EVIDENCE OF A SYMMETRIC DISORDERED ICE ABOVE ~62 GPA FROM RAMAN SCATTERING AND X-RAY DIFFRACTION INVESTIGATIONS. E. Wolanin,^* Ph. Pruzan,^* M. Gauthier,^* J.C. Chervin,^* B. Canny,^* D. Hausermann,^** M. Hanfland^**, ^*Physique des Milieux Condenss (URA 782), Universit P. & M. Curie, B 77, 4, place Jussieu F-75252 Paris Cedex 05, France, ^**E.S.R.F., B.P. 220 F-38043 Grenoble Cedex, France

Experimental investigations of ice and deuterated ice, using Raman scattering at low temperature and x-ray diffraction at 300 K, were performed in the diamond anvil cell at pressure on excess to 100 GPa.

One of the main feature of the ice phase diagram above 2 GPa is the VII-VIII order-disorder antiferro-paraelectric transition whose locus was determined by Raman scattering. Analysis of the behavior of the VII-VIII transition temperature T_c vs pressure, points out three pressure regimes for the proton disordering. In particular the drop of T_c to 0 K at 62 GPa is consistent with the proton delocalization between two neighbouring oxygens.

Our x-ray investigation performed at 300 K confirms the Pn3m structure for ice VII for pressures above 100 GPa. Using a treatment based on the universal equation of state (EOS) of Vinet, our x-ray data exhibits three pressure regimes for the EOS consistent with our Raman results. Ice adopts a disordered structure with a proton delocalization along the O-O direction. This structure is very likely a step before the ordered symmetric ice.