Trigonal crystal structure

Q-Quartz, which has a trigonal crystal structure, undergoes a rapid, reversible transition to hexagonal /J-quartz at 573 °C and then slowly changes to hexagonal /3-tridymite at about 870 °C tridymite in turn goes over slowly to cubic /3-cristobalite at 1470 °C, and this melts at 1713 °C. The reversion of cristobalite and tridymite to quartz is slow, so that these forms can exist at room temperature (as a-modifications). In addition, dense modifications with six-coordinate Si are found in shocked rocks associated with meteorite impact craters coesite forms only above 450 °C and 3.8 GPa, and stishovite requires over 1200 °C and 13 GPa. Survival of those metastable polymorphs on the geological timescale is evidence of an extremely slow recrystallization rate. 

Valluzzi, R. Gido, S.P. Zhang, W. Muller, W.S. Kaplan, D.L. A trigonal crystal structure of Bombyx mori silk incorporating a threefold helical chain conformation found at the air-water interface. Macromolecules (1996). 

UiZnn. This material, having a rhombohedral (trigonal) crystal structure (space group R3m) is another example of a well-established HF compound (Ott et al. 1984a)... 

Silicas (Si02) exist in crystalline and amorphous forms. Crystalline silica has three diiferent aUotropic crystalline forms hexagonal with trigonal crystal structure, which occurs in the mineral quartz cubic with tetragonal crystal structure, which occurs in the mineral crysta-bolite and a second hexagonal form in the mineral tridymite. Crystalline sihcas various aUotropic crystal structures are created by pressure and temperature in the crystal forming stage. 

Figure 3.43 Polarized Light micrographs of isotractic polypropylene crystalhzed at 135°C for 50 minutes (a) and 62 minutes (b) in a ca. 25-pm-thick film. The weakly birefringent object 5 is based on monoclinic a crystals and is not of interest here. All other objects have the trigonal crystal structure. Object 1 is a flat-on view of a hedrite. Objects 2,3, and 4 are other views of hedrites, subject to possible growth restrictions. Note particularly the evolution of the small plate-like aggregate 2 into an oval shape 3. From Varga and Ehrenstein [61] with kind permission from Springer Science-tBusiness Media B.V.

Figure 10.2 Crystal structure of GeF2 (a) projection along the chains, and (b) environment of Ge (pseudo trigonal bipyramidal). The bond to the unshared F is appreciably shorter (179 pm) than those in the chain and there is a weaker interaction (257 pm) linking the chains into a 3D structure.

Turning finally to compounds with bonds from the heavier Group 14 elements to heavier Group 15 elements we may note compounds such as [Sn C(PMe2)3)2] which has the pseudo trigonal bipyramidal structure (6). This complex, which has Sn bonded exclusively to four P atoms, is formed as yellow crystals by the... 

Crystal Structure and Lattice Parameters (nm) Orthorhombic, a = 0.283, b = 0.554, c = 1.1470 Cr3C2 is an intermediate carbide having carbon chains with C-C distance approximately 0.165 nm running through distorted metal lattice where the Cr atoms are at the corners of trigonal prisms and the carbon atoms in the center of the prisms.i li" ... 

Fig. 21.3 The crystal structure of BasBPO . The dark tetrahedrons represent [P04] groups the light planar trigons represent [BOs] groups the light spheres represent Ba atoms.

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