Anisotropic compression in the high-pressure regime of pure and chromium-doped vanadium dioxide

We present structural studies of V${}_{1\ensuremath{-}x}$Cr${}_{x}$O${}_{2}$ (pure, 0.7 $%$, and 2.5 $%$ Cr-doped) compounds at room temperature in a diamond anvil cell for pressures up to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied show a persistence of the monoclinic ${M}_{1}$ symmetry between 4 and 12 GPa. Above 12 GPa, the monoclinic ${M}_{1}$ symmetry changes to isostructural ${M}_{x}$ phase (space group $P{2}_{1}/c$) with a significant anisotropy in lattice compression of the $b$-$c$ plane of the ${M}_{1}$ phase. This behavior can be reconciled invoking the pressure-induced charge delocalization.