Theoretical and Experimental Evidence for a New Post-Cotunnite Phase of Titanium Dioxide with Significant Optical Absorption

We report the discovery of a post-cotunnite phase of TiO₂ by both density-functional ab initio calculations and high-pressure experiments. A pressure-induced phase transition to a hexagonal Fe₂P-type structure (space group P6̅ 2m) was predicted to occur at 161 GPa and 0 K and successfully observed by in situ synchrotron x-ray diffraction measurements at 210 GPa and 4000 K with a significant increase in opacity. This change in opacity is attributed to a reduction of band gap from 3.0 to 1.9 eV across the phase change. The Fe₂P-type structure is proved to be the densest phase in major metal dioxides.