Atomic distance bond-length, lattice constant

The unit cell of ThCr2Si2-type is displayed in fig. 1. This type of crystal structure consists of tetrahedra composed of X atoms with a transition metal inside. The X-X distances are usually close to the sum of covalent radii of X atoms, similarly as are the T-X contacts. Thus strong chemical interactions are expected within the layers composed of tetrahedra which in turns, consist of 4X atoms. The bond lengths are critically dependent on the magnitude of the z parameter and the da ratio (a, c are the lattice constants). 

From Fig. Ic one notices that in the NaTl structure both the Na and Tl atoms form diamond-like sublattices (Fig. Id). The diamond structure arises from the NaTl structure by leaving vacancies V in one sublattice (Table 1). Therefore in the B32 structure (Fig. Ic) the bond lengths d(A-A), d(A-B) and d(B-B) are equal and the atoms A and B must have equal sizes to get a spacefilling distribution . It follows that the atomic radii in B32 type compounds rB32 should be half of the distance to the nearest neighbours. In Table 2 the lattice constants for the binary Zintl phases are listed. The resulting atomic... 

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