Glide in the NaCl structure

Since the elastic stiffness is related to the electronegativity difference density (Gilman, 2003) so is the hardness. Thus, like the covalent solids, the hardnesses of the alkali halides depends on the strength of the chemical bonding within them. 

It is also worth noting that Equation 9.1 indicates a connection between C44, hardness, and e. The dielectric constant, e depends on the polarizability, a of each alkali halide through the Clausius-Mossotti equation  

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