Four modern theoretical models have been used to predict Vickers hardness (H V ). The thermodynamic (T) model , which is based on thermodynamic properties and crystal structure, which generally shows good agreement with experiment, and is therefore recommended for hardness evaluation of superhard and ultrahard phases. The values of Vickers hardness and bulk moduli calculated using this model are summarized in Table . The Lyakhov-Oganov (LO) model takes into account the topology of the crystal structure, the strength of covalent bonding, the degree of ionicity and directionality; and the empirical models, Mazhnik-Oganov (MO) and Chen-Niu (CN) , are based on elastic properties, namely, bulk and shear moduli. As shown previously , in the case of superhard compounds of light elements, the Lyakhov-Oganov model gives somewhat underestimated values of hardness, while empirical models are not reliable.