  The complexity of strongly correlated electron physics in vanadium dioxide is
exemplified as its rich phase diagrams of all kinds, which in turn shed light
on the mechanisms behind its various phase transitions. In this work, we map
out the hydrostatic pressure - temperature phase diagram of vanadium dioxide
nanobeams by independently varying pressure and temperature with a diamond
anvil cell. In addition to the well-known insulating M1 (monoclinic) and
metallic R (tetragonal) phases, the diagram identifies the existence at high
pressures of the insulating M1' (monoclinic, more conductive than M1) phase,
and two metallic phases of X (monoclinic) and O (orthorhombic, at high
temperature only). Systematic optical and electrical measurements combined with
density functional calculations allow us to delineate their phase boundaries as
well as reveal some basic features of the transitions.
