Space groups isometric

Chromium(III) oxide crystallizes in the rhombohedral structure of the corundum type space group D3d-R3c, Q 5.2 g/cm3. Because of its high hardness (ca. 9 on the Mohs scale) the abrasive properties of the pigment must be taken into account in certain applications [3.44], It melts at 2435 °C but starts to evaporate at 2000 °C. Depending on the manufacturing conditions, the particle sizes of chromium oxide pigments are in the range 0.1-3 pm with mean values of 0.3-0.6 pm. Most of the particles are isometric. Coarser chromium oxides are produced for special applications, e.g., for applications in the refractory area. 

Gallium(III) arsenide (GaAs) is not naturally occurring, but it is manufactured and extensively used as a semiconductor (Chapter 5). The crystalline structure of GaAs resembles sphalerite (ZnS, isometric, F43m space group), where each arsenic atom is coordinated by four gallium atoms (Klein, 2002), 339 (Bloss,... 

Isaacs studied the structure of zaratite [63ISA], The cell dimensions are a = 6.16 A, Z = 1 ceii = 233.74 p(calc.) = 2.31 g-cm" , crystal system isometric, space group unknown (in part amorphous). 

The group corresponding to the complete set of equivalent, isometric conformations is thus of order 24x 16 = 384. However, since the complete conformational space has a translational period of jt along each of the four directions, we can always consider a cell with this smaller periodicity. In terms of this cell the general positions of Table 2.8 still apply but have to be taken modulo n instead of 2rt. 

Crick and Watson were the first to suggest that small viruses were built up of small protein subunits packed together symmetrically to form a protective shell for the nucleic acid. They reasoned that formation of small identical molecules was an efficient use of the limited information contained in the virus nucleic acid. They also realized that, of the types of symmetry possible for a three-dimensional structure enclosing space, only the cubic point groups could lead to an isometric particle, which was the known symmetry of many viruses at the time. Three types of cubic symmetry exist namely,... 

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