Models of the giant planets mostly rely on our knowledge of a few simple molecules under extreme P-T conditions. For Neptune and Uranus, the existence of a thick layer of ice (a mixture of water, ammonia and methane), located between a small rocky core and a thin gaseous atmosphere, is generally assumed. Pressure and temperature conditions in this ice layer range from about 10 GPa and 2000 K up to 700 GPa and 6000 K. The properties of these planets are thus expected to be largely influenced by the behavior of these ices under extreme P-T conditions. In particular, the ice layer is suspected to be the source of the non-dipolar, non-axisymmetric magnetic fields measured by the Voyager II spacecraft. This magnetic field is believed to originate from the dissociation of the water molecules into ionic species at high P/T, which is supported by several first-principles calculations predicting the ionization of water both in the fluid and solid phases. Dissociation of water ice has been inferred from a number of experiments, although a direct proof is still lacking. Furthermore, the presence of other molecules, in particular ammonia and methane, in the ice layer has so far been largely neglected, and it is not known how they influence its properties.

                Here I will present the results of our investigations on pure water and ammonia ices and their mixtures from static high P-T experiments and computer simulations.  In particular I will discuss the melting lines, crystal structures and chemical stability of the pure ices in light of our synchrotron x-ray diffraction and Raman spectroscopy experiments. I will also discuss and compare the high P-T phase diagrams of the three stable ammonia hydrates.