Mechanical Properties and Electronic Structures of Cotunnite TiO 2

Based on the first-principles plane-wave basis pseudopotential calculations, we investigate mechanical properties and electronic structures of the hardest known oxide, cotunnite TiO 2 . The calculated results show that cotunnite TiO 2 has the highest bulk modulus (348 GPa) and hardness (32 GPa) among the high-pressure phases of TiO 2 , but its mechanical properties are not superior to those of c-BN. Moreover, the high hardness of cotunnite TiO 2 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.