Pressure-induced phase transitions and metallization inVO2

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, P21/c) and the high temperature rutile phase (R, P42/mnm) of VO2 undergo phase transitions to a distorted M1 monoclinic phase (M1 ′ , P21/c) above 13.0 GPa and to an orthorhombic phase (CaCl2-like, Pnnm) above 13.7 GPa, respectively. Upon further compression, both highpressure 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.

Pressure–Temperature Phase Diagram of Vanadium Dioxide

“…We mark this metallic phase 9.6 GPa as X phase following previous work. 9 Plotting the pressure dependence of the three most prominent Raman peaks reveals new information, as shown in Figure 2b. It can be seen that the V-O mode at = 617.5 cm -1 (2.5 GPa) blue shifts linearly with an initial rate of d /dP = 4.1 cm -1 /GPa, but then the rate changes to a much shallower, 2.4 cm -1 /GPa when the pressure is higher than 16.5 GPa.…”

“…20 As pressure further increases beyond ~ 38 GPa, VO2 transforms into the metallic X phase showing very low resistance. 9 It can be seen from Figure 4 that, although these data points were taken with different techniques (Raman, electrical, reflectance or XRD), along different axes of variables (isobaric or isothermal), and on different samples, they clearly define a consistent P-T phase diagram over a wide pressure and temperature range. This phase diagram share some similarities to the largely schematic diagram of phases reported in Ref.…”

“…This effect points to a continuous phase transition from M1 to a new, insulating M1' phase at this pressure, as suggested also by our previous room-temperature microx-ray diffraction (XRD) experiments. 9 The M1' phase is isostructural to M1, yet with very different electrical resistance as discussed below. In order to probe the electronic nature of these phases, electrical transport measurements were also performed as a function of pressure or temperature.…”

“…Transitions between these phases can be driven by temperature, photo-excitation, hydrostatic pressure, uniaxial stress, or electrical gating. 1,[4][5][6][7][8][9][10][11][12] For example, the well-known, insulating M1 (monoclinic, space group P21/c) phase transforms to the metallic R (tetragonal, P42/mnm) phase at 68 o C under ambient pressure. 1 This M1-R transition can also be driven by hydrostatic pressure 6 or uniaxial compression.…”

“…In the following we analyze these features. 9 The black triangle data points are from Ref. 6 The error bars include the temperature and pressure uncertainties.…”