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 ${\mathrm{TiO}}_2$ by both density-functional ab initio calculations and high-pressure experiments. A pressure-induced phase transition to a hexagonal ${\mathrm{Fe}}_2\mathrm{P}$-type structure (space group $P\overline{6}2m$) 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 ${\mathrm{Fe}}_2\mathrm{P}$-type structure is proved to be the densest phase in major metal dioxides.

Figure 1

Crystal structures of two high-pressure phases of ${\mathrm{TiO}}_2$ and enthalpy relationships among several phases. (a) The orthorhombic cotunnite-type (left) and hexagonal ${\mathrm{Fe}}_2\mathrm{P}$-type structures (right). The red and blue spheres represent oxygen and titanium atoms, respectively. Two different kinds of ${\mathrm{TiO}}_9$ polyhedra are colored in blue (Ti1) and light blue (Ti2) for the ${\mathrm{Fe}}_2\mathrm{P}$-type structure. (b) Static enthalpy differences relative to the cotunnite-type phase as a function of pressure. Those between the ${\mathrm{Fe}}_2\mathrm{P}$-type and cotunnite-type are focused in the inset along with the results within the LDA.

Figure 2

(a) An in situ XRD profile of ${\mathrm{Fe}}_2\mathrm{P}$-type ${\mathrm{TiO}}_2$ after heating with a two-dimensional image presenting circular Debye rings (inset). Simulated patterns for ${\mathrm{Fe}}_2\mathrm{P}$-type (red) and cotunnite-type (blue) are also provided. (b) Volume compression curves of the cotunnite-type and ${\mathrm{Fe}}_2\mathrm{P}$-type phases at 300 K calculated with the GGA (solid lines) and the LDA (dashed line). The red diamond represents the experimental volume of a ${\mathrm{Fe}}_2\mathrm{P}$-type phase (present study), and open symbols represent those of a cotunnite-type phase [10, 11, 12]. A vertical line represents the static GGA transition pressure. (c) High-$(P,T)$ phase diagram of ${\mathrm{TiO}}_2$ with phase boundaries between the cotunnite-type and ${\mathrm{Fe}}_2\mathrm{P}$-type phases calculated within the GGA (red solid line) and LDA (dashed line). Blue and red squares represent the present experimental conditions where the cotunnite-type phase and ${\mathrm{Fe}}_2\mathrm{P}$-type, respectively, were observed. The vibrational density of states of the cotunnite-type (blue) and ${\mathrm{Fe}}_2\mathrm{P}$-type (red) phases calculated at 160 GPa within the GGA (inset).

Figure 3

Electronic properties of the two high-pressure polymorphs of ${\mathrm{TiO}}_2$. (a) Electronic band structures calculated at 160 GPa within the GGA for the cotunnite-type (upper) and ${\mathrm{Fe}}_2\mathrm{P}$-type (lower) structures. The top of valence bands is set to zero. (b) Kohn-Sham probability densities ($|\varphi {|}^2$) of a Ti $3d$ state at the conduction band bottoms in the cotunnite-type and ${\mathrm{Fe}}_2\mathrm{P}$-type phases at 160 GPa. Dashed cubes show the unit cells. (c) Optical images of a sample in the DAC before the phase transition (left) and after synthesis of ${\mathrm{Fe}}_2\mathrm{P}$-type ${\mathrm{TiO}}_2$ (right). Dashed circles represent the sample chamber. The center of the sample chamber (gray opaque area in the right panel) was heated by lasers.