Homoatomic Polyanions of Group 15 Elements in Solid State Compounds

2 Homoatomic Polyanions of Group 14 Elements in Solid State Compounds 

Obviously, it is possible to produce bigger clusters at Aig = 4.76 by stabilizing them with suitably sized cations, available by electroequivalent substitution of alkali metal by alkaline earth metal cations. 

For the binary alkaline earth sihcide SiSr, two different structures have been reported. One contains one-dimensionally extended zigzag chains beside isolated Si atoms [73]. Schafer et al. prepared a modification with the same composition, which instead contained isolated sUicide units of ten atoms. In these units, planar hexagons are substituted in the 1-, 2-, 4-, and 5-ring positions by four additional Si atoms. An isostructural compound was found for germanium as well, but showing defects in this unit in the positions 1, 2, 4, and 5. Both materials could not be obtained from stoichiometric approaches, and their formation obviously is coupled to strontium excess [69] (Fig. 3). 

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. 

Homoatomic chains are a fascinating group of polyanions, due to the possibility to study multiple bonding of heavier main group elements in the solid state and the coordination behavior of differently charged atoms in the same molecular structure. 

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