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

We present structural studies of V${}_{1-x}$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 $P{2}_1\mathrm{/}c$) 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.