  Controlling electronic population through chemical doping is one way to tip
the balance between competing phases in materials with strong electronic
correlations. Vanadium dioxide exhibits a first-order phase transition at
around 338 K between a high temperature, tetragonal, metallic state (T) and a
low temperature, monoclinic, insulating state (M1), driven by electron-electron
and electron-lattice interactions. Intercalation of VO2 with atomic hydrogen
has been demonstrated, with evidence that this doping suppresses the
transition. However, the detailed effects of intercalated H on the crystal and
electronic structure of the resulting hydride have not been previously
reported. Here we present synchrotron and neutron diffraction studies of this
material system, mapping out the structural phase diagram as a function of
temperature and hydrogen content. In addition to the original T and M1 phases,
we find two orthorhombic phases, O1 and O2, which are stabilized at higher
hydrogen content. We present density functional calculations that confirm the
metallicity of these states and discuss the physical basis by which hydrogen
stabilizes conducting phases, in the context of the metal-insulator transition.
