Fig. 79: Evolution of the (111)/(222) intensity ration which reflects the changes in proton ordering. The quantum behaviour of the proton in ice VII is illustrated in the inset by a schematic drawing of the expected proton density (dotted line) between two oxygen atoms along the (111) direction. The potential energy (full line) changes from a double to a single well with pressure. The sequence is (a) orientationally-disordered ice VII, (b) translationally-disordered asymmetric ice VII, (c) proton-disordered symmetric ice, and (d) symmetric ice X (see text).