Structure of Single-Crystal Rutile (TiO₂) Prepared by High-Temperature Ultracentrifugation

We report the preparation of single-crystal rutile (TiO₂) with a unique structure prepared by ultracentrifugation at high temperature (strong gravitational field). Single-crystal rutile was subjected along the c-axis direction to a gravitational field of 0.4 × 10⁶ G at 400 °C, and the uniquely structured single-crystal rutile, which did not conform to Pauling's third rule, was quenched at ambient conditions. The anisotropy (a/c ratio) of the tetragonal phase increased by 2%. The (Ti-Oa)/(Ti-Ob) and Os/Ou ratios increased by 1.6% and 3%, respectively, and approached 1; Ti-Oa and Ti-Ob are the two Ti-O interatomic distances, and Os and Ou are the shared edge Oa-Oa and the unshared edge Oa-Ob, respectively. This means that the TiO₆ octahedral group became isotropic. Os was expanded from its normal size, in contradiction to the usual laws, by the strong gravitational force. Such a structural change has not previously been achieved under high-pressure or high-temperature conditions, and may be related to structural stabilization induced by a unique uniaxially distorted crystalline state under ultracentrifugation. Ab initio simulations supported the experimental results.