Widened Diamond Lattices. SiO2 Structures

The insertion of the oxygen atoms widens the silicon lattice considerably. A relatively 

Left snapshot the numbers indicate the height of the atoms in the direction of view as multiples of j. 

The phase diagram for SiO2 is shown in Fig. 12.11. The transition between a- and ji-quartz only requires minor rotations of the SiQ4 tetrahedra, the linkage pattern remaining unaltered (Fig. 18.9, p. 224) this transition takes place rapidly. The other transitions, on the other hand, require a reconstruction of the structure, Si-O bonds being untied and rejoined they proceed slowly and thus render possible the existence of the metastable mod- 

Only SiO4 tetrahedra are shown. Numbers designate the heights of the Si atoms in the tetrahedron centers as multiples of of 

The symbols X for 82 screw axes mark the axes of the helical chains. The slight tilting of the tetrahedra relative to the direction of view (c axis) vanishes in /3-quartz (stereo image) 

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