Elastic anisotropy and atomic structure

Atomic trends in anisotropy also exist for crystals with lower symmetry than cubic but these changes are more difficult to visualize. For hexagonal crystals, Eq. (2.64) can still be used to assess the anisotropy but, clearly, additional anisotropy parameters would be needed to completely describe the behavior. This increase in complexity is beyond the scope of this text except for a couple of simple examples, discussed below. 

For the parallel arrangement, both types of spring are subjected to the same displacement. It is left to the reader to show that the stiffness constant Cpfor the parallel system with an equal number of the two spring types is given by 

Hlllllllllllllllllll-Hllllllllllllllllll -iiiiiiiiii-iiiiiiim -llllllllllllllllllll-llllllllllllllllllf--IIIIIIMIMMIIlim -llllllllllllllllllll-Hllllllllllllllllll -iiiiiiiiii-iiiiiiim 

Comparison of the elastic constants for graphite and diamond, as given in Table 3.1, shows that although the elastic constants are comparable for uniaxial strains in the x, and directions (strongly covalent), the value of Cjj for graphite 

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