X-Ray Signature of the Superionic Transition in Warm Dense fcc Water Ice

The fcc superionic phase of ice is a key component of the warm dense water phase diagram. While a few x-ray diffraction studies, under dynamic and static compressions, have reported the stability of the fcc structure, the transition to the superionic state has not been investigated in detail. Here, a remarkable thermal volume expansion is disclosed, which is interpreted as being directly related to the superionic transition. This could be achieved by implementing a heating capsule geometry within the laser-heated diamond anvil cell. Fcc ice is recovered metastable at ambient temperature, allowing us to observe that superionicity in the fcc phase emerges at a slightly lower temperature than for the bcc-fcc structural transition. The crossover in volume thermal expansion at the superionic transition agrees with recent ab initio calculations; however, its magnitude is larger than predicted.