Stability of ice XII relative to ice V and ice VI at high pressures

The Gibbs energy difference between ice XII and ice V and between ice XII and ice VI at high pressures has been estimated in their fully orientationally disordered states from the available data. The Gibbs energy of ice XII is 183 J/mol higher than that of ice V at 0.5 GPa, and therefore ice XII is metastable with respect to ice V. The Gibbs energy of ice XII is 180-475 J/mol higher than that of ice VI at 1.1 GPa and 100 K. This is inconsistent with the recent deduction [T. Loerting, I. Kohl, C. Salzmann, E. Mayer, and A. Hallbrucker, J. Chem. Phys. 116, 3171 (2002)] that ice VI in the range 0.7-1.5 GPa and 158-212 K is metastable with respect to XII, and also with the speculation that proton (or orientationally)-ordered ice XII could have formed in Johari and Whalley's [J. Chem. Phys. 70, 2094 (1979)] search for orientationally ordered ice VI. An examination of the available dielectric data and Raman spectral features show no indication of ice VI to ice XII transformation at high pressures. Therefore, ice VI, not ice XII, is the stable phase at approximate to1 GPa and in the 158-212 K range. Ice polymorphs coexist in a thermoelastic equilibrium when the strain energy increase at the two-phase interface of the crystals growing in the parent phase becomes equal to the Gibbs energy decrease resulting from the growth. Hence a multiplicity of phases may coexist at high pressures. (C) 2003 American Institute of Physics.