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 oxygen and deuterium atoms in D₂O ice VII, and give the magnitudes of the site separations. An O-D distance and D-\^O-D angle closest to those found in ice VIII are obtained with oxygen sites displaced along $〈111〉$ directions. Such displacements imply an H-bond geometry significantly different from that found in ice VIII, and this may have implications for the centering transition to ice X.