  Using a membrane-driven diamond anvil cell and both ac magnetic
susceptibility and electrical resistivity measurements, we have characterized
the superconducting phase diagram of elemental barium to pressures as high as
65 GPa. We have determined the superconducting properties of the recently
discovered Ba-VI crystal structure, which can only be accessed via the
application of pressure at low temperature. We find that Ba-VI exhibits a
maximum Tc near 8 K, which is substantially higher than the maximum Tc found
when pressure is applied at room temperature.
