Table shows the crystal parameters, density and Vickers hardness calculated from the thermodynamic model. For template C 4 , all values are close to those of diamond and lonsdaleite, as expected. The hardness of tetragonal C 9 and C 12 is lower, but in both cases it exceeds 80 GPa, which allows us to assign these allotropes to the family of ultrahard phases . We also calculated hardness of two sqc allotropes from the SACADA database, sqc3051 and sqc6952. The hardness values were found to be 51 and 67 GPa, respectively, which is a correlation with the density.