Transformation of rutile to TiO₂-II in a high pressure hydrothermal environment

The high pressure transformation of rutile to TiO₂-II with the α-PbO₂ structure is known to be kinetically hindered. In this study we show that a hydrothermal environment at 6 GPa and 650 C provides appreciable rates for producing single phase bulk samples of TiO₂-II. So obtained TiO₂-II was characterized by scanning electron microscopy, powder X-ray diffraction, Raman and Far-IR spectroscopy. The structural properties are identical to TiO₂-II from dry transitions. Transmission electron microscopy studies strongly indicate that Ostwald ripening processes play an important role in the hydrothermally assisted transformation and subsequent growth of TiO₂-II crystals. TiO₂-II is thermally stable to about 550 C. At 600 C the onset of the transformation to rutile is observed. The thermal expansion in the temperature range from room temperature to 500 C is highly anisotropic, virtually affecting only the c unit cell parameter (α_c=7.1(2)×10^-6 C ^-1). The pressure-temperature conditions for the hydrothermally assisted transformation of rutile are viable for industrial production settings, and in light of the large technological significance of TiO₂, TiO₂-II may present an interesting target for large-scale synthesis.