Silicon polyanions

Fig. 4a. Structure of Caj jLij jSij (skew [001] view) with planar silicon polyanions of para-linked flat six-membered rings the trigonal prismatic cation array is indicated by the polyhedral representation trigonal prisms and extended trigonal prismatic coordination of the Zintl anion (CN6 (Ca) light blue, CN6 (Li, Ca) green, CN7 (Li, Ca) purple) trigonal prisms coordinating the central part of the anion are exclusively composed of Ca atoms...

Fig. 4b. ELF in the plane of the silicon polyanion clearly showing bonding and lone electron pair regions (white areas) Fig. 4c. ELF cross sections through the Si-Si bonds as numbered in 4b the ELF is more ellipsoidal for the bonds I, 3, and 4 indicating a n-type contribution while 2 and 5 tend to be more o-iike...

Figure 6,4 Ternary plot of singly bonded oxygen 0 , silicon Si and bridging oxygen in each kind of polyanion. Shaded areas compositional ranges 6.31 and 6.33. Reprinted from Ottonello (1983), with kind permission of Theophrastus Publishing and Proprietary Co.

Muller deposited cationic poly(L-lysine) (PLL) and polyanions like poly(maleic acid-co-a-methylstyrene) (PMA-MS) or poly(vinylsulfate) (PVS) by consecutively adsorbing on unidirectionally scratched silicon substrates... 

The polyanions produced on condensation can incorporate non-metal species such as phosphorous and silicon in which case the resulting clusters are referred to a heteropolyanions... 

In CaSi2, a layered polyanion [Sf] is present which consists of puckered Si rings and which is related to a layer in the a-arsenic structure (Fig. la, [14]). SrSi2 forms a cubic arrangement wdiere the triply silicon atoms [(3b)Si] coimect to a threedimensional framework which is a type of its own (Fig. lb,... 

On the basis of the known variety of compoxmds and structures it was reasonable to expect new polyanionic Si arrangements in ternary silicides. The different interactions metal to silicon and the specific spatial requirements of different metal atoms was anticipated to have structure directing influences. We report here about compoimds MxM ySi with M = Ca-Ba and M = Mg. 

Magnesium seems to play a key role in the formation of such new silicides which show a series of novel polyanionic silicon arrangements with fascinating bonding patterns and electronic structures compared to those known hitherto. 

The compounds CaSi, SrSi, BaSi, SrMgSi2, and Ca7Mg725Sij4 are very close to being isoelectronic and thus, on the average, should contain twofold bonded doubly charged silicon atoms (2b)Si , which are isoelectronic with sulfur. Some structural data are given in Table 1 and representative plots of the polyanions in Fig. 2a-2e. 

The new ternary compound SrMgSi2 shows a considerable disproportionation of the silicon atoms into terminal (lb)Si , bridging (2b)Si, and branching (3b)Si" species (Fig. 2b, [31]). As shown in figure 2b the whole polyanion contains a one dimensional planar chain of silicon atoms with syndiotactically arranged side chains [Si(Si3)] . The Si-Si distances vary somewhat between 235 and 249 pm, with the shortest distances between the side chain bridging atoms and those of the main backbone. 

The longer bonds occur at the end of the side chains (245 pm) and between the atoms of the central one dimensional backbone (249 pm). The "functionalisation" of the silicon atoms is not only reflected but seemingly initiated by the distribution of the Mg and Sr atoms. Clearly the latter coordinate exclusively the Si atoms of the central backbone of the polyanion. Those silicon atoms which are in the bridging side chain have a 4-1-3 coordination of Sr and Mg atoms while the terminal (lb)Si groups have an outer coordination of four Mg atoms. The whole one dimensional polyanion looks like a blossomed CaSi chain" with its syndiotactic side chains. 

Cai4Sii9 is an overlooked phase in the binary system with nearly an even number of (3b)- and (2b)-Si atoms per formula unit [31, 48]. It contains a novel two dimensional polyanion of silicon which is displayed in Fig. 6a. The central unit is a 3,3,3-barrelane Sin which is linked via six Si3 bridges to six neighboring barrelanes. The whole polyanion contains seven layers of silicon atoms and has a width of ca. 900 pm. The barrelane part is shown in Fig. 6b with its trigonal planar (3b)-silicon atoms. The calcium atoms are distributed not only between but also inside the layers. 

The consecutive layer-by-layer adsorption of polyanions and polycations built on glass or silicon wafers first (by chemisorption) with a monolayer of positively charged poly(ethyleneimine) (PEI) was recently reported [47]. Analogous hybrid multilayer architectures were prepared in which polycationic polymers... 

The silver silicide (landelli 1985) shows an ordering between the silver and silicon atoms. It crystallizes with the ZrNiAl type. This is one of the rare cases where the silver atoms show no random distribution within the polyanion. 

Finally, the unusual mineral condensed ions (MCIs) certainly qualify as inorganic salts, and these polyanions formed from tungstate or molybdate (as MO4") and central ions such as silicon and antimony are being studied... 

Fig. 6 Force curves measured with a Colloidal Probe AFM. The outer surfaces (Sdicon colloidal probe and Silicon wafer) were coated with different numbers of polyfethylene imine) (PEI, poly-cation)/poly (styrene sulfonate) (PSS, polyanion) double layers. The solid lines correspond to fits by the Eq. 1. Due to sake of clarity the curves are shifted in vertical direction. The polyanion (PSS) was always the outermost layer The abscissa corresponds to the distance between colloidal probe (microsphere) and the Silicon wafer. The graph is taken from [41]...

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