral unit charge. Regular substitutions with distinct chemistries and structures lead to the formation of groups of discrete minerals called aluminosilicates. 

A further enhanced degree of reduction leads to chains of group 14 elements with Ne values between 6 and 6.6 e/atom. These values cover the range between twofold bound silicon atoms in one-dimensionally extended zigzag chains and 2 dumbbells. Isolated chains reside in between these border structures and therefore compete with them [14]. This leads to the expectation of an extreme cation size dependence of the anion formation. Table 4 contains the reported isolated homoa-tomic chains in solid state compounds. 

Compounds closely related to silica that are found in most rocks, soils, and clays are the silicates. Like silica, the silicates are based on interconnected Si04 tetrahedra. However, in contrast to silica, where the 0 Si ratio is 2 1, silicates have 0 Si ratios greater than 2 1 and contain silicon-oxygen anions. This means that for the formation of the neutral solid silicates, cations are needed to balance the excess negative charge. In other words, silicates are salts containing metal cations and polyatomic silicon-oxygen anions. Examples of important silicate anions are shown in Fig. 16.30. 

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