Structural stability of a golden semiconducting orthorhombic polymorph of Ti2O3 under high pressures and high temperatures

An orthorhombic polymorph of titanium oxide (Ti2O3) has been synthesized at high pressure–high temperature (HP–HT) conditions. It has been refined in the Pnma space group and the Th2S3 structural type with the unit cell parameters as follows: a = 7.8248(6) A, b = 2.8507(4) A, c = 8.0967(3) A, V = 180.61(1) A3 and Z = 4. The samples of Pnma-Ti2O3 were of a golden colour, in contrast to the conventional black corundum-structured Ti2O3. The structural stability of this polymorph has been examined by simultaneous Raman and x-ray diffraction studies under high pressure over 70 GPa and high temperature over 2200 K. No phase transformations or chemical reactions have been established. The electrical resistivity of Th2S3-structured Ti2O3 samples showed a semiconducting behaviour and, at ambient conditions, was equal to 0.20–0.46  Ω cm. Conventional near-infrared absorption spectroscopy established the absence of energy gaps above 0.25 eV.