M. Mattesini

We report on an in situ observation of a cubic titanium dioxide polymorph. Experiments conducted on an anatase sample as starting material using a laser-heated diamond-anvil cell have shown that at a pressure of $48\mathrm{GPa}$ and temperatures between 1900 and $2100\mathrm{K}$, the x-ray diffraction pattern contains reflections of two phases: the known cotunnite-structured $\mathrm{Ti}{\mathrm{O}}_2$ and the new cubic phase with lattice parameter $a=4.516\left(1\right)\mathrm{\AA }$. Reflections of the cubic phase can be followed on decompression at ambient temperatures down to about $9\mathrm{GPa}$. Our observations suggest that at high temperature and elevated pressure both IV $AB$ and IV $A$ element dioxides follow a common transformation path.

Materials are currently sought for use in the photo-induced decomposition of water on crystalline electrodes. Titanium dioxide is valuable in this respect. The electronic structural properties of cubic ${\mathrm{TiO}}_2$ polymorphs were investigated by means of first-principles methods. We demonstrate that both fluorite- and pyrite-type ${\mathrm{TiO}}_2$ have important optical absorptive transitions in the region of the visible light. A cubic ${\mathrm{TiO}}_2$ phase that can efficiently absorb the sunlight would be an important candidate material for the development of the solar cells. Also, we present results on the $\mathrm{Ti}$ $L$ edges for the two different titania forms. We predict that a qualitative spectroscopic discrimination of the cubic polymorphs can be achieved by following the $\mathrm{Ti}$ $2p\to 3d$ x-ray transitions.