The theory of C-axis elastic moduli of graphite and their strain dependence using simple models of the interplanar interaction

Theoretical values for the c-axis elastic moduli C₃₃, C₄₄ and their strain derivatives $\frac{1}{C_{33}}\frac{?C_{33}}{?{\in }_3}$, $\frac{1}{C_{44}}\frac{?C_{44}}{?{\in }_3}$ in hexagonal graphite have been computed as a function of c-axis strain using the Lennard-Jones and the exponential core pairwise interplanar potentials. Fair agreement with experiment was found in the compressive behavior for the Lennard-Jones potential, good aggrement for the exponential core potential. The largest measured shear moduli in graphite, probably representative of the ideal hexagonal material, are larger than the theoretical shear moduli by a factor of at least ten. The theoretical shear c-axis strain derivatives are in somewhat better agreement. The notable discrepancy between elementary interplanar potential theory and experiment for the c-axis shear modulus in graphite appears to be an intrinsic flaw of such theories.