Body-centered cubic packing bep

Body-centered cubic packing (bep) is a common structure type among metals. The space group is 0 -Im3m, and the atomic coordinates of equivalent positions are listed in Table 10.3.1. 

The positions of interstices in the bcp structure (a) octahedral interstices (small black spheres), (b) tetrahedral interstices (small open spheres). 

Distorted tetrahedral interstices also occur on the faces of the unit cell. Each face has four such distorted tetrahedral interstices with a radius ratio r/R = 0.291, as shown in Fig. 10.3.1(b). Each sphere corresponds to six of these tetrahedral interstices. 

These two types of interstitial polyhedra are not mutually exclusive i.e., a particular part of volume in the unit cell does not belong to one particular polyhedron. In addition, distorted trigonal-bipyramidal interstitial polyhedra are each formed from two face-sharing tetrahedra. The number of such interstices that corresponds to one sphere is 12. Therefore, each sphere in the bcp structure is associated with three octahedral, six tetrahedral, and twelve trigonal-bipyramidal interstices, i.e., a total of 21 interstices. The size and distribution 

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