High-pressure and high-temperature synthesis of the cubic $\mathrm{Ti}{\mathrm{O}}_2$ polymorph

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.

Figure 1

Examples of x-ray diffraction patterns obtained in experiments with titanium dioxide at (a) $2.8\left(1\right)\mathrm{GPa}$, (b) $6.2\left(1\right)\mathrm{GPa}$, (c) $29\left(1\right)\mathrm{GPa}$, (d) $27\left(1\right)\mathrm{GPa}$ after heating between $1300\mathrm{K}$ and $1500\mathrm{K}$, (e) $48\left(1\right)\mathrm{GPa}$ after heating between $1900\mathrm{K}$ and $2100\mathrm{K}$, (f) on decompression at $8.9\left(1\right)\mathrm{GPa}$, and after complete release of pressure (g) (Au for gold, An for anatase, Bd for baddeleyite, OI for orthorhombic $Pbca$ phase, OII for orthorhombic cotunnite-type phase, C for cubic $\mathrm{Ca}{\mathrm{F}}_2$-like phase, T for $\mathrm{Ti}{\mathrm{O}}_2\mathrm{II}$, and R for rutile).

Figure 2

Pressure dependence of volume of the cubic fluoritelike $\mathrm{Ti}{\mathrm{O}}_2$ phase (dots). The Birch-Murnaghan equation of state [Eq. (1)] is plotted as solid lines with parameters $K_{300}=202\left(5\right)\mathrm{GPa}$, $K^{\prime }=1.3\left(1\right)$, and $V_0=115.50\left(1\right){\mathrm{\AA }}^3∕\text{unit}$ cell. Dashed and dot-dashed lines show relations for OII and MI phases, respectively (Refs. 4, 5).