Evidence of a Pressure-Induced Metallization Process in Monoclinic ${\mathrm{VO}}_2$

Raman and combined infrared transmission and reflectivity measurements were carried out at room temperature (RT) on monoclinic ${\mathrm{VO}}_2$ over the 0–19 GPa and 0–14 GPa pressure ranges. Both lattice dynamics and optical gap show a remarkable stability up to $P^{*}\sim 10\mathrm{GPa}$ whereas subtle modifications of V ion arrangements within the monoclinic lattice, together with the onset of a metallization process via band gap filling, are observed for $P>P^{*}$. Differently from $P=0$, where the ${\mathrm{VO}}_2$ metallic phase is found only in conjunction with the rutile structure above 340 K, a new RT metallic phase within a monoclinic structure appears accessible in the high pressure regime.

Figure 1

(a) MIR reflectivity $R(\omega )$ and (b) transmittance $T(\omega )$ of ${\mathrm{VO}}_2$ at selected pressures. Fringes were averaged out in the reflectivity spectra, except at the lowest pressure. Arrows indicate increasing pressure. Dashed lines replace the data at the frequencies where the diamond absorption is very high. Inset: correspondingly low-frequency optical density $O_d(\omega )$.

Figure 2

Optical conductivity ${\sigma }_1(\omega )$ of ${\mathrm{VO}}_2$ at different pressures. Dashed lines are guides to the eyes. Inset: normalized spectral weights $S{W}_L^{*}(P)$ and $S{W}_H^{*}(P)$.

Figure 3

Raman spectra of ${\mathrm{VO}}_2$ at different pressures. Arrows mark the phonon peaks ${\omega }_{V1}$, ${\omega }_{V2}$, and ${\omega }_O$. Inset: pressure dependence of ${\omega }_{V1}$ and ${\omega }_{V2}$ (dashed lines are guide to the eyes).