|
The titanium oxyhalides TiOCl and TiOBr are low-dimensional materials which undergo unconventional transitions to a spin-Peierls state via a structural incommensurate state. With the electronic configuration 3d1 these compounds are Mott-Hubbard insulators with a charge gap of approximately 2 eV. They have been discussed to exhibit a resonating valence bond state and high-temperature superconductivity upon doping. However, despite several attempts, up to now a metallization of TiOX (X= Cl, Br) upon doping was not successful. Our recent pressure-dependent infrared spectroscopic investigations on TiOX [1] suggest that the application of external pressure is an alternative way to induce an insulator-to-metal transition in TiOX. By infrared microspectroscopy we could show the filling of the Mott-Hubbard gap at the critical pressure 16 GPa (14 GPa) for TiOCl (TiOBr) at room temperature. According to our pressure-dependent x-ray powder-diffraction data, the gap closure coincides with a structural phase transitions. These findings are discussed in the context of recent electric transport experiments on TiOCl under pressure [2] and theoretical predictions of pressure-induced phase transitions in TiOCl by an ab initio molecular dynamics study [3]. [1] C. A. Kuntscher et al., Phys. Rev. B 74, 184402 (2006); C. A. Kuntscher et al., Phys. Rev. B 76, 24110 (R) (2007); C. A. Kuntscher et al., Phys. Rev. 78, 035106 (2008). [2] Forthaus et al., Phys. Rev. B 77, 165121 (2008). [3] Y-Z. Zhang et al., Phys. Rev. Lett. 101, 136406 (2008). |
|