Since the thermodynamic model is the most reliable in the case of superhard boron compounds and shows perfect agreement with the available experimental data for cubic boron nitride , it is obvious that the hardness values calculated within the empirical Mazhnik-Oganov and Chen-Niu models are not reliable. As for the Oganov-Lyakhov model , it gives slightly underestimated values, as has already been observed for the superhard compounds of the B-C-N system . It is evident (see Table ) that the hardness of B 2 N 3 (52 GPa for both tetragonal and hexagonal polymorphs) is only slightly lower than that of cubic (55 GPa) and wurtzite (54 GPa) BN polymorphs. In contrast, the hardness of B 3 N 3 is significantly lower, especially for the tetragonal polymorph (42 GPa). However, all four new boron nitrides have Vickers hardness exceeding 40 GPa, making them members of the superhard phases family.