Pressure-induced phase transitions and metallization in ${\mathrm{VO}}_2$

We report the results of pressure-induced phase transitions and metallization in ${\mathrm{VO}}_2$ 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 $(M1,P{2}_1/c)$ and the high-temperature rutile phase $(R,P{4}_2/mnm)$ of ${\mathrm{VO}}_2$ undergo phase transitions to a distorted $M1$ monoclinic phase $(M{1}^{\prime },P{2}_1/c)$ above 13.0 GPa and to an orthorhombic phase (${\mathrm{CaCl}}_2$-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 $M1\text{−}M{1}^{\prime }$ phase transition structurally resembles the $R\text{−}{\mathrm{CaCl}}_2$ 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 $M{1}^{\prime }$ phase, is responsible for the metallization under pressure. The metallization mechanism is discussed based on the proposed crystal structure.

Figure 1

High-pressure x-ray diffraction patterns of ${\mathrm{VO}}_2$ collected at (a) room temperature and (b) 383 K, where $R$ and $O$ denote rutile and ${\mathrm{CaCl}}_2$ phases. The pattern at 1.7 GPa in (b) is collected at room temperature for comparison. Stars indicate diffraction peaks from the Ne pressure medium.

Figure 2

Pressure dependence of $a,b$, and $c$ unit cell parameters of the monoclinic structure. Solid red circles are x-ray diffraction data from this study; solid lines are results from ab initio GGA+$U$ ($U=1$ black, 2 red, 3 green, 4 blue; $J=0.68$) calculations in this study. Open circles are from Ref. [27]. (b) Pressure dependence of unit cell parameters of the rutile and ${\mathrm{CaCl}}_2$ structures. To compare with the compression behavior of monoclinic phases, equivalent lattice parameters (${\mathit{a}}_{M1}^R=2{\mathit{c}}_R,{\mathit{b}}_{M1}^R={\mathit{a}}_R,{\mathit{c}}_{M1}^R={\mathit{a}}_R-{\mathit{c}}_R$) are used.

Figure 3

Pressure versus volume data for ${\mathrm{VO}}_2$ at (a) room temperature and (b) 383 K; dashed vertical lines denote transition pressures from (a) $M1$ to $M{1}^{\prime }$ phases and (b) rutile to ${\mathrm{CaCl}}_2$ phases. (a) Solid blue squares are data from present XRD; open red squares are previous data from Ref. [15]. The solid and dash blue curves indicate our EOS for M1 and $M{1}^{\prime }$ phases respectively, the solid red curves indicate the refitting EOS using data from Ref. [15]. (b) Solid black squares are data from present XRD; the solid and dashed curves indicate our EOS for rutile and ${\mathrm{CaCl}}_2$ phases respectively.

Figure 4

Raman spectra of ${\mathrm{VO}}_2$ measured at high pressures and ambient temperature. The phonon peaks are labeled by letters from A to J.

Figure 5

Pressure dependencies of phonon frequencies of ${\mathrm{VO}}_2$ at ambient temperature. Phonon modes from A to J corresponding to those in Fig. 4. (a) Phonon modes between 100 and $350 {\mathrm{cm}}^{-1}$, and (b) phonon modes between 350 and $800 {\mathrm{cm}}^{-1}$. Changes in Raman peaks are indicated by the vertical dashed lines.

Figure 6

Pressure dependence of electrical resistivity of ${\mathrm{VO}}_2$ at ambient temperature. The transition pressures are indicated by the vertical lines.

Figure 7

(a) Band gap $E_g$ as a function of Hubbard parameter $U$. (b) The band gap as a function of pressure (solid squares: calculations; dashed line: deduced from resistivity).