Evasive ice X and heavy fermion ice XII: facts and fiction about high-pressure ices

Recent theoretical and experimental results on the structure and dynamics of ice in wide regions of pressure and temperature are compared with earlier models and predictions to illustrate the evasive nature of ice X, which was originally introduced as completely ordered form of ice with short single centred hydrogen bonds isostructural Cu2O. Due to the lack of experimental information on the proton ordering in the pressure and temperature region for the possible occurrence of ice X, effects of thermal and quantum delocalization are discussed with respect to the shape of the phase diagram and other structural models consistent with present optical and X-ray data for this region. Theoretical evidences for an additional orthorhombic modification lice XI) at higher pressures are confronted with various reasons supporting a delocalization of the protons in the form of a heavy fermion system with very unique physical properties characterizing this fictitious new phase of ice XII. (C) 1999 Elsevier Science B.V. All rights reserved.