Seven-Coordinated “Metallic” WO₃ at High Pressures

Abstract

We have studied the pressure-induced structural and electronic phase transitions in WO3 to 60 GPa using micro-Raman spectroscopy, synchrotron X-ray diffraction, and electrical resistivity measurements. The results indicate that WO3 undergoes a series of phase transitions with increasing pressure: triclinic WO3-I initially transforms to monoclinic WO3-II (P21/c) at 1 GPa, involving a tetrahedral distortion in a corner-shared octahedral framework, and then to a mixed corner and edge-shared seven-coordinated WO3-III (P21/c) at 27 GPa with a large volume change of ∼6% and further to WO3-IV (Pc) above 37 GPa. These structural phase transitions also accompany a significant drop in resistivity from insulating WO3-I to semiconducting WO3-II, and poor metallic WO3-III and IV, arising from the Jahn–Teller distortion in WO6 and the hybridization between O 2p and W 5d orbitals in WO7, respectively. Unlike its molecular analogue of MoO3, the transitions in WO3 show little effect on the use of different pressure transmitting media.