Mechanical Properties and Electronic Structures of Cotunnite TiO₂

Based on the first-principles plane-wave basis pseudopotential calculations, we investigate mechanical properties and electronic structures of the hardest known oxide, cotunnite TiO₂. The calculated results show that cotunnite TiO₂ has the highest bulk modulus (348 GPa) and hardness (32GPa) among the high-pressure phases of TiO₂, but its mechanical properties are not superior to those of c-BN. Moreover, the high hardness of cotunnite TiO₂ can be understood from both the dense crystal structure (high valence electron density and short bond lengths) and the unusual mixtures of covalent and ionic bonding of Ti--O.