Optical properties ofV1−xCrxO2compounds under high pressure

Raman and infrared transmission and reflectivity measurements were carried out at room temperature and high pressure (0--15 GPa) on ${\text{V}}_{1\ensuremath{-}x}{\text{Cr}}_{x}{\text{O}}_{2}$ compounds. Raman spectra were collected at ambient conditions on the $x=0.007$ and 0.025 materials, which are characterized by different insulating monoclinic phases ($M3$ and $M2$, respectively), while infrared spectra were collected on the $x=0.025$ sample only. The present data were compared with companion results on undoped ${\text{VO}}_{2}$ [E. Arcangeletti et al., Phys. Rev. Lett. 98, 196406 (2007)], which is found at ambient conditions in a different, third insulating monoclinic phase, named $M1$. This comparison allowed us to investigate the effects of different extents of structural distortions (Peierls distortion) on the lattice dynamics and the electronic properties of this family of compounds. The pressure dependence of the Raman spectrum of ${\text{VO}}_{2}$ and Cr-doped samples shows that all the systems retain the monoclinic structure up to the highest explored pressure, regardless the specific monoclinic structure ($M1$, $M2$, and $M3$) at ambient condition. Moreover, the Raman spectra of the two Cr-doped samples, which exhibit an anomalous behavior over the low-pressure range $(P<8\text{ }\text{GPa})$, merge into that of ${\text{VO}}_{2}$ in the high-pressure regime and are all found into a common monoclinic phase (a possible fourth kind phase). Combining Raman and infrared results on both the ${\text{VO}}_{2}$ and the present data, we found that a common metallic monoclinic phase appears accessible in the high-pressure regime at room temperature for both undoped and Cr-doped samples independently of the different extents of Peierls distortion at ambient conditions. This finding differs from the behavior observed at ambient pressure, where the metallic phase is found only in conjunction with the rutile structure. The whole of these results suggests a major role of the electron correlations, rather than of the Peierls distortion, in driving the metal-insulator transition in vanadium dioxide systems, thus opening to new experimental and theoretical queries.