SiN4 tetrahedra

Both modifications are based on SiN4 tetrahedra connected at the corners. The dimension of the SiN4 tetrahedron is very similar to that of Si04. Therefore... 

Summary Systematic investigations enabled a broad synthetic access to the nitridosilicate class of compounds. Typical building blocks in these compounds are SiN4 tetrahedra, which may be linked via common vertices forming complex network structures. The first results suggest that the structural variabilities of nitridosilicates are more versatile as compared to the oxosilicates. Highly cross-linked network structures are accessible, which cause the remarkable stability of nitridosilicates. 

Sr2SisN8 and Ba2SisNg are isotypic and have a network structure of corner-sharing SiN4 tetrahedra similar to Ca2SisN8. However both structure types are topologically different [10] as the... 

The SiN4 tetrahedra form layers built up by Vierer and Achter rings. These layers are stacked along [001] and are connected by double tetrahedra bridging the Achter rings (Fig 6). The contact distances M -N correspond to the sum of the ionic radii... 

BaYbSi4N7 containes a network structure of comer sharing SiN4 tetrahedra... 

Figure 24. Crystal structures of trigonal a-Si3N4 (space group / 31c) and hexagonal P-S13N4 (P6 /m), emphasizing the comer-sharing SiN4 tetrahedra.

Figure 30. Crystal structures of monoclinic Ca2Si5N8 (space group Cc) and tetragonal CesSisNn (Sm3Si6Nii type, space group PAbm). The three-dimensional networks of comer-sharing SiN4 tetrahedra are emphasized.

Only a few ternary silicon nitrides have been prepared in a pure form and characterized. The isotypic compounds MSiN2 (M = Be, Mg, Mn, Ca, Zn), with the same valence electron concentration of 4, can be considered as ternary substitution variants of AlN [247]. These compounds contain three-dimensional (3-D) infinite network structures, with SiN4 tetrahedra linked through all four vertices by comer-sharing, which forms condensed [SieNe] twelve-membered rings in MgSiN2 [248]. 

The compound LaSi3Ns was first reported by Inoue et ol. [35], who prepared it at high pressures at 2000 °C. The unit cell of LaSisNs is orthorhombic, with a = 0.784, h = 1.124 and c = 0.481 nm, while the structure (Figure 5.19) is another variant of 3-D- linked [SiN4] tetrahedra, where some of the nitrogens are 3-coordinated by... 

Fig. 16.34 Structure of CaAlSiNs. a Viewed along [001], Ca light gray spheres, [SiN4] and tetrahedra gray. b View showing layers of condensed dreier rings [SIN4] tetrahedra up gray) and down black). Reproduced from Ref. [56] by permission of John Wiley Sons Ltd...

A different class of nitrides also discovered by Schnick et al. are the open framework silicon nitrides CejSigNii andPrjSigNii [100, lOlj.Here the voids of the covalent framework formed by SiN4 tetrahedra are filled with the rare earth cations. The biggest channels that can be found in these structures are formed by six-membered rings. From the point of view of the zeolite chemist this does not seem too large. However, silicon nitrides offer a variety of features that find several applications, as they are very inert and thermally stable. 

While sp hybridization of silicon realizes SiN4 tetrahedra to form the structure, nitrogen with a ground-state electron structure of ls 2s 2p must have a filled p orbital perpendicular to the plane of the sp hybridized valence orbitals necessary to produce the planar Si3N groups that are the... 

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