  Silica, water and hydrogen are known to be the major components of celestial
bodies, and have significant influence on the formation and evolution of giant
planets, such as Uranus and Neptune. Thus, it is of fundamental importance to
investigate their states and possible reactions under the planetary conditions.
Here, using advanced crystal structure searches and first-principles
calculations in the Si-O-H system, we find that a silica-water compound
(SiO2)2(H2O) and a silica-hydrogen compound SiO2H2 can exist under high
pressures above 450 and 650 GPa, respectively. Further simulations reveal that,
at high pressure and high temperature conditions corresponding to the interiors
of Uranus and Neptune, these compounds exhibit superionic behavior, in which
protons diffuse freely like liquid while the silicon and oxygen framework is
fixed as solid. Therefore, these superionic silica-water and silica-hydrogen
compounds could be regarded as important components of the deep mantle or core
of giants, which also provides an alternative origin for their anomalous
magnetic fields. These unexpected physical and chemical properties of the most
common natural materials at high pressure offer key clues to understand some
abstruse issues including demixing and erosion of the core in giant planets,
and shed light on building reliable models for solar giants and exoplanets.
