High cristobalite

Fig. 7. Structure of high cristobalite, where O and 0 represent Si and O, respectively (44).

Crystalline Silica Three principal polymorphic forms exist at atmospheric pressure. These are quartz, tridymite, and cristobalite. Quartz is stable below 870°C. It transforms to tridymite form at about 870°C. Tridymite is stable up to 1,470°C and transforms to cristobahte at 1,470°C. High cristobalite melts around 1,723°C. Other than these three polymorphs, there are also three high pressure phases of crystalline sihca keatite, coesite, and stishovite. 

The structure of boron phosphate prepared under normal atmospheric conditions consists of tetragonal bipyrimids analogous to the high cristobalite form of silica. Both the boron and phosphorus are tetrahedrally coordinated by oxygen. Similar silicalike structures are found for BAs04 and TaB04 (156). A quartzlike form of boron phosphate can be prepared by heating the common form to 500°C at 5.07 GPa (50,000 atm) (157). 

Refinement of the framework coordinates was difficult because of the acentricity and pseudosymmetry. A stable least-squares solution was obtained for the framework atoms in which the T-0 distances indicate alternation of A1 and P atoms. However, the 0 atoms showed large displacements from the centroid, particularly 0(2) for which a difference-Fourier map indicated three spearate peaks (Figure 3). Because there is no optical or X-ray evidence for symmetry lower than hexagonal, it is assumed that there are microdomains with tilted tetrahedra, as proposed for high-cristobalite(6) and high-tridymite( 7). For convenience, the displacements of the oxygen atoms are approximated by ellipsoids. 

TrT.oo 10 79 250 K(high quartz high cristobalite) 1996 5 K(high cristobalite liquid)... 

High cristobalite is the stable phase near It becomes metastable with respect to quartz at lower temperatures but... 

The enthalpy of formation is calculated from that of quartz using Aj.H (970 K) 0.45 0.15 kcal mol" for high quartz high cristobalite, as determined by Holm (1 ) from is an oxide melt. This value reduces to A H (298.15 K) = 0.60 kcal mol for... 

H is derived from enthalpy data (, l ) for high cristobalite using the adopted... 

FIG. 5. Differences in standard Gibbs energy of formation among various forms of silica using high cristobalite as a reference state (mainly after Mosesman and Pitzer, 1941 from Muanand Osborn, 1965 ). 

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. 

Fig. 6 Atomic arrangement in the high-cristobalite unit cell viewed down an a-axis. Small darker and large lighter spheres represent oxygen and silicon ions respectively. As in Fig. 4, the relative sizes of these ions correspond to the significant degree of covalency in the Si-O bond [11]...

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]). 

Based on Pauling s radii, how do you expect the lattice parameters of Si and Si02 (high cristobalite) to... 

How do the densities of high cristobalite and silica glass compare You will need to dig for the data on this one—the library or the Internet. 

Ring silicates Beryl Rings of Si04 tetrahedra High cristobalite 0.713 2.32... 

Boron Phosphate. BPO4 vaporizes at 1400 C sp. gr. 2.81 related structurally to high-cristobalite. It has been used as a constituent of a ceramic body that fires to a translucent porcelain at 1000°C. Boron Phosphide. There are two compounds BP, Sp. gr. 2.97 Knoop Hardness (lOOg) 3200 decomposes at 1130 C. B13P2, sp. gr. 2.76 Knoop Hardness (100 g) 3800 melts or sublimes at approx. 2000 C. 

Figure 3.16 Structures of (a) HP-tridymite (high tridymite) and (b) P (high-)-cristobalite.

---