  A new phase of ice, named ice XV, has been identified and its structure
determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart
of ice VI and is thermodynamically stable at temperatures below ~130 K in the
0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure
range of the phase diagram have thus been finally mapped, with only
hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is
antiferroelectric, in clear disagreement with recent theoretical calculations
predicting ferroelectric ordering.
