  H2O is an important constituent in planetary bodies, controlling habitability
and, in geologically-active bodies, plate tectonics. At pressures within the
interior of many planets, the H-bonds in H2O collapse into stronger, ionic
bonds. Here we present agreement between X-ray diffraction and Raman
spectroscopy for the transition from ice-VII to ice-X occurring at a pressure
of approximately 30.9 GPa by means of combining grain normalizing heat
treatment via direct laser heating with static compression. This is evidenced
by the emergence of the characteristic Raman mode of cuprite-like ice-X and an
abrupt 2.5-fold increase in bulk modulus, implying a significant increase in
bond strength. This is preceded by a transition from cubic ice-VII to a
structure of tetragonal symmetry, ice-VIIt at 5.1 GPa. Our results
significantly shift the mass/radius relationship of water-rich planets and
define a high-pressure limit for release of chemically-bound water within the
Earth, making the deep mantle a potential long-term reservoir of ancient water.
