Cristobalite, atomic structure

Figure 11.1 Comparison of the atomic structure of cristobalite (high temperature form of Si02) with that of silicon (diamond structure using tetrahedral unit cell).

The product from (4.188) is an ionic compound which contains discrete PN4 tetrahedra and can be formulated as 7(Li+) (PN4) (4.190a) [27]. The product from (4.189) on the other hand has an isoelectronic Si02 cristobalite-type structure built from a network of PN4 tetrahedra with each N atom linked to two P atoms and each of the latter linked tetrahedrally to four N atoms (cf. the P2NH structure) (Figure 4.17). The short bond length of 1.64A, determined experimentally, indicates a possible contribution from P=N linkages (4.190b) [28]. 

FIGURE 1437 the structure of cristobalite is like that of diamond except that an O atom (light red) lies between the Si atoms (purple). The arrangement about each Si atom is shown in structure 11. 

The insertion of the oxygen atoms widens the silicon lattice considerably. A relatively large void remains in each of the four vacant octants of the unit cell. In natural cristobalite they usually contain foreign ions (mainly alkali and alkaline earth metal ions) that probably stabilize the structure and allow the crystallization of this modification at temperatures far below the stability range of pure cristobalite. To conserve electrical neutrality, probably one Si atom per alkali metal ion is substituted by an A1 atom. The substitution of Si... 

Fig. 3.11. Schematic two-dimension representation of the structure of cristobalite (a crystalline form of Si02) and of vitreous Si02. Si atoms are represented by full circles, oxygen by open circles. A, B and C represent three cases of double possible equilibrium positions for the atoms of the material in the amorphous state A, transversal displacement B, longitudinal displacement C, small-angle rotation of the Si04 tetrahedron. 

Cadmium cyanide, CdCCN), is analogous to Si02 with respect to the AB2 composition, the tetrahedral confiugration of A, the bridging behavior of B between a pair of A atoms, and the ability to build a three-dimensional framework in which cavities of molecular scale are formed. Cadmium caynide itself crystallizes in a cubic system of the anticuprite type, in which two identical fr-cristobalite-like frameworks interpenetrate each other without any cross-connection the cavity formed in one framework is filled by the other. When we replace one of the frameworks by appropriate guest molecules such as those of CCl, CCl CH, etc., we may obtain a novel clathrate structure with an adamantane-like cavity, as shown in Fig. 1 [1], Our results including those recently obtained are summarized in Table 1. 

In earlier literature reports, x-ray data of a-based ceramics, the /3-like phase observed in certain silica minerals was explained by a structural model based on disordered Q -tridymite. However, others have suggested that the structure of the stabilized jS-cristobalite-like ceramics is closer to that of a-cristobalite than that of Q -tridymite, based on the 29Si nuclear magnetic resonance (NMR) chemical shifts (Perrota et al 1989). Therefore, in the absence of ED data it is impossible to determine the microstructure of the stabilized jS-cristobalite-like phase. ED and HRTEM have provided details of the ceramic microstructure and NMR has provided information about the environments of silicon atoms in the structure. Infrared spectroscopy views the structure on a molecular level. 

Crystalline Silica. Silica exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a liquid. The literature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of silica, there are three main forms at atmospheric pressure quartz, stable below about 870°C tridymite, stable from about 870—1470°C and cristobalite, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the structures are based on Si04 tetrahedra linked in such a way that every oxygen atom is shared between two silicon atoms. The structures, however, are quite different in detail. In addition, there are other forms of silica that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of silicon is six rather than four. 

The yff-cristobalite structure is named after one mineral form of silicon dioxide, S102. The silicon atoms are in the same positions as both the zinc and sulfurs in zinc blende (or the carbons in diamond, which we look at later in Section 1.6.5) each pair of silicon atoms is joined by an oxygen midway between. The only metal halide adopting this structure is beryllium fluoride, Bep2, and it is characterized by 4 2 coordination. 

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