Pressure-induced phase transitions and metallization in VO2

Abstract

We report the results of pressure-induced phase transitions and metallization in VO2 based on synchrotron x-ray diffraction, electrical resistivity, and Raman spectroscopy. Our isothermal compression experiments at room temperature and 383 K show that the room temperature monoclinic phase (M1, P2(1)/c) and the high-temperature rutile phase (R, P4(2)/mnm) of VO2 undergo phase transitions to a distorted M1 monoclinic phase (M1', P2(1)/c) above 13.0 GPa and to an orthorhombic phase (CaCl2-like, Pnnm) above 13.7 GPa, respectively. Upon further compression, both high-pressure phases transform into a new phase (phase X) above 34.3 and 38.3 GPa at room temperature and 383 K, respectively. The room temperature M1-M1' phase transition structurally resembles the R-CaCl2 phase transition at 383 K, suggesting a second-order displacive type of transition. Contrary to previous studies, our electrical resistivity results, Raman measurements, as well as ab initio calculations indicate that the new phase X, rather than the M1' phase, is responsible for the metallization under pressure. The metallization mechanism is discussed based on the proposed crystal structure.