Table shows the hardness values calculated using other models in addition to the thermodynamic model. As in the case of other ultrahard phases , the Lyakhov-Oganov model gives underestimated hardness values, while both empirical models do not work properly in the case of C 9 and C 12 , which are characterized by low (0.82 and 0.71) values of the Pugh's modulus ratio (G/B).