Cristobalite structure

By mutual rotation of the tetrahedra the a-ZnCl2 structure is converted to the cristobalite structure... 

Figure 21. Experimentally (trydimite) and theoretically (a-cristobalite) structures proposed for the polymeric carbon dioxide.

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. 

Figure 8.4 Structure of CsTaN2 and CsNbN2. Examples of ternary nitrides in the jS-cristobalite structure. Large white = Cesium, Black = Tungsten/Niobium, Small white =...

Until recently, when Peri 155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Br nsted and Lewis acid sites, Peri 156) had proposed the existence of a sites on the Si02—A1203 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HC1, are adsorbed very selectively on these a sites, whereas NH3 and H20 are also held by many other sites 157). To rationalize the formation of these sites, Peri 155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of A1C13 with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure 158). It should be mentioned in this connection that Peri s surface model of silica may... 

Si-O cristobalite-, tridymite-structure e.g. quartz-, tridymite-, cristobalite-structure... 

These mixed oxides have been considered as nonlinear optical materials (see Nonlinear Optical Materials). They crystallize as ordered derivatives of the cristobalite structure. As early X-ray structural studies were performed on twinned crystals grown from congment melts, their true symmetry was not firmly established. Untwinned single crystals have been produced by hydrothermal methods, and the space group has been convincingly established as 14. 

From its sensitivity to short-range structure and low-frequency dynamics, NMR spectroscopy provides some constraints on the nature of the P-cristobalite structure. For cristobalite, only powder NMR techniques can be applied, because large single crystals do not survive the a-P transition intact. It is helpful to consider also AlP04-cristobalite, because it is similar in essential respects to Si02 and the cubic point symmetry of the Al-position in the average structure of the P-phase provides some additional constraints. 

We noted that BP crystallizes with the zinc-blende structure. Replacement of alternate Si atoms in the cristobalite structure by B and P atoms gives the structure of BPO4 in fact, BPO4 crystallizes with all the structures of the normal... 

In the silica structures tetrahedral Si04 groups are linked together through their common oxygen atoms. We noted earlier (Fig. 3.32) that the same topological possibilities are presented by the H2O molecule and by a molecule or ion of the type 02M(0H)2- There are a number of forms of crystalline H2O (ice). In addition to those stable only under higher pressures, which are described on p. 537, there are two forms of ice stable under atmospheric pressure. Ordinary ice (ice-I/,) has the tridymite structure, but at temperatures around — 130°C water crystallizes with the cristobalite structure. The tridymite structure is related to the net of Fig. 3.35(c), 106... 

All the layers are in fact slices of the cristobalite structure, which is the end-member of the series, and the Si307 layer is the anion in Na2Si307. This A3X7 layer, in which all circuits are rings of six tetrahedra, may be compared with the simple A3X7 layer in melilite and related compounds which is composed entirely of rings of five tetrahedra. We referred to this layer under pyrosilicates it is illustrated in Fig. 5.7(c), p. 163. In both A3X7 layers two-thirds of the tetrahedra share 3 vertices and the remainder 4 vertices. 

GaP04 (cristobalite structure) GaSb04 (statistical rutile structure)... 

Cristobalite has been well-characterized since the late fifties [7,9], The high-cristobalite structure is characterized by a continuously connected network of (Si04)4-tetrahedra and is summarized in Table 3. The atomic model of the high-cristobalite structure in Fig. 6 [11] was generated with Accelrys Catalysis 3.0.0. Also, the Si-0 distances have been noted to range between 0.158nm and 0.169nm. 

LiPN2 forms with the filled cristobalite structure [57, 58], The PN, array mimics that of the isoelectronic Si02, with the Li ions in tetrahedral interstices. Alternatively, the structure may be considered a ternary derivative of the wurtzite structure (as exemplified by AIN). 

Calcium forms numerous ternary nitrides. CaGeN, crystallizes with the stuffed p-cristobalite structure [126]. Gray CaSiN- has been synthesized but not analyzed with diffraction techniques [127J. although it is likely isostructural. 

Phospham or phosphonitrilic amide, PNNH, is a fascinating material also isoelectronic with Si02 [213], Its formula suggests a relationship to LiPN2 with the filled high cristobalite structure, but it is amorphous to X-ray analysis. Phospham has been used in the synthesis of phosphorus-containing nitrides [111], There is an extensive literature on phosphonitrile polymers (for a review, see [214]). 

A co-ordination of 4 2 is found among AX% halides only in BeF2, which has the (idealized) / -cristobalite structure, named after one of the forms of Si02. The cubic unit cell of this structure is shown in... 

Fig. 8.07. Clinographic projection of the unit cell of the (idealized) cubic / -cristobalite structure.

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