A good agreement of the bulk moduli of three new C 6 allotropes and rh-C 4 estimated using the thermodynamic model (B 0 ) and calculated from the set of elastic constants (B V ) is observed. For all three phases K Ic is almost twice higher than the 2.8 MPa•m½ value for single-crystal cubic BN and close to the experimental value of diamond fracture toughness (5 MPa•m½) .