High Pressure Crystalline Structure and Resistance of Vanadium Dioxide to 13.5 GPa*

We have investigated the insulator-to-metal transition in thin film vanadium dioxide as a function of pressure at ambient temperature using a designer diamond anvil cell (DAC). Four-point probe resistance measurements show a monotonic decrease over the entire pressure range studied with no significant discontinuity. High-pressure X-ray diffraction measurements observe an $\mathrm{M_1}$ ($\mathrm{P2_1/c}$) phase at 0 GPa, an $\mathrm{M_2}$ (C2/m) phase from 0.8 GPa to 1.1 GPa, and a reentrant $\mathrm{M_1}$ phase from 1.1 GPa to 13.5 GPa. Crystal refinement above 1.1 GPa shows a monotonically decreasing $a$, $b$ and $c$ lattice constants and a minimum in the monoclinic angle, $\beta$, near 8.5$\pm$0.5 GPa. The atomic positions show that the first V-V nearest neighbor distance ($d$) decreases over the entire pressure range, the second nearest neighbor distance ($s$) increases until 5 GPa after which it is constant with $s$$\approx$$f$$\approx$3.2 {\AA}. The next most closely spaced V-V distance ($f$), which corresponds to V atoms in different unit cells, is approximately constant across the entire pressure range measured.