High-pressure phases of ${\mathrm{VO}}_2$ from the combination of Raman scattering and ab initio structural search

Despite more than 50 years of investigation, the understanding of the metal-insulator transition in ${\mathrm{VO}}_2$ remains incomplete and requires additional experimental and theoretical works. Using Raman scattering under pressure, we first confirm the known transition around 11 GPa affecting the V orbital occupancy in the absence of structural changes. Moreover, we disclose a transition around 19 GPa involving the V orbitals together with a structural distortion, revealed by the splitting of a phonon branch associated with the V chains. The high-pressure metallic $X$ phase is found to be of low symmetry and becomes the lowest-enthalpy structure at high pressure by ab initio structural prediction calculations. In contrast to a well-established picture of the metal-insulator transition (i.e., the Peierls transition), the high-pressure metallic phase here is of lower symmetry than the ambient pressure insulating phase.

Figure 1

(a) Raman spectra for pressure from 1 GPa (bottom) to 26 GPa (top) (open circles) and associated Gaussian fit (lines) at room temperature. The spectra show the two phonons associated with the V chains (${\omega }_{V1}=191{\mathrm{cm}}^{-1}$ and ${\omega }_{V2}=222{\mathrm{cm}}^{-1}$ at ambient pressure). (b) Evolution of the ${\omega }_{V1}$ (red points, left scale) and the ${\omega }_{V2}$ (blue points, right scale) modes as a function of pressure. The split mode ${\omega }_V^{\prime }$ observed above $\sim 19$ GPa is shown in green (right scale).

Figure 2

Assignment of the phonon symmetry for 1 GPa (bottom) and 26 GPa (top) Raman spectra at room temperature. As predicted by group theory, nine $A_g$ and nine $B_g$ symmetry branches are identified by the blue and red discontinuous lines, respectively. The split branch at 236 ${\mathrm{cm}}^{-1}$ pointed out by the black arrow does not come from a preexisting phonon at ambient pressure. This increase in the number of vibrational modes indicates the symmetry breaking of the lattice.

Figure 3

High-pressure diffractogram of ${\mathrm{VO}}_2$. The arrows show the splitting of the Bragg peaks around 20 GPa.

Figure 4

Enthalpy per atom of the low-energy ${\mathrm{VO}}_2$ crystal structures as a function of pressure with respect to the ground-state (at zero pressure) reference phase ($M_1$, space group $P{2}_1/c$).

Figure 5

Enthalpy per atom of the low-energy ${\mathrm{VO}}_2$ crystal structures as a function of pressure calculated with the PBE functional. The zero corresponds to the rutile structure at ambient pressure.

Figure 6

Structure of monoclinic $M_1$ VO2 (left) compared with the new monoclinic $M_3$ phase ($P1n1$) (right).

Figure 7

Phonon band structure and density of states for the $X$ monoclinic phase.