MULTISITE DISORDERED STRUCTURE OF ICE-VII TO 20 GPA

Neutron diffraction measurements on ice VII and ice VIII at pressures up to 20 GPa provide direct evidence of multisite disorder of both oxy gen and deuterium atoms in D2O ice VII, and give the magnitudes of the site separations. An O-D distance and D-(O) over cap-D angle closest to those found in ice VIII are obtained with oxygen sites displaced al ong [111] directions. Such displacements imply an H-bond geometry sign ificantly different from that found in ice VIII, and this may have imp lications for the centering transition to ice X. [S0031-9007(98)07166- X].