Crystal atomic distance, bond-length

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). 

The X-ray data and theoretical B3LYP/6-311g modeling show that the introduction of a bromine atom into the 4- -position of THF-l-boraadamantane adduct framework (THF - tetrahydrofuran) leads to the shortening of the B-O bond length (162.1(3) pm instead of 164.7(2) in parent compound) due to the intramolecular polarization or polarization by polar crystal media ( tail-to-tail dimers, Br—Br distance is equal to 374.3 pm) <2004MC189>. 

Clarity requires that a distinction be made between elastic strain and plastic deformation. They both have units of length/length, but they are physically different entities. Elastic strain is recoverable (conservative) plastic deformation is not (non-conservative). At a dislocation core, where atoms exchange places via shear, the plastic displacement gradient is a maximum as it passes from zero some distance ahead of the core, up to the maximum, and then back to zero some distance back of the core. In crystals with distinct bonds, the gradient becomes indefinite (infinite) at the core center. 

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