This proposal is a continuation of proposal HC-4677 which aimed at measuring the boundary line between bcc insulating and fcc-superionic (fcc-SI) phases of ice. A sample assembly with two boron-doped diamond absorbers has been optimized and it enabled a controlled laser heating of the ice sample up to 200 GPa. A negative slope for the bcc to fcc-SI transition and a metastable quenched fcc-SI phase at ambient temperature have been observed. That hints to the stability of the fcc-SI phase below 1000K by going to 300 GPa. Also, compact phases with greater packing density than bcc should be observed by compressing water ice up to 400 GPa at 300K. We plan to continue the exploration of the detailed structural properties of dense water ice above 200 GPa. These are very challenging measurements of great impact to model planetary interiors of Uranus, Neptune and other ice giant exoplanets.