  Water is abundant in natural environments but the form it resides in
planetary interiors remains uncertain. We report combined synchrotron X-ray
diffraction and optical spectroscopy measurements of H2O in the laser-heated
diamond anvil cell up to 150 gigapascals (GPa) and 6500 kelvin (K) that reveal
first-order transitions to ices with body-centered cubic (bcc) and
face-centered cubic (fcc) oxygen lattices above 900 (1300) K and 20 (29) GPa,
respectively. We assigned these structures to theoretically predicted
superionic phases based on the distinct density, increased optical
conductivity, and greatly decreased enthalpies of fusion. Our measurements
address current discrepancies between theoretical predictions and various
static/dynamic experiments on the existence and location of melting curve and
superionic phase(s) in the pressure-temperature phase diagram indicating a
possible presence of the conducting fcc-superionic phase in water-rich giant
planets, such as Neptune and Uranus.
