Modulated phases and proton centring in ice observed by X-ray diffraction up to 170 GPa

Because of its open hydrogen-bonded structure, ice shows many structural ch anges between different crystalline forms under high pressure. Crystallogra phic studies of these transitions have been pursued largely by neutron scat tering, which allows the positions of the hydrogen atoms to be identified(1 ,2). Such studies have previously been extended to pressures of up to 20 GP a, which is however insufficient tea permit the investigation of ice X, a ' symmetric ice' in which the protons are thought to reside midway between th e oxygen atoms(3-5). So far, information about ice X has therefore come fro m indirect methods such as infrared(6,7) or Brillouin(8) spectroscopy. Here we show that single-crystal X-ray diffraction is able to reveal the signat ure of hydrogen-bond symmetrization. The 111 reflection can be assigned to the hydrogen atoms alone, and we can measure it up to 170 GPa in a diamond anvil cell. This diffraction line (normalized against the intensity of the 222 line, which is due mostly to oxygen atoms) indicates that the proton ce ntring in ice X occurs from about 60 to 150 GPa; at this latter pressure th e intensity increases sharply, signalling a further structural change. At l ower pressures, we see ice VII ordering in a sequence of spatially modulate d phases between 2.2 and 25 GPa, which suggests an analogy with the incomme nsurate phases of the frustrated Ising model(9).