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

The high pressure transformation of rutile to TiO2-II with the alpha-PbO2 structure is known to be kinetically hindered. In this study we show that a hydrothermal environment at 6 GPa and 650 degrees 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 degrees C. At 600 degrees C the onset of the transformation to rutile is observed. The thermal expansion in the temperature range from room temperature to 500 degrees C is highly anisotropic, virtually affecting only the c unit cell parameter (alpha(c)=7.1(2) x 10(-6) degrees 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. (C) 2013 Elsevier Inc. All rights reserved.